RHEOLOGY AND STABILITY OF OLIVE OIL CREAM …repository.um.edu.my/1181/1/Thesis- Tan Hsiao Wei.pdf · rheology and stability of olive oil cream emulsion stabilized by sucrose fatty

RHEOLOGY AND STABILITY OF OLIVE OIL CREAM EMULSION STABILIZED BY SUCROSE FATTY ACID

ESTERS NONIONIC SURFACTANTS

TAN HSIAO WEI

FACULTY OF SCIENCE UNIVERSITY OF MALAYA

KUALA LUMPUR

JULY 2009

RHEOLOGY AND STABILITY OF OLIVE OIL CREAM EMULSION STABILIZED BY SUCROSE FATTY ACID

ESTERS NONIONIC SURFACTANTS

TAN HSIAO WEI

THESIS SUBMITTED IN FULFILMENT OF THE REQUIREMENTS

FOR THE DEGREE OF MASTER OF SCIENCE

FACULTY OF SCIENCE UNIVERSITY OF MALAYA

KUALA LUMPUR

JULY 2009

ii

ABSTRACT

The influence of oil and surfactant concentration to the stability and rheological behavior of

the olive oil emulsions stabilized with sucrose fatty acid ester was evaluated through an

accelerated aging test at 45oC. The stability of the emulsion in this study was examined by

the appearance of phase separation in the emulsion, mean droplet size and Zeta potential

over one month. The effect of accelerated ageing on the emulsions rheological properties

was investigated using oscillatory measurements and a viscometry test at the interval of one

day, one week and one month of storage time. The droplet size of the emulsions was found

to decrease with the increase in the oil and surfactant concentrations which give effect on

the viscosity and yield stress of the emulsions. The flow curve of the emulsions always

exhibited shear thinning behavior and obeys the Power Law viscosity. The shear thinning

effect of the emulsions was found to be decreased when the oil and surfactant concentration

increased due to the smaller droplet size and narrower size distribution. The dynamic

properties of the emulsions were also affected by the oil and surfactant content which

indicates the stronger structural integrity and greater interdroplet interactions. The

viscoelasticity of the emulsions was enhanced by the increased in the oil and surfactant

concentrations. The emulsions with higher oil composition show greater elasticity which

implies strong dynamic rigidity of the emulsions. The emulsions with 80% oil were the

most stable emulsions with longest shelf-life.

iii

ABSTRAK

Pengaruhan kepekatan minyak dan surfaktan terhadap kestabilan dan sifat rheologi bagi

emulsi yang distabilkan dengan Sucrose Fatty Acid Ester dikaji di bawah keadan terkawal

pada suhu 45oC. Penilaian terhadap kestabilan emulsi dijalankan melalui pengujian tahap

pisahan, saiz titisan, dan Keupayaan Zeta bagi emulsi tersebut dalam tempoh masa satu

bulan. Kesan menua keatas emulsi disiasat dengan menggunakan teknik rincihan secara

berayun dan suatu ujian viscometrik selepas masa penyimpanan sepanjang satu hari, satu

minggu dan satu bulan. Saiz titisan emulsi didapati menurun apabila peratusan kandungan

minyak dan surfaktan yang digunakan dalam penyediaan emulsi meningkat. Penurunan saiz

titisan ini telah memberikan kesan pada kelikatan dan tegasan alah emulsi tersebut.

Lengkungan aliran bagi emulsi tersebut mempamerkan sifat penipisan secara rincihan dan

mengikuti kelikatan Power-Law. Kesan penipisan secara rincihan bagi emulsi ini didapati

menurun apabila peratusan kandungan minyak dan surfaktan yang digunakan meningkat.

Ini disebabkan oleh titsan emulsi yang lebih kecil dan taburan saiz titisan emulsi yang lebih

sempit. Sifat dinamik bagi emulsi tersebut juga dipengaruhi oleh komposisi minyak dan

surfaktan yang menunjukkan kekuatan integriti struktur serta interaksi yang lebih besar

antara titisan emulsi. Sifat viskoelastik bagi emulsi juga dipertingkatkan dengan

peningkatan penggunaan minyak dan surfaktan. Emulsi yang disediakan dengan peratusan

minyak yang lebih tinggi mempunyai sifat keanjalan yang lebih besar dan ini merupakan

suatu tanda kekuatan ketegaran dinamik. Emulsi yang disediakan dengan 80% minyak

merupakan emulsi yang paling stabil dengan tempoh penggunaan yang terpanjang.

iv

ACKNOWLEDGEMENTS

I would like to take the opportunity to express my appreciation to many people that

have made this dissertation possible. First of all, I would like to express my sincere

gratitude to my supervisor Assoc. Prof. Dr. Misni Misran for his valuable guidance,

brilliant discussion, supervision and patience throughout the course of this research.

Special thanks to the Ministry of Science, Technology and Innovation (MOSTI) and

the University of Malaya that have generously been giving financial support towards my

master studies. My heartfelt gratitude also goes out to all lecturers and staffs in the

Department of Chemistry for their assiduous dedication and also the University of Malaya

management.

I would like to render my appreciation to Ms Loh Mei Ying, Mr. Tan Cok King and

Mr. Tiong Ngik Seng for their experiences, advices and guidance on theories and operation

of the instruments. I would also like to thank the members of Colloid and Surfaces

Laboratory for their encouragement and assistance throughout the research.

Last but not least, I would like to extend my deepest gratitude to my beloved

parents, my sister and my dearest friend Mr. Tay Kheng Soo for encouraging and inspiring

me all these years.

v

Table of Contents

ABSTRACT…………………………………………………………………………. ii

ABSTRAK…………………………………………………………………………... iii

ACKNOWLEDGEMENTS………………………………………………………… iv

TABLE OF CONTENTS…………………………………………………………… v

LIST OF FIGURES………………………………………………………………… vii

LIST OF TABLES………………………………………………………………….. xi

LIST OF ABBREVIATIONS……………………………………………………… xii

Chapter 1 General Introduction Page

1.1 Surfactant……………………………………………………………………... 1

1.2 The importance of glycolipid…………………………………………………

1.2.1 Sucrose fatty acid ester………………………………………………...

4

4

1.3 Aggregation of Surfactant to the Formation of Micelle………………………

1.3.1 The Dynamic Micellization……………………………………………

1.3.2 The Micellar Kinetics………………………………………………….

1.3.3 Micellar Kinetics and Emulsion……………………………………….

7

8

9

11

1.4 Emulsion

1.4.1 Hydrophile-Lipophile Balance (HLB)………………………………...

1.4.2 Determination of Emulsion Type……………………………………...

(a) Conductivity Measurement……………………………………….

(b) Dye Solubility Method…………………………………………...

(c) Dispersion in Water………………………………………………

(d) Microscopic Imaging……………………………………………..

1.4.3 Emulsion Stabilization………………………………………………..

1.4.4 Emulsion Applications………………………………………………..

11

14

15

16

16

17

17

18

22

1.5 Olive Oil……………………………………………………………………… 25

vi

1.6 Property of Emulsion…………………………………………………………. 27

1.7 Rheology……………………………………………………………………...

1.7.1 Historical perspective…………………………………………………

1.7.2 Application of Rheology……………………………………………...

29

29

32

1.8 Objective……………………………………………………………………... 34

Chapter 2 Materials and Methods

2.1 Materials ……………………………………………………………………. 35

2.2 Emulsion preparation…………………………………………………………. 35

2.3 Zeta potential measurement…………………………………………………... 36

2.4 Morphology and droplet size analysis………………………………………... 37

2.5 Rheological measurement…………………………………………………… 38

Chapter 3 Results and Discussion

3.1 Phase Separation……………………………………………………………… 40

3.2 The Effect of Surfactant, Oil Concentration and Storage Time on the Droplet

Size.…………………………………………………………………………...

3.2.1 Effect of Surfactant Concentration to the Droplet Size……………….

3.2.2 Aging effect…………………………………………………………...

3.2.3 Effect of oil concentration…………………………………………….

3.3 Zeta potential………………………………………………………………….

43

46

52

55

62

3.4 Rheological Properties of Emulsion System………………………………….

3.4.1 Steady State Behavior…………………………………………………

3.4.1.1 Effect of emulsion compositions on the flow properties……………...

3.4.1.2 Aging effect to the emulsions flow properties………………………..

66

66

66

81

3.5 Dynamic Characteristic ……………………………………………………… 82

4.0 Conclusion…………………………………………………………………

4.1 Future Research………………………………………………………

108

110

vii

5.0 References……………...………...………………………………………. 112

Appendix……………..………………………………………………….. 125

List of Figure Chapter 1 Page

Figure 1.1 Surfactants consist of two parts: (a) the hydrophilic part (head

group) and (b) the hydrophobic part (lipid tail)…………………….

1

Figure 1.2 The structure of Sucrose palmitate, which is a type of sucrose fatty

acid ester…………….........................................................................

5

Figure 1.3 The arrangement of surfactant in different types of aggregation (a)

micelle and (b) inverse micelle..........................................................

7

Figure 1.4 The mechanism of adsorption and desorption of surfactant

monomers under the dynamic equilibrium condition (above CMC),

where 1 1k k−= ……………………………………………………….

8

Figure 1.5 The schematic to show the second relaxation time, τ2 of the micelle 9

Figure 1.6 Schematic illustration of the effect of bulk concentration to

micellization process. (a) When the surfactant concentration is

below the CMC, the solution is very dilute and the kinetic diffusion

of the monomer is fairly important and the diffusion rate follow the

Fick’s First Law where the flux of diffusion (Jx) is proportional to

the concentration gradient ( dCdx

) and gives xdCJ Ddx

= − . (b) The

micelle concentration is small (just above the CMC), a dissemble

boundary is found, where there were two potential regimes, the

micelle zone and the micelle free zone. (c) When the bulk

concentration is above the CMC, a large diffusion flux drives the

micelle directly to the sublayer, continually supplying the

monomer to the surface……………………………………………..

10

Figure 1.7 Distribution of emulsion droplets (a) monodisperse, and (b)

viii

polydisperse………………………………………………………… 12

Figure 1.8 The schematic of (a) oil-in-water emulsion (o/w), (b) water-in-oil

emulsion (w/o), (c) water-in-oil-in-water emulsion (o/w/o) and (d)

oil-in-water-in-oil emulsion (w/o/w)………………………………..

13

Figure 1.9 The chemical structure of (a) Sudan III and (b) crystal violet........... 17

Figure 1.10 The schematic of emulsion destabilization mechanisms…………… 19

Figure 1.11 The schematic of interdroplet pair potential, w(h), where the wA

and wR are the attractive and repulsive interaction potential

respectively; hmin is the minimum separation of the emulsion

droplets which corresponds to the minimum interdroplet interaction

potential, wmin……………………………………………………………………………………

20

Figure 1.12 The schematic to illustrate the kinetic stability of an emulsion with

the activation energy before the system comes to a thermodynamic

stable state………………………………………………………......

20

Figure 1.13 The typical viscoelastic spectrum which is the master showing the

viscoelastic respond of non-Newtonian material…………………...

31

Chapter 2

Figure 2.1 The emulsion dispersed in (a) water very well, but resists the

dispersion in (b) olive oil indicating that the emulsion is an o/w

emulsion……………………………………………………………

36

Chapter 3

Figure 3.1 The emulsion volume fractions for emulsions at (a) day 1, (b) day

7, and (c) day 30…………………………………………………….

40

Figure 3.2 The mean droplet size for emulsions with (a) 50%, (b) 60%, (c)

70% and (d) 80% of disperse phase ……………………..…………

44

Figure 3.3 Micrographs of emulsion in 80% oil with (a) 2 wt%, (b) 5 wt%

and (c) 10 wt% of SFAE surfactants showing the decrease of

droplet size with increase in surfactant concentration……..………..

48

ix

Figure 3.4 The micrographs of dilute emulsion (50% oil) with (a) 2 wt%, (b) 5

wt%, and (c) 10 wt% of surfactant concentration..............................

51

Figure 3.5 The micrographs of emulsions with 50% disperse phase captured at

(a) day 1, (b) day 7 and (c) day 30, showing the coalescence

destabilization……………………………………………………….

53

Figure 3.6 The effect of oil concentration to the droplet size of emulsions over

30 days of storage under accelerating condition. (a) First day, (b)

7th day, and (c) 30th day.…………………………….........................

56

Figure 3.7 The micrographs for emulsions with (a) 50%, (b) 60%, (c) 70%,

and (d) 80% of oil and stabilized with 2 wt% of surfactant. These

micrographs were obtained after one day of storage under

45oC.………………………………………………………………...

60

Figure 3.8 The Zeta potential of emulsions measured in the presence of 0.01

M NaCl solutions for (a) 50%, (b) 60%, (c) 70%, and (d) 80% of

oil concentration ……….…………………………...........................

63

Figure 3.9 The shear rate dependence viscosity of emulsions with a series of

surfactant concentration for (a) 50%, (b) 60%, (c) 70%, and (d)

80% of oil…………………………………………...........................

67

Figure 3.10 The effect of oil concentration to the zero shear viscosity of

emulsion after (a) 1 day, (b) 7 days, and (c) 30 days of storage

under an accelerated condition of 45oC……………………………..

70

Figure 3.11 The yield stress of the emulsions determined using the Herschel-

Bulkely model from the Bohlin rheometer software, showing the

correlation between the mean droplet size and the yield stress of

the emulsion as a function of surfactant concentration for (a) 1st

day, (b) 7th day, and (c) 30th day………..…………………………...

73

Figure 3.12 Schematic representation of structural change when shear applied.

(a) The shear thickening region, (b) the First Newtonian region, (c)

the shear thinning region, and (d) the Second Newtonian region…..

77

Figure 3.13 The droplet size distribution of the emulsions stabilized with 2 wt%

of SFAE for (a) 1st day, (b) 7th day, and (c) 30th day of storage

time.....................................................................................................

77

x

Figure 3.14 The packing of emulsion droplets in the (a) polydisperse system

and (b) monodisperse system showing the droplet packing

efficiency……………………………………………………………

80

Figure 3.15 The γc of the emulsions for (a) 1st day, (b) 7th day, and (c) 30th

day......................................................................................................

83

Figure 3.16 The morphology of emulsions with 80% oil which stabilized by (a)

7 wt%, (b) 8 wt%, (c) 9 wt% and (d) 10wt% of SFAE……………..

85

Figure 3.17 The morphology for the emulsions with 70% oil stabilized with (a)

7 wt%, (b) 8 wt%, (c) 9 wt%, and (d) 10 wt% of SFAE…………...

87

Figure 3.18 The elastic modulus of emulsions at the first day of age obtained

from the strain sweep to establish the linear viscoelastic range. (a)

50%, (b) 60%, (c) 70%, and (d) 80% oil……………………………

90

Figure 3.19 The magnitude of elastic modulus of the emulsions obtained from

the strain sweep measurement for (a) 1st day, (b) 7th day, and (c)

30th day...............................................................................................

93

Figure 3.20 The cohesive energy of the emulsions obtained at (a) 1, (b) 7, and

(c) 30 days of storage periods……………………………………….

95

Figure 3.21 Frequency sweep profile of the emulsions with (a) 50%, (b) 60%,

(c) 70%, and (d) 80% of oil…………………………………............

97

Figure 3.22 The Tan δ of emulsions with (a) 50%, (b) 60%, (c) 70%, and (d)

80% of oil obtained after one day of storage………………………..

102

Figure 3.23 The effect of oil concentration to the Tan δ of emulsions stabilized

with 2 wt% of SFAE after (a) 1 day, (b) 7 days, and (c) 30

days………………………..……………………….......................

105

xi

List of Table Chapter 1 Page Table 1.1 The examples of different types surfactants …………………………. 2

Table 1.2 Some common hydrophilic surfactants found in commercial available

cosmetic products …………………………………………………….

3

Table 1.3 Microbial biosurfactants and their general structures ……………….. 6

Table 1.4 The classification of surfactants accordingly to the HLB values and

the applications………………………………………………………..

15

Table 1.5 The hydrophilic and lipophilic group number obtained by Davies

which are used in the calculation of HLB value of a surfactant..……..

15

Table 1.6 The type of fatty acid present in olive oil and its composition (in the

form of methyl ester)………………………………………………….

26

Table 1.7 The typical shear rate range for various physical operations with

examples………………………………………………………………

33

Chapter 3

Table 3.1: The Power Law index, n of the emulsions for over 30 days of storage

time. All n were smaller than 1 indicating that the emulsions exhibits

shear thinning behavior………………………………………………..

76

xii

List of Abbreviations SFAE Sucrose Fatty Acid Ester

CMC Critical Micelle Concentration

o/w Oil-in-water

w/o Water-in-oil

o/w/o Oil-in-water-in-oil

w/o/w Water-in-oil-in-water

HLB Hydrophilic-Lipophilic Balance

FDA Food and Drug Administration

EU European Union

GC/MS Gas chromatography/mass spectrometry

CHAPTER 1

INTRODUCTION

Chapter 1 Introduction

________________________________________________________________________ - 1 -

1.0 General Introduction

1.1 Surfactant

The word surfactant (Figure 1.1) comes from an abbreviation for surface active

agent. They are amphiphilic molecules, containing both the hydrophilic and

hydrophobic parts. This duality character enables the surfactant to adsorb at the surface

or interface of two or more immiscible phases thereby reducing the surface or interfacial

tension. The ability of surfactant to adsorb at interface may also be used to prevent

aggregation, flocculation and coalescence of emulsion droplets to enhance the emulsion

stability. They replaced the energy rich bulk phase molecules and reduced the surface

free energy of the system, thus reduce the surface/interface tension [1]. Surfactants are

often classified according to the charge of their head group into anionic, cationic,

nonionic, and zwiterionic (Table 1.1). Surfactants have been widely used in household

products, cosmetic, food and pharmaceutical industries [2] as emulsifiers, wetting

agents, cleansers, foaming agents [3], and drug delivery applications [4, 5]. Surfactants

are also used for creating dispersion system such as emulsions and suspensions for

nanoparticles synthesis [3].

Figure 1.1: Surfactants consist of two parts: (a) the hydrophilic part (head group) and (b)

the hydrophobic part (lipid tail).


________________________________________________________________________ - 2 -

Table 1.1: The examples of different types of surfactants

Surfactant Type Example

(a) Anionic

(b) Cationic

(c) Nonionic

(d) Zwiterionic

In cosmetic industry, surfactants are used as emulsifier, wetting agent, cleanser,

foaming agents, solubilizers, conditioners, thickener and also to produce emollients [2,

3]. Thus, these surfactants used in cosmetic formulations are expected to be human

friendly. Ionic/hydrophilic surfactants (Table 1.2) especially anionic surfactants are

commonly used surfactants in most cosmetic formulations. However, there were studies

proven that the surfactants are harmful to human and environment [6-8]. That is because

the toxicity of these anionic surfactants can harm the cell membrane, enzyme activity,

the binding properties of proteins, and other cell components of aquatics and human [6,

7]. Application of these surfactants in pharmaceutical formulations influences the

biological efficiency of the active ingredient in the formulations [9]. It has been reported


________________________________________________________________________ - 3 -

that these ionic surfactants will either bind directly to the drug [10] or by influencing the

adsorption and absorption processes and the partition of drugs between hydrophobic and

hydrophilic compartments in the organ and organisms [11]. In other words, the

surfactants penetrate through the skin and interfere with the function of the cell

membrane [3, 4, 12].

Table 1.2: Some common hydrophilic surfactants found in commercial available

cosmetic products [13].

Class of Surfactant Head Group Structure

Sulfate Carboxylate

Phosphate

Sulfonate

Betaines

Besides that, disposal of these ionic surfactants mentioned earlier has become

another major concern due to their potential to induce ecotoxicity [3]. Owing to the

extensive use of surfactants in our daily life and industrial world, a considerable amount

of these ionic surfactants has been released into the environment causing serious

pollution to the river and sea [6, 14, 15]. Most of the synthetic surfactant are

biorecalcitrant and the ecological impact of these surfactants is the major concern [16].

Thus, there is a preference to use biosurfactants for formulations such as glycolipids

[17], which are nonionic surfactants, biocompatible and biodegradable in nature.


________________________________________________________________________ - 4 -

1.2 The importance of glycolipid

Glycolipid is one type of the biosurfactant [18] and is a popular class of

biosurfactant used in the industrial applications. In recent years, biosurfactants such as

glycolipid have drawn a large attention among the formulators and researches to replace

the synthetic surfactants in house hold products, personal care products, and

pharmaceutical products due to the destructive cause of the synthetic surfactants to the

environment and human [18]. Glycolipids can be naturally produced by bacteria, fungus,

and yeasts as shown in Table 1.3 [18]. They are known for biocompatibility [19],

therefore, is a promising alternative to replace synthetic surfactants in our daily used

products such as cosmetics, pharmaceuticals and foods. Besides that, the glycolipids are

also environmental friendly due to their biodegradable property in nature [20].

1.2.1 Sucrose fatty acid ester

Sucrose fatty acid ester (SFAE) is a glycolipid surfactant (Figure 1.2) and is

non-ionic. It is widely used as an emulsifier in cosmetic and pharmaceutics industries

for a number of years to replace the petroleum based surfactants. The non-toxic nature

of this compound has also led to the extensive use in the formulation of food products

[21], which due to the naturally occur sugar head group. The interests of developing the

sucrose ester were started from the late 1950s as a natural alternative and is

commercially produced [22]. Their amphiphilic character can be controlled within wide

limits by altering both the degree of esterification and the chain length of the ester group,

so that extensive permutations are possible to obtain a required hydrophile-lyophile

balance [23]. Sucrose esters have a range of applications in the food, cosmetics, oral-


________________________________________________________________________ - 5 -

care, detergent, and pharmaceutical industries [24, 25]. Their properties as

antimicrobials are useful in food storage [26], antitumorals [27] and insecticidals [28].

This carbohydrate-based surfactant [29] can be easily synthesize by

esterification process in the presence of a biocatalyst [30, 31], fermentation with

Corynebacaterium Hydrocarboclastus [32]. There is also no concern about the

environmental pollution of this surfactant, due to the biocompatibility and

biodegradability [18, 33].

HO

HO

O

HO

O

HO OH

OH

OH

OO O

Figure 1.2: The structure of Sucrose palmitate, which is a type of sucrose fatty acid ester.


________________________________________________________________________ - 6 -

Table 1.3: Microbial biosurfactants and their general structures [18].

Microorganism Biosurfactant General Structure

Rhodococcus

erythropolis

Sucrose and

fructose lipids;

Trehalose

lipids;

Trehalose

mycolates

Torulopsis

species Sophorolipids

Psedomonas

species Rhamnolipids

Candida

Petrophilum Peptidolipid


________________________________________________________________________ - 7 -

1.3 Aggregation of Surfactant to the Formation of Micelle

Surfactants tend to form aggregates spontaneously in solution to form a

thermodynamically stable structure, so called micelle [34]. The increase of surfactant

concentration in the bulk phase leads to micelle formation. Concentration when the

micelles starts to form is the Critical Micelle Concentration (CMC) [35]. Generally,

there the surfactant aggregates to form the micelle or inverse micelle (Figure 1.3).

However, the shapes and structures of the aggregation are dependent on the nature of

surfactant and the molecular geometry [36]. In a micelle, the hydrophilic head group of

the surfactant is directed to the aqueous phase (polar) allowing the formation of the

hydrogen bonding between the polar head group and water molecules; while the

hydrophobic tail is protected in the core of the micelle from polar environment to reduce

the free energy of the system (Figure 1.3(a)) [37].

Fig. 1.3(a)

oil

Fig. 1.3(b)

water

Figure 1.3: The arrangement of surfactant in different types of aggregation (a) micelle

and (b) inverse micelle.


________________________________________________________________________ - 8 -

1.3.1 The Dynamic Micellization

Figure 1.4: The mechanism of adsorption and desorption of surfactant monomers under

the dynamic equilibrium condition (above CMC), where 1 1k k−= .

The micelles are the reservoir for the surfactant monomer in solution and surface

activity is reduced due to the shielding of the hydrophobic tail in the core of the micelle,

which is sensitive to non-polar environment [34, 38]. In other words, the micelle cannot

adsorb directly to the available surface and interface and only the monomers are

responsible to the surface, interfacial tension reduction and dynamic phenomena, such

as emulsification, wetting and foaming [34]. As the surfactant concentration exceeds

CMC, the amount of micelle and surfactant monomers are in dynamic equilibrium

indicating that the rates of adsorption and desorption between monomers and micelles

are the same (Figure 1.4) [39]. The rate of association and dissociation are in a very fast

time scale (~ 10-6s), and is called the fast relaxation process of micelle [39, 40].


________________________________________________________________________ - 9 -

1.3.2 The Micellar Kinetics

There is another relaxation process of micelle to explain the life-time of micelles

(Figure 1.5). The relaxation time in this process is very important, because the time

scale can vary from millisecond to minutes [39]. The longer the relaxation time scale,

the more stable the micelle is. However, that is not a favor able condition, because the

more stable the micelle becomes, the longer the time that will be taken by the system to

reach dynamic equilibrium at the interface. Figure 1.6 shows the transportation model

proposes by Song et. al. [37]. Besides, there were also some other adsorption kinetic

theories having almost the same concept, saying that the micelles have to be broken into

surfactant monomers before adsorption to the interface [41-43].

Figure 1.5: The schematic to show the second relaxation time, τ2 of the micelle.


________________________________________________________________________ - 10 -

(a)

(b)

(c)

Figure 1.6: Schematic illustration of the effect of bulk concentration to micellization

process. (a) When the surfactant concentration is below the CMC, the solution is very

dilute and the kinetic diffusion of the monomer is fairly important and the diffusion rate

follows the Fick’s First Law where the flux of diffusion (Jx) is proportional to the

concentration gradient ( dCdx

) and gives xdCJ Ddx

= − . (b) The micelle concentration is

small (just above the CMC), a dissemble boundary is found, where there were two

potential regimes, the micelle zone and the micelle free zone. (c) When the bulk

concentration is above the CMC, a large diffusion flux drives the micelle directly to the

sublayer, continually suppling the monomers to the surface [37, 44].


________________________________________________________________________ - 11 -

1.3.3 Micellar Kinetics and Emulsion

The micelle kinetic adsorption has a correlation with the emulsification and

several other dynamic processes such as wetting and foaming. During emulsion

preparation, energy is applied to break down the oil layer into small oil droplets. The

process increased the oil-water interfacial area. Thus, more surfactant monomers are

required to stabilize these oil droplets against destabilization process. Therefore, the

amount of surfactant used in emulsification has to be well above the CMC, in order to

maintain the equilibrium between micelles and monomers in the interfacial layer and

bulk phase. For this reason, a micelle with shorter life time is favorable.

Certainly, very stable micelles face a difficulty to supply sufficient amount of

monomers onto the large interfacial area. Owing to their long relaxation time, τ2 they are

only able to release a small amount of surfactant molecules to the bulk, creating low

monomer flux. As a result, the oil droplets are not well covered by the surfactant,

increasing the interfacial tension and eventually the oil droplet size increases [45-48].

1.4 Emulsion

An emulsion is a system consisting of two immiscible liquids dispersed in one

another and is thermodynamically unstable [1]. An emulsion can be kinetically stable

(long-term stability) with the presence of surfactant in the system by creating an energy

barrier to flocculation and coalescence [1, 49] and exists in a metastable state [50]. The

droplets size of emulsion is generally greater than 100 nm [1, 51]. The appearance of an

emulsion is turbid, because the droplets are larger than the wavelength of visible light

[51]. In real emulsions, the size distribution of droplets is generally polydisperse as

described in Figure 1.7 [52].


________________________________________________________________________ - 12 -

The mean droplet size (d) and polydispersity (p) are normally used to characterize

emulsions (Eq. 1 and 2) [53]. According to Leal-Calderin, F., et. al. [53], an emulsion

with P ≤ 25% is considered as monodisperse system. However, other researchers have

different opinion about the polydispersity [54]. They believe that monodispersity is a

relatively term. They had proposed that a system with 10% of polydispersity in the

droplets size is acceptable to characterize as a monodisperse system.

4

3

i ii

i ii

N dd

N d=∑

∑ (1)

3

3

1 i i ii

i ii

N d d dp

d N d

−=

∑

∑ (2)

d is the median droplet diameter and iN is the total number of droplets with diameter id .

(a)

(b)

Figure 1.7: Distribution of emulsion droplets (a) monodisperse, and (b) polydisperse.


________________________________________________________________________ - 13 -

The emulsion can be dispersed in several manner and the most common are oil-in-

water (o/w), water-in-oil (w/o), and double emulsion such as oil-in-water-in-oil (o/w/o)

and water-in-oil-in-water (w/o/w) (Figures 1.7 and 1.8) [1]. The dispersion type of

emulsion is highly dependent on the nature of surfactant used in the system and

emulsion preparation [1, 55]. According to Bancroft’s postulate, the phase in which the

surfactant is most soluble is the continuous phase [1, 56]. In order words, hydrophobic

surfactant will form w/o emulsion; hydrophilic surfactant will form o/w emulsion. As

indicated above, the double emulsions can be prepared (Figure 1.8(c) and 1.8(d)) by

dispersing w/o in water continuum or o/w in oil continuum. For example, a w/o/w

emulsion is prepared by emulsifying simple w/o emulsion in an aqueous solution of a

high Hydrophile-Lipophile Balance value surfactant and vice versa [57, 58].

(a) (b)

(c)

(d)

Figure 1.8: The schematic of (a) oil-in-water emulsion (o/w), (b) water-in-oil emulsion

(w/o), (c) water-in-oil-in-water emulsion (o/w/o) and (d) oil-in-water-in-oil emulsion

(w/o/w).


________________________________________________________________________ - 14 -

1.4.1 Hydrophile-Lipophile Balance (HLB)

The HLB value is an indication of the solubility of surfactant. The concept was

introduced by Griffin in 1954 [59] and is extended by Davies in 1957 [1, 60]. This

theory proposed that the hydrophobic surfactants have relatively low HLB value

compared to hydrophilic surfactants (Table 1.4). These hydrophobic surfactants are

predicted to be suitable for the formation of w/o emulsion, whereas the hydrophilic

surfactants are good for the formation of o/w emulsion. Therefore the continuous phase

is in no need to be predominating in quantity of the material. For example, an o/w

emulsion can be formed by using oil to water ratio of 8:2 with hydrophilic surfactant.

There were several methods to calculate the HLB value of a surfactant. Equation

(3) shows the calculation of HLB value from the structure of the surfactant. Davies who

extended the concept in 1957 proposed another calculation of HLB value (Eq. (4))

which is based on the group contribution method (Table 1.5) [1, 60], where Σ

hydrophilic = n × hydrophilic group number; Σ hydrophobic = n × hydrophobic group

number and n is the repeating number of the particular group. However, Davies’s

method is not suitable for some of the surfactant especially nonionic surfactants due to

the lack of group numbers [61, 62].

20 H

H L

MHLBM M

= ×+

(3)

HM is the molecular weight of the hydrophilic part and LM is the molecular weight of

the lipophilic part [1, 63].

7HLB hydrophilic lipophilic= ∑ − ∑ + (4)


________________________________________________________________________ - 15 -

Table 1.4: The classification of surfactants accordingly to the HLB values and the

applications [64].

Solubility of Surfactant in Water HLB Value Applications

Insoluble 4-5 w/o

Partially soluble 6-9 Wetting agent

Translucent to clear 10-12 Detergent

Very soluble 13-18 o/w

Table 1.5: The hydrophilic and lipophilic group number obtained by Davies which are

used in the calculation of HLB value of a surfactant [36].

Hydrophilic group HLB Value Lipohilic Group HLB Value

-SO4Na 35.7 -CH- -0.475

-COOK 21.1 -CH2- -0.475

-N 9.4 =CH- -0.475

Free Ester 2.4 -CH3 -0.475

Free Alcohol 1.9

-CO2H 2.1

-O- 1.3

1.4.2 Determination of Emulsion Type

As mentioned in section 1.2, there are several types of emulsions. In general,

o/w and w/o emulsions is the most common emulsion we came across. Therefore it is

necessary to differentiate these two types of emulsions. There are several methods that

have been used to differentiate types of emulsions such as conductivity measurement

[65], dye, microscopic imaging [1] and dispersion in water.


________________________________________________________________________ - 16 -

(a) Conductivity measurement

The test is based on the principle of the good conductivity of the aqueous phase

[66]. This method is commonly used where high resistance indicated oil as the

continuous phase, and low resistance indicated water as the continuous phase [65].

According to Hanna, S.A. [67], the o/w emulsion passes a current of 10-13 mA, while

for the w/o emulsion it is only 0.1 mA or less. Therefore a crude experiment is

performed by dipping two electrodes attached to an electric circuit with a light indicator

into the test emulsion (in the present of some electrolyte). Owing to the ability of the

o/w to conduct electric, the light goes on if the test emulsion is an o/w emulsion and

vice versa [67]. This method is useful during the study of an emulsification technique so

called the Phase-Inversion Temperature (PIT) emulsification technique to produce a

fine and more stable emulsion [68, 69].

(b) Dye Solubility Method

This method has the same purpose as the previous method that is to identify the

type of continuous phase of an emulsion [67]. Generally, water or oil soluble dyes such

as Sudan III and crystal violet which is oil-soluble and water-soluble dyes respectively

are chosen to perform the test (Figure 1.9). For example, Sudan III which is the oil

soluble dye will turn w/o emulsion into red colour, while an o/w emulsion will turn to

blue colour as the water soluble is used. Although this method is relatively easy to

perform, nevertheless there are risks and safety precautions when using these dyes.

According to Refat, N.A. et. al., the International Agency for Research on Cancer has

classified the sudan dye as category 3 carcinogen [70], therefore this dye has been


________________________________________________________________________ - 17 -

categorized as illegal food additives (according to FDA and EU) [71] and is advised not

to have any contact with eyes and skin.

(a)

(b)

Figure 1.9: The chemical structure of (a) Sudan III and (b) crystal violet.

(c) Dispersion in water

This method is a crude method which operates on the basis of the ability of an

emulsion to be diluted by its continuous phase. For an o/w emulsion, the emulsion is

expected to able being diluted with water and gives a well dispersion. On the other hand,

if an oil phase of the o/w emulsion is used to dilute this emulsion, then the emulsion

tend to resist dispersion [1, 66].

(d) Microscopic imaging

Observation through the microscope is the direct way to determine the type of an

emulsion. Owing to the difference of reflective index for two liquids, it helps to

distinguish the type of emulsion. A liquid with higher reflective index will appear

brighter under a simple optical microscope and vice versa [1, 72]. For example, the


________________________________________________________________________ - 18 -

reflective index for olive oil ranges from 1.46 to 1.47 [73], while the reflective index for

water is 1.33. According to the principal rule that higher reflective index shows brighter

image, therefore it is expected that the oil droplet will look brighter than the water

droplet [74-76]. This microscopic technique is a more detail technique as compared to

others. This is because one can observe the microscopic structure and also determine if

there is any formation of mix emulsion (o/w and w/o emulsion) which other methods

are not providing this information. Beside, this technique also allows a direct

observation of the droplet shape and measurement of the droplet size.

1.4.3 Emulsion Stabilization

As discussed before, emulsions are thermodynamically unstable and there are

various factors affecting the stability, eventually phase separation. These factors do not

only come from the internal such as the interfacial properties, but also from the external

such as the storage time and conditions (temperature and humidity), action of bacteria,

and mechanical agitation [77].

Emulsion stability is refered to the ability of an emulsion to resist change with

time. Since the emulsion is thermodynamically unstable, they are expected to undergo

destabilization after a period of time leading to a total phase separation [51]. For this

reason, an emulsifier is used to increase the stability of the emulsion system as

discussed in section 1.1. The instability of emulsion discussed is referred to physical

instability such as sedimentation/creaming, flocculation, coalescence, and Ostwald

ripening.


________________________________________________________________________ - 19 -

Figure 1.10: The schematic of emulsion destabilization mechanisms.


________________________________________________________________________ - 20 -

Figure 1.11: The schematic of interdroplet pair potential, w(h), where the wA and wR are

the attractive and repulsive interaction potential respectively; hmin is the minimum

separation of the emulsion droplets which corresponds to the minimum interdroplet

interaction potential, wmin [78].

Figure 1.12: The schematic to illustrate the kinetic stability of an emulsion with the

activation energy before the system comes to a thermodynamic stable state.


________________________________________________________________________ - 21 -

(i) Sedimentation/creaming of emulsion droplets happens due to the density

difference between the two phases and are forms of gravitational separation [50]. For

example, in creaming process (Figure 1.10(d)), the oil droplet (o/w emulsion) moves

upward to the surface due to its lower density as compared to that of water. This

gravitational instability rate is affected by the flocculation mechanism [79]. Flocculation

(Figure 1.10(a)) is an aggregation process of two or more droplets to form flocs [80,

81]. Flocculation only happens after collision of droplets. After collision, particles may

either move away from each other or form permanent aggregate [82]. This is highly

dependent to the types of interaction (attractive and repulsive) between the droplets [81].

When the attractive force is dominant, collision of droplets will leads to floc formation

(Figure 1.11). The flocculation is a reversible process, since the droplets will re-

disperse after subjected to a gentle agitation. This flocculation process enhances the

gravitational separation rate and is a significant destabilization process in dilute

emulsion, It decreases the shelf life of the emulsion [83].

(ii) Coalescence is another emulsion destabilization mechanism. Coalescence is a

process whereby two or more droplets merge together to form a single larger droplet

which is the most thermodynamically stable condition (Figure 1.10(b)) [84]. This

process can only happen when the droplets are close together and the interfacial

membrane between the droplets is disrupted [84]. In general, the forces acting between

the droplets and the resistance of droplets against membrane rupture are the major

factors affecting the coalescence process and is important for a concentrated emulsion

[84]. The stiffness of the interfacial layer is the key to the droplet coalescence, which

creates an energy barrier that has to be overcome before the thermodynamic stable state

is reached (Figure 1.12). Therefore, it is necessary to introduce a strong interfacial layer

to an emulsion in order to enhance the emulsion stability. According to Figure 1.11,


________________________________________________________________________ - 22 -

the coalescence often happens when the droplets are in a very close distance. At this

point, the attractive force is greater than the repulsive force and causes failure of the

interfacial layer to protect the droplets. Consequently, the droplets will merge and the

energy of the droplet will fall into a deep minimum wmin, which is an irreversible

process.

(iii) Ostwald ripening (Figure 1.10(c)) is a diffusive transfer of disperse phase from

smaller droplets to larger droplets under the influence of Laplace pressure difference [85,

86]. The Ostwald ripening destabilization occurs especially in a polydisperse emulsion

[87] facilitated by the presence of micelle in the continuous phase [88-90]. The micelles

solubilize the oil molecules and transported them from one droplet to another [88]. In

other words, the micelles enhance the Ostwald ripening by increasing the solubility of

oil in water, allowed the oil molecules diffused from the small droplet to the larger one.

However, the rate of destabilizations can decrease by having small emulsion

droplet size, increasing the viscosity of the disperse medium, lowering the difference in

density between the two phases or/and creating an energy barrier at the oil and water

interface [1].

1.4.4 Emulsion Applications

Emulsions are widely used as commercial products (such as foods, cosmetics,

and paints) [91, 92] and oil recovery processes [93, 94]. Emulsions are used in cosmetic

industry as lotions, creams, moisturizers, and shower gels. The main functions of

emulsion in cosmetic industry are moisturizing and occlusion to prevent the loss of

water from the skin [1, 95]. The emulsion is a good system for both the hydrophobic


________________________________________________________________________ - 23 -

and hydrophilic substance (active ingredients) such as drugs and vitamins in cosmetic

and pharmaceutical formulations for various applications [96].

As in pharmaceutical industry, emulsions act as a carrier for active ingredients

or drugs [96]. Although most of the drugs are water soluble, and can be injected through

aqueous solutions or water-in-oil emulsions, nevertheless reports have shown that the

delivery of drugs through fatty emulsions are more effective [97-99]. The application of

emulsion in pharmacy as parental emulsion is relatively important especially for the

treatment of critical ill patient [100]. A parental emulsion is a special oil-in-water

emulsion that used to feed the patients whose medical condition makes them unable to

eat normally [101]. Hence, careful selection of component for the formulation has to be

stringently considered.

The oil phase of the formulation has to be either paraffinic or vegetable base,

and only nonionic emulsifiers are suitable. Besides that, the emulsion viscosity has to be

as low as 1 mPa s and the droplet size of the emulsion has to be small (of less than 5μm)

in order to avoid blockage of the vessels during drug delivery and reduce the risk of

toxicity [102]. Certainly, the emulsion has to remain stable at least for 1-2 years and

also at high temperature. At high temperature, destabilization process will be more

pronounced especially coalescence of emulsion droplets can lead to an increase of

droplet size which is totally undesirable during application. For this reason, the

emulsions are always kept under refrigerated condition (~ 3-4oC).

O/W emulsions are often used for intramuscular injection which is the injection

of drugs directly into muscle. Besides that, these emulsions are also used in

chemotherapy treatment for cancer [102]. Therefore, the emulsions have to be able to

reach the target and breakup to release the active compounds on site efficiently. The

efficiency of these emulsions is highly depending on the type of emulsifier used. In


________________________________________________________________________ - 24 -

some condition, modifying the applied emulsifiers is necessary in order to enhance the

efficiency of drug loading and delivering.

Cosmetic products had a very large global market and valued at $125.7 billion in

the year of 1998 [103]. Today, the Asia-Pacific region has become the most valuable

cosmetic market in the world and is leading in global skin care products market.

According to the Euromonitor International’s market report, the Asia-Pacific is the

world’s second largest cosmetics and toiletries market with sales of $62.1 billion in the

year of 2005 [104]. Emulsion is generally used as a vehicle carrying the cosmetic active

ingredients in cosmetic products. Unlike pharmaceutical emulsions, most cosmetic

emulsions are care products which achieved caring and preventing effect on the site of

application. The skin and hair are the most common sites of application.

Playing almost the same role as the pharmaceutical emulsions, cosmetic

emulsions also delivered cosmetically active compounds to target side of application.

Since the cosmetic emulsions are applicable through the skin, hair and nails, emulsions

are more likely to play roles of caring and preventing to the outer layer of our human

body. Besides carrying specific component for special effects such as anti-aging and

cleansing, the cosmetic emulsions have more important function which is to create a

protective layer against external potential damaging factors such as UV radiation. By

applying a suitable and appropriate cosmetic emulsion containing specific active

ingredients helps to improve the appearance of the outermost organs. For examples, skin

dehydration can be prevented through the application of moisturizer which contain

hydrating agent that can retard moisture loss from the skin [105].

An effective cosmetic emulsion has to fulfill several criteria in order to have

maximum performances. First and foremost, the cosmetic emulsions should have long

shelf life and stable at a wide range of temperature. Since the action of cosmetic

emulsions is limited to the outer organ, therefore a pleasant feeling upon application and


________________________________________________________________________ - 25 -

the consistency to achive this pleasant feeling are relatively important such as

moisturizers. The purpose of using moisturizer is to increase the water-holding capacity

and protect the deposit of oily material from the environment that we are exposed to

[106]. Therefore, sticky and greasy product definitely influences consumer preference.

It is also an important point that the emulsions should not cause any skin irritation or

allergenic effect upon application. For this reason, the raw ingredients for the

formulation have to be biocompatible.

Emulsions are also applicable in other industries such as food, agriculture, paint,

paper coating, lubrications, petroleum extraction, bitumen emulsion and etc [102].

Emulsions can easily entrap both hydrophobic and hydrophilic active ingredients by

reducing the usage of organic solvent where some of them are very toxic and is a

potential carcinogenic compound. Moreover, the homogeneity of emulsions makes it to

be more easily spread thereby enhances the spreadability of the active ingredients at the

application site. This helps in reducing toxicity of the concentrate active ingredients.

1.5 Olive Oil

Olive, which is famous for its valued fruit and oil, has long been cultivated in

the Mediterranean Basin and is one of the important parts in the Mediterranean-style

diet. The Mediterranean-style diet is famous in reducing the risk of heart and other

chronic diseases by lowering the low-density lipoprotein cholesterol level due to the

high content of monounsaturated and polyunsaturated fatty acids in nuts [107, 108].

The olive oil is widely used as cooking oil, salad dressing, fuel for traditional oil lamp,

and more importantly some applications in formulating cosmetics and pharmaceuticals

products such as skin and hair moisturizer. This is due to the ability of olive oil to


________________________________________________________________________ - 26 -

promote smooth and radiant complexion, maintaining the elasticity of the skin, and also

conditioning and adding shine to hair.

Table 1.6: The type of fatty acid present in olive oil and its composition (in the form of

methyl ester).

Composition Percentage (mol/mol) IUPAC Name

Oleic acid 56.0 – 83.0 Octadec-9-enoic acid

Palmitic acid 7.5 – 20.0 Hexadecanoic acid

Linoleic acid 3.5 – 20.0 9,12-octadecadienoic acid

Stearic acid 0.5 – 3.5 Octadecanoic acid

Palmitoleic acid 0.3 – 3.5 Hexadec-9-enoic acid

Linolenic acid 0.0 – 1.5 9,12,15-octadecatrienoic

acid

Myristic acid 0.0 – 0.5 Tetradecanoic acid

Others Minor -

Olive oil is the richest source of the monounsaturated fatty acid, the oleic acid.

According to the Recommended International Standard for Olive Oil, Virgin and Refine

(Table 1.6), olive oil has a high composition of unsaturated fatty acid especially oleic

acid (56% - 83%) [109]. These monounsaturated fatty acids are proven to be beneficial

to human which help to reduce canser risk. According to epidemiological studies, they

had report and show evidences that the monounsaturated fatty acids reduce the cancer

risk especially in breast canser [110-114]. There are also several studies discussing and

proposing method and formula using olive oil to skin canser [115, 116]. Their results

show that the number of tumors in the mice pretreated with olive oil has significantly


________________________________________________________________________ - 27 -

reduced as compared to the untreated mice, which indicated that the olive oil can

effectively protect the skin and reduce the risk of having skin cancer [117, 118].

Beside internal use, olive oil which is a type of essential oil is also suitable for

external application such as carrier oil in aromatherapy treatment for body massage. The

function of carrier oil is to dilute the concentrate essential before applying to the skin.

This is because the potential of the concentrate essential oil such as Lavender and

Jasmine to cause skin irritation if applied to the skin without further dilution.

The olive oil is also a rich source of antioxidants such as flavenoid polyphenols

and vitamins A, D, E, and K. These natural antioxidants are able to provide protection

to the skin against the sun. The flavenoid polyphenols are also effective in healing of

sunburn, lowering the cholesterol level, blood pressure, and the risk of coronary disease

[119, 120]. Owing to the extensive antioxidant and benefits of olive oil to human

especially the skin, olive oil is selected to be the oil phase of the emulsions in this study.

1.6 Properties of Emulsion

The basic condition for the formation of emulsion is by mixing two immiscible

liquid such as oil and water. As mentioned in section 1.4, that system is not stable.

Therefore another component, the surfactant is employed to ‘hold’ the droplets, ensure

that the droplets are well dispersed in the continuous phase and stable against

destabilization process. Upon addition of surfactant, the complexity of emulsion system

increases which involves various types of interaction forces affecting the microstructure

of the emulsion such as droplet size and droplet size distribution. Since there is a strong

relation between microstructure and macroscopic properties of an emulsion, the changes

that happen in the emulsion microstructure will affect the macroscopic properties of the

emulsion such as their rheology. The rheology is related to the study of the flowability


________________________________________________________________________ - 28 -

and elasticity of the emulsion. For examples, there are differences in the viscosity of the

coarse and fine emulsions, by which the fine emulsion will have a higher viscosity and

elasticity as compared to the coarse emulsion.

At the commercialization view point, high viscosity, low oil content and fine

droplet size emulsion is required. This is because the high viscosity will enhance the

emulsion stability; lower oil content will decrease greasy feeling on the skin; while fine

droplet size enhance the delivery of active ingredients into the skin. For these reasons,

additives such as thickeners, co-surfactants and other additives for specific application

are added into the emulsion during formulation. These additives’ are commonly used in

formulating cosmetic creams and food emulsions. The additives such as thickeners are

often used to act as viscosity modifiers in order to modify the viscosity of the

continuous phase to obtain more stable emulsion. Besides, it also enhances the

properties of the emulsion and promotes the pleasant feeling upon applications. For

example, in cosmetic, the viscosity of the creams is related to the spreading of the

creams on the skin; in food, the emulsion should have good mouthfeel and chewability.

The spreading and chewing properties can be monitored by studying the

rheology of the emulsions such as the viscosity. Beside the viscosity, there is another

important rheological property of emulsion which is the viscoelasticity that tells the

elasticity of the emulsion whether it is more solid-like or liquid-like. The study of

viscoelastic properties of an emulsion is also providing useful information about long-

term stability of the emulsions and the stability of the droplets against destabilization

processes such as coalescence and flocculation [121]. Consequently, the study of

rheology of emulsion is essential especially in the industries of cosmetic and

pharmaceutics to evaluate the properties and stability of the products.


________________________________________________________________________ - 29 -

1.7 Rheology

1.7.1 Historical perspective

The word rheology comes from the Greek words which carry the meaning of

flow for “rheo” and “-ology” for study of [122]. The term rheology was first introduced

by Professor Bingham from Lafayette College. Rheology is an independent scientific

discipline, studying the deformability and flow properties of a matter under an applied

stress or strain [123, 124]. The definition of rheology mentioned above was accepted by

the American Society of Rheology in 1929 [124].

For the past 300 years, behaviors of perfect liquids and solids were represented

mathematically by Newton’s law and Hooke’s law respectively, assuming that these

were the universal laws [125, 126]. However, there are a lot of materials in the world

that have an intermediate mechanical behavior in between the perfect solids and liquids

which cannot be described by the classical theories. Scientists began to have doubt

when Wilhelm Weber and James Clerk Maxwell found the non-ideal behavior of silk

threads and fluids respectively. Wilhelm Weber’s experimental results show that the silk

threads are not fully elastic material; Maxwell’s fluids were found to have some elastic

properties which were mathematically proved [125].

According to the response of silk threads and elastic fluid, they were classified

as the typical viscoelastic and non-Newtonian materials. A viscoelastic material will

recover after being subjected to small deformation showing the elasticity of the material,

while a non-Newtonian material has a time and stress dependent viscosity providing

information on the continuous flowability [127]. In order to have better understanding

of the terms viscoelastic and Newtonian or non-Newtonian, one may consider the

behavior of cheese and honey. According to the data collected from rheological

measurements, cheese was showing a viscoelastic response [128, 129], while honey is


________________________________________________________________________ - 30 -

the typical Newtonian fluid which the viscosity is independent of time and applied

stress [130-132].

The typical viscoelastic response of non-Newtonian liquid as a function of

frequency is presented in Figure 1.13. This viscoelastic spectrum (Figure 1.13) is

obtained from the frequency sweep at a wide range of frequency with a fix strain. The

spectrum can be divided into five regions, which are the terminal, transitions, plateau

and glassy regions. Owing to the capability of the instrument used in this study, only

regions II and III were detected for all the emulsions in this study. Region I, which is

the terminal region, is obtained at a very low frequency range and the material will

behave as a liquid showing predomination of G’. As the frequency increases, the

magnitude of G’ increases and become dominant against the G”, which is the plateau

zone. Before this plateau zone, there is a transition zone where the two G’ and G”

curves crossover at t*. This crossover point represented the relaxation time of the tested

sample. This point is important for designing products in industries because it provides

curing information especially in polymer industry producing epoxy resins [133].


________________________________________________________________________ - 31 -

Figure 1.13: The typical viscoelastic spectrum which is the master showing the

viscoelastic respond of non-Newtonian material [133].

These rheological properties discussed above, can be studied by using a

rheometer. In the earlier days, viscometer was the only developed as an instrument to

measure the flow behavior of liquid. Due to the advancement of technology in recent

years, development of advanced rheometer is no longer a dream. The very first

rotational rheometer was produced in the year 1990 [134]. This type of rheometer not

only allows temperature control, but also able to vary the rotational speeds which enable

the scientist and rheologist to investigate the material properties as a function of shear

rate. Soon later, the oscillation rheometer is available with greater function. This

modern rheometer is able to control the stress in the range of ≤ 1 m N/m to more than 1

N/m with a wide range of temperature control from -150oC to 300oC. With the help of

these advanced instruments, more and more information about the rheological behavior

of materials can be obtained.


________________________________________________________________________ - 32 -

1.7.2 Application of Rheology

Owing to the fact that rheology can give a better picture of the behavior of a

material, therefore it is widely used as a tool to test the texture and flow behavior of

industrial products especially in the processing industries such as foods [135-138],

cosmetic [139-143], pharmaceutical [144-146], polymer [147-149], coating [150, 151],

and oil processing [152, 153]. The rheological results can help the scientists and the

manufacturers to have better understanding of the products. The rheological results also

enable scientists to estimate the products’ quality such as elasticity, viscosity,

deformability, storage, shelf life and intermolecular interactions, due to the ultra-

sensitivity toward microstructure of a material.

Also, in commercial emulsions, factors such as appearance and flow properties

of the final products are very important. Consumers are often very sensitive to the

texture, the mouth feel, and flavor perception of the products. In order to influence the

consumers’ buying preference, the rheological properties such as the stickiness, the

viscosity, and the elasticity of the products have to be controlled with care. Sometimes,

it is necessary to modify the properties of their products in order to improve product

quality. In this case, rheology is a good tool to perfectly accomplish the task. For

example, the residue emptying behavior of products from its container and this is very

important for the products that packed in pump disperser bottle. This type of products

such as body shampoo, hair shampoo and body lotion has to be able to flow to the

bottom of the container to guarantee the optimal of emptying. When the residues

emptying are low, that means the sagging viscosity of the product is high and has to be

modified. By referring to the shear rate range shown in Table 1.7, the viscosity of

specific situation such as sagging, levelling and rubbing can be identified and enable the


________________________________________________________________________ - 33 -

researchers or formulators to modify the flow behavior in order to maximize the

products performance.

Table 1.7: The typical shear rate range for various physical operations with examples

[154, 155].

Situation Shear Rate Range (s-1) Examples

Sedimentation of fine

powders in liquids 10-6-10-3

Medicines, paints, salad

dressing

Leveling 10-2-10-1 Paints, printing ink

Draining of under gravity 10-1-101 Toilet bleaches, paints,

coatings

Extruders 100-102 Foods

Dip coating 101-102 Paints, confectionery

Mixing and stirring 101-103 Liquid manufacturing

Pipe flow 100-103 Pumping liquids, blood

flow

Brushing 103-104 Painting

Rubbing 104-105 Skin creams, lotions

High-speed coating 104-106 Paper manufacture

Spraying 105-106 Atomisation, spray drying

Lubrication 103-107 Bearings, engines

Besides food and cosmetic creams, rheology is also applicable to study of

natural phenomena such as deformation of natural rocks [156-159], wastewater

treatment industry [160, 161], and biological problems [162, 163]. Overall, rheology is

a useful and informative discipline in science which had provided a lot of information

for the food, cosmetic and pharmaceutical industries as well as the natural phenomena

studies.


________________________________________________________________________ - 34 -

1.8 Objective

1. To prepare olive oil cream emulsions stabilized by sucrose fatty acid esters.

2. To study the stability of the emulsions through physically phases separation and

rheological properties.

3. To determine the effect of oil–water ratio, surfactant concentration and emulsion

age on the stability and rheological properties of the emulsion.

CHAPTER 2

MATERIALS AND METHODS


________________________________________________________________________ - 35 -

2.0 Materials and Methods

2.1 Materials

Deionized water with resistivity of 18.2 Ωcm-1, supplied from a Barnstead

Diamond Nanopure Water Purification unit couple with a Barnstead DiamondTM RO

unit from Barnstead International, Iowa USA was used in all emulsion preparation. A

commercial grade extra virgin olive oil from brand Laleli with maximum acidity 0.8%,

density 0.9091 g/ml, and viscosity 9.562 m Pas at 30oC was used as received. Nonionic

surfactant sucrose fatty acid ester (SFAE) was obtained from TCI, Tokyo Kasei Co.

LTD, Japan. GC/MS analysis of SFAE was found to be containing sucrose myristate

0.29%, sucrose palmitate 83.99%, and sucrose stearate 15.72%. Polyethylene glycol

dodecyl ether (Brij35P) was purchased from Fluka.

2.2 Emulsion preparation

Preparation of aqueous phase was performed by dissolving 2 – 10 wt% of SFAE

and 0.5 wt% Brij35P (acting as a solubilizer due to low solubility of SFAE in water)

into different volumes of deionized water (20%, 30%, 40% and 50% from total volume)

followed by sonication for 5 minutes and then heated with stirring at 60oC until clear gel

like appearance was observed. Preparation of oil phase was performed by homogenizing

2 – 10 wt% of SFAE into different volumes of olive oil (50%, 60%, 70% and 80% from

total volume) with an IKA Labortechnik homogenizer model T25 from IKA, Germany

and followed by treatment at 60oC for 5 minutes. The emulsions were prepared by

pouring the oil phase into the aqueous phase and homogenized with the homogenizer at

fix 13,000 rpm for 5 minutes. As the speed of homogenizer and the homogenation time


________________________________________________________________________ - 36 -

are factors affecting the droplet size of the emulsions, therefore these parameters were

fixed in order to obtain a more comparable and reliable results. The emulsions were

then kept under accelerating condition at 45oC for further analysis. Emulsion stability

test was performed by observing emulsion phase separation over 30 days.

Dispersion test was used to identify the type of emulsion. The dispersion test

was carried out by placing a drop of emulsion on water and oil. The emulsion tends to

disperse in water (Figure 2.1 (a)), but no dispersion was observed in oil (Figure 2.1

(b)). Thus, the prepared emulsion is an o/w emulsion. Beside, the micrographs of these

emulsions were also a good evidence to prove that these are o/w type of emulsions.

(a) (b)

Figure 2.1: The emulsion dispersed in (a) water very well, but resists the dispersion in

(b) olive oil indicating that the emulsion is an o/w emulsion.

2.3 Zeta potential measurement

Zeta potential of the emulsion droplets was measured by using Malvern

ZetaSizer Nano Series (Malvern Instruments, UK). The samples were prepared by

diluting approximately 0.02 g of emulsion with 5 ml of 0.01 M of sodium chloride

solution. Finally, the solutions were conditioned for 20 minutes at 30oC before carrying


________________________________________________________________________ - 37 -

out the measurements. The Zeta potential of the emulsions was measured on the 1st day,

7th days and 30th days after the emulsions were prepared. The results were obtained by

applying the Henry equation,

2 ( )3Ef kaU εζη

= (5)

where EU is the electrophoretic mobility, ε is the dielectric constant, ζ is the zeta

potential, η is the viscosity of the continuous phase, and ( )f ka is the Henry’s function

[164]. In this study, the calculation of Zeta potential is done by using the Smoluchowski

approximation where the ( )f ka is 1.5. That is due to the properties of the emulsion

droplets which were dispersed in electrolytes containing more that 10-3 molar of salt and

droplet size greater than 0.2 μm. According to equation (5), the Zeta potential of the

emulsions was calculated from the electrophoretic mobility of the droplets. Under an

electrical field, the droplets tend to move towards the electrode with opposite charge.

Their velocity is then measured by a technique call Laser Doppler Velocimetry and is

expressed in unit of field strength as their mobility.

2.4 Morphology and droplet size analysis

The integrity, aggregation and the droplet size distribution of the emulsion were

observed and determined by using a light polarizing microscope (Leica model PM RXP ,

New York) equipped with a JVC Color Video camera (model KY F550) and Leica

QWin image analysis software. The measurements of droplets size was performed

without any further dilution of the emulsions. Approximately 3000 droplets were


________________________________________________________________________ - 38 -

randomly chosen from 10 micrographs captured from 10 different drops of the

emulsions. The droplet size was reported as the volume-weighted mean droplet size (d4,3)

as shown in the following equation, where in is the number of droplets with diameter

id [165].

4

4,33

i i

i i

n dd

n d=∑

∑ (6)

2.5 Rheological measurement

A stress/rate controlled Bohlin CVO-R Rheometer (Malvern Instrument UK)

with a temperature controller, was employed to measure the rheological properties of

the emulsion. The measurements were performed under the temperature of 30.0 ± 0.1oC

(-40 to 180oC, Peltier Plate system from Bohlin Instrument Ltd.) with 4o/40 mm cone

and plate geometry and gap of 0.150 mm. In order to obtain the proper parameter and

reliable data from the frequency sweep, amplitude sweep was first performed at a

controlled strain mode with applied strain in the range of 0.0001 to 1 unit and at fixed

frequency of 1 Hz; followed by the frequency sweep, which was performed at a

controlled strain mode (a very low deformation strain was chosen from the linear

viscoelastic profile from amplitude sweep) with frequency varying from 0.001 to 10 Hz.

All the measurements were performed on day 1, 7, and 30.

n

Kσ γ= & (7)


________________________________________________________________________ - 39 -

The steady rheological behavior of the emulsions was measured at a controlled

rate mode varying from 0.0001 to 50 s-1. The samples were allowed relaxing for 10

minutes after loaded to the plate before the measurement started. All the shear data

obtained from the increasing shear rate measurement were fitted to the Power- Law (Eq.

7) and Hershel-Bulkley model (Eq. 8) using the Bohlin rheometer software (Gemini

150). The σ and γ& are the shear stress and shear rate respectively; K is the consistency

index and n is the flow behavior index, so called the Power-Law index describing the

non-Newtonian behavior of liquids . For a shear thinning fluid, 0 < n < 1; for a shear

thickening fluid, n > 1, and a Newtonian fluid give n = 0.

no Kσ σ γ= + & (8)

γ& is the shear rate, the oσ is the yield stress; K is the dimensionless constant, n is the

power index measuring the degree of thinning (n < 1 for shear thinning behavior).

CHAPTER 3

RESULTS AND DISCUSSION


________________________________________________________________________ - 40 -

3.0 Results and Discussion

3.1 Phase Separation

The stability of emulsion was first determined by observing the physical phase

separation which greater phase separation indicates lower emulsion stability with

shorter shelf life. According to Figure 3.1, the emulsions phase separation was

dependent on the oil-water ratio, surfactant concentration and storage time. As the

surfactant concentration and oil-water ratio increased, the emulsions phase separation

decreased which indicated the increase of emulsion stability. This happens to all

emulsions except the emulsion with 80% oil. These emulsions with 80% oil were the

most stable emulsions that showing no phase separation even after 30 days of storage

under 45oC. This result shows that the emulsions with 80% oil have a long shelf life.

Fig. 3.1(a)

1 2 3 4 5 6 7 8 9 10 110.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Surfactant concentration (wt%)

Em

ulsi

on V

olum

e Fr

actio

n (%

)

50% Oil 60% Oil 70% Oil 80% Oil


________________________________________________________________________ - 41 -

Fig. 3.1(b)

1 2 3 4 5 6 7 8 9 10 110.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0


Em

ulsi

on V

olum

e Fr

actio

n (%

)

50% Oil 60% Oil 70% Oil 80% Oil

Fig. 3.1(c)

1 2 3 4 5 6 7 8 9 10 110.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Em

ulsi

on V

olum

e Fr

actio

n (%

)


50% Oil 60% Oil 70% Oil 80% Oil

Figure 3.1: The emulsion volume fractions for emulsions at (a) day 1, (b) day 7, and (c)

day 30.


________________________________________________________________________ - 42 -

The emulsions which were prepared with 50%, 60%, and 70% oil showed a

significant phase separation. Among these emulsions, emulsions with 50% oil were the

most unstable. According to the experimental result, emulsions with 50% oil which

were stabilized by 2- 4 wt% of surfactant show immediate phase separation (Figure

3.1(a)). After 30 days of storage under 45oC, all emulsions with 50% oil show phase

separation even the one that was stabilized with 10wt% of SFAE (Figure 3.1(c)). This

means that these emulsions were having a short shelf life.

The major factor causing the emulsion phase separation is the creaming process

which is a type of gravitational destabilization process (Figure 1.10(d)). In the case of

oil-in-water emulsion, the oil tends to move upward under gravity due to the difference

in density between the continuous phase and the disperse phase [166]. The droplets are

accumulated at the top of the emulsion due to the creaming effect, resulting in oiling off

(formation of an oil layer on top of the emulsion) after some time [167]. The creaming

rate of emulsion is highly dependent on the density of the disperse and continuous phase,

viscosity of the continuous phase, droplet size and the droplet concentration [168].

These will be discussed in the following sections.


________________________________________________________________________ - 43 -

3.2 The Effect of Surfactant, Oil Concentration and Storage Time on

the Droplet Size.

The droplet size of an emulsion is an important parameter which influences the

colloidal stability and rheological properties such as the flow and deformations of the

emulsion. Emulsion with smaller droplet size enhances the skin penetration of drugs and

active ingredients through skin [169, 170]. The droplet size of the emulsions decreased

when the oil and surfactant concentration increased (Figures 3.2 and 3.3). In this study,

results (Figures 3.1 and 3.2) show that the smaller the droplet size, the greater the

emulsion stability and this agreed with many other research works on emulsion with

different composition and type of oil [168, 171]. The droplet size of emulsions with

50% and 60% oil decreased dramatically (about 60%) with increase from 2 to 10 wt%

of surfactant concentration. On the other hand, there was only a slight decrease in

droplet size of emulsions with 70% and 80% oil, showing a trend that no significant

decrease in the droplet size as the surfactant concentration further increased from 7 to

10 wt%. Besides, the aging of emulsion also affected the droplet size very much

especially in more dilute emulsions (50% and 60% oil). The experimental result shows

that there was a significant increase in the droplet size after 30 days of storage (Figures

3.2(a) and (b)). However, the aging effect was not considerable observed in the

concentrated emulsions of 70% and 80% oil content. The decreased of droplet size of

concentrated emulsions was insignificant as compared to the dilute emulsions (Figure

3.2).


________________________________________________________________________ - 44 -

Fig. 3.2(a)

1 2 3 4 5 6 7 8 9 10 110

10

20

30

40

50

60

70

80

90

Surfactant Concentration (wt%)

Mea

n D

ropl

et S

ize,

d (m

m)

Day 1 Day 7 Day 30

Fig. 3.2(b)

1 2 3 4 5 6 7 8 9 10 110

10

20

30

40

50

60

70

80

90

Mea

n D

ropl

et S

ize,

d (m

m)


Day 1 Day 7 Day 30


________________________________________________________________________ - 45 -

Fig. 3.2(c)

1 2 3 4 5 6 7 8 9 10 110

10

20

30

40

50

60

70

80

90


Mea

n D

ropl

et S

ize,

d (m

m)

Day 1 Day 7 Day 30

Fig. 3.2(d)

1 2 3 4 5 6 7 8 9 10 110

10

20

30

40

50

60

70

80

90

Mea

n D

ropl

et S

ize,

d (m

m)


Day 1 Day 7 Day 30

Figure 3.2: The mean droplet size for emulsions with (a) 50%, (b) 60%, (c) 70% and (d)

80% of disperse phase.


________________________________________________________________________ - 46 -

3.2.1 Effect of Surfactant Concentration to the Droplet Size

There were several methods used to control the droplet size of the emulsion. One

is introducing the surfactant into the system to reduce the interfacial tension, so that the

homogenizer can easily disrupt the droplets under low interfacial tension leading to the

formation of smaller droplet [172]. Another is to vary the speed of homogenizer and the

duration of emulsification during sample preparation through secondary

homogenization [173]. However, the second technique is dependent on the

homogenizer’s capability and is not an economic method. Besides that, the age of

emulsion is also affecting the emulsion droplet size, due to the thermodynamic

instability of the emulsions as presented in Figure 3.2.

Before proceeding further to the discussion on the effect of the surfactant

concentration on the emulsions properties, it is necessary to discuss about the reason of

choosing 2 wt% of SFAE surfactant to be the minimum amount of surfactant to stabilize

the emulsions. This is to make sure that there was enough supply of surfactant

molecules to the oil and water interface. That is also a basic condition to attain in the

formation of emulsion, because the formulation will fail if insufficient amount of

surfactant is used. According to the experimental results (Figure 3.1), a full emulsion

was obtained with this minimum surfactant concentration and was stable for 30 days

under 45oC of storage condition. This indicates that the minimum amount of surfactant

concentration used in this study is sufficient and able to produce a stable emulsion.

The decrease of emulsion stability with respect to the decrease of droplet size

when increased of surfactant concentration is expected and can be explained with

creaming destabilization process. According to the ideal Stokes’ Law for the creaming

rate of emulsion, the gravitational instability rate of emulsion is directly proportional to

the square root of the droplet radius (Eq. 9) [172]. For example, when the droplet size


________________________________________________________________________ - 47 -

increased from 25 μm (8 wt% of surfactant in emulsion with 60% oil) to 50 μm (3 wt%

of surfactant in emulsion with 60% oil), the creaming rate for the emulsion with droplet

size of 50 μm is four times greater than the smaller one. Thus, the emulsion with droplet

size of 50 μm (3 wt% of surfactant, 60% oil) is much more unstable compared to 25 μm

droplet (Figure 3.1). Equations 10 and 11 predict the minimum droplet size and the

surfactant concentration used in order to produce an emulsion with longer shelf life

[172].

2

2 1

1

2 ( )9Stokes

grv ρ ρη

−= − (9)

Stokesv is the Stokes’ creaming velocity, g is the acceleration due to gravity, r is the

droplet radius, 1ρ and 2ρ are the density of the continuous phase and disperse phase

respectively, 1η is the viscosity of the continuous phase. The sign for the v indicating

the type of destabilization processes, creaming (+) or sedimentation (-).

min3 sat

s

rc

φΓ= (10)

minr (m) is the minimum size of stable droplet that can be produced, satΓ (kg m-2) is the

excess surface concentration of the surfactant at saturation at the oil-water interface, φ

is the disperse phase volume fraction (Eq. 3), sc (kg m-3) is the concentration of

surfactant in the emulsion.


________________________________________________________________________ - 48 -

D

E

VV

φ = (11)

DV is the volume of emulsion droplets and EV is the total volume of the emulsion.

Fig. 3.3(a)


________________________________________________________________________ - 49 -

Fig. 3.3(b)

Fig. 3.3(c)

Figure 3.3: Micrographs of emulsion in 80% oil with (a) 2 wt%, (b) 5 wt% and (c) 10

wt% of SFAE surfactants showing the decrease of droplet size with increase in

surfactant concentration.


________________________________________________________________________ - 50 -

When the surfactant concentration increased, the amount of surfactant adsorb at

the droplet surface increased. The droplets were well protected due to close arrangement

of surfactant layer at the droplet surface. When droplets approach at proximately close

enough to each other, repulsive force dominated and the droplets repelled each other

keeping them apart. At low surfactant concentration, the energy barrier created by the

surfactant layer at droplet surface is relatively low due to the dynamic property of the

interfacial layer. The hydrophobic interaction between the droplets came into

consideration due to the increased exposure of the nonpolar surface (oil) with the polar

region (water). The interaction between the uncovered droplet surface areas with the

aqueous continuous phase is thermodynamically unfavorable. Therefore, the system will

try to minimize these unfavorable energy by minimizing the contact area [174, 175].


________________________________________________________________________ - 51 -

Fig. 3.4(a)

Fig. 3.4(b)


________________________________________________________________________ - 52 -

Fig. 3.4(c)

Figure 3.4: The micrographs of dilute emulsion (50% oil) with (a) 2 wt%, (b) 5 wt%,

and (c) 10 wt% of surfactant concentration.

3.2.2 Aging effect

The changes of the droplet size as a function of time is obviously a kinetic effect,

which can be affected by the dynamics, colloidal interactions and interfacial properties

of the emulsion [176, 177]. Kinetic effect is used to elaborate the long term stability of

the emulsions. Generally, the emulsions with smaller droplet size have longer shelf life,

but are the more thermodynamically unstable due to the large interfacial area (Eq. 12).

formation configG A T SγΔ = Δ − Δ (12)


________________________________________________________________________ - 53 -

formationGΔ is the Gibbs free energy of the system, γ is the interfacial tension, AΔ is the

difference in the total interfacial area before and after emulsification, T is the absolute

temperature and configSΔ is the configurational entropy of the droplets in the system.

As previously mentioned in Chapter 1, the kinetic stability of emulsion depends

on its activation energy, which is attributed to the adsorbed surfactant layer (Figure

1.12). According to McClements, D.J. [176], the emulsions have to overcome this

activation energy before they reach the thermodynamic stable state. For a long term

stability emulsion, the activation energy is generally 20 times greater than the thermal

energy of the system (> 20kT) [178]. However, due to the dynamic property of the

surfactant layer, the stability of droplets is changing from time to time.

Fig. 3.5(a)


________________________________________________________________________ - 54 -

Fig. 3.5(b)

Fig. 3.5(c)

Figure 3.5: The micrographs of emulsions with 50% disperse phase captured at (a) day

1, (b) day 7 and (c) day 30, showing the coalescence destabilization.


________________________________________________________________________ - 55 -

For dilute emulsions (< 70% oil), the droplets were in continual motion, because

of the low viscosity of the emulsions and droplets were not closely packed (Figure 3.5).

The droplets collided with each other when they move around under Brownian motion

and gravity [84]. The motion may lead to coalescence destabilization after collisions

when there was insufficient surfactant to perfectly cover the droplets’ surface at low

surfactant concentration (< 4 wt%) [179, 180]. As droplets moving toward each other,

the surface-to-surface distance between the droplets was decreased. At closer distance,

there is an energy barrier created by the adsorbed surfactant, which is the repulsive

force is dominating. As previously discussed, the droplets has to overcome the energy

barrier (the activation energy) before they coalescence and fall into the deep minimum

(w(hmin)) (Figures 1.11 and 1.12) [176]. Due to desorption of surfactant resulting from

the dynamic behavior of the interfacial layer, the activation energy decreased [179] and

creating a greater contact area between the droplets. As a result, the droplets can easily

overcome the energy barrier and merge to form a larger droplet [81].

3.2.3 Effect of oil concentration

In this study, the mean droplet size is found to be decreasing when the oil

concentration increased from 50% to 80% (Figures 3.1 and 3.4) [181]. The droplets

were also becoming close packed when the oil concentration increased from 50% to

80% (Figures 3.6 and 3.7) due to the increase of droplet concentration. However, no

flocs were observed even though they were all closely packed. The most concentrated

emulsion (80% oil) was the most stable emulsion showing high droplet concentration

compared to others (Figure 3.7). The high droplet concentration enhances the stability

of the emulsion, because their movements were blocked by each other [167]. Thus, this

slows down the creaming rate, followed by the destabilization of emulsion.


________________________________________________________________________ - 56 -

From the droplet size analysis results, the less concentrated emulsion (lower oil

concentration) has greater droplet size (Figures 3.4 and 3.5). Since the minimum

amount of surfactant used in this study was sufficient to produce a stable emulsion with

80% oil, in general, the droplet size of the emulsions with 50% oil at fix surfactant

concentration should be smaller than those with 80%. But experimental results show the

other way round. This may due to the effect of the emulsion viscosity.

Fig. 3.6(a)

1 2 3 4 5 6 7 8 9 10 110

10

20

30

40

50

60

70

80

90


Mea

n D

ropl

et S

ize,

d (μ

m)

50% oil 60% oil 70% oil 80% oil


________________________________________________________________________ - 57 -

Fig. 3.6(b)

1 2 3 4 5 6 7 8 9 10 110

10

20

30

40

50

60

70

80

90


Mea

n D

ropl

et S

ize,

d (μ

m)

50% oil 60% oil 70% oil 80% oil

Fig. 3.6(c)

1 2 3 4 5 6 7 8 9 10 110

10

20

30

40

50

60

70

80

90


Mea

n D

ropl

et S

ize,

d (μ

m)

50% oil 60% oil 70% oil 80% oil

Figure 3.6: The effect of oil concentration to the droplet size of emulsions over 30 days

of storage under accelerating condition. (a) First day, (b) 7th day, and (c) 30th day.


________________________________________________________________________ - 58 -

In this study, the emulsions were prepared by varying the water to oil ratio

(WOR). When the emulsion was said to be low oil concentration, it is to understand that

the water or aqueous part of that emulsion increased. Certainly, when the water portion

increased, the viscosity of the emulsions will decrease as the emulsions were diluted. In

other words, the viscosity of the continuous phase ( 1η ) is decreased. In addition, the

droplet concentration was relatively low in the emulsion with low oil concentration;

they were not in a close pack condition. That indicates the mobility of the droplets

increases which enhances the collision frequency (FG) [81]. As the droplets were able to

move freely in the emulsion induced by the gravitational force, the impact of collision

between the droplets is high and may result in the disruption of the interfacial layer,

consequently the droplets merge to form a larger droplet [84]. The formation of larger

droplet enhanced the creaming rate of the droplets, thus increased the phase separation

and decreasing the emulsion stability. The above explanations were supported by

equation 13 which shows the relationship between the FG and 1η . The equation 13

shows very obvious that the FG increased with the decrease of 1η . Beside 1η , a

polydisperse system also contributes to the FG which due to the fact that larger droplet

will move to the top more quickly than the smaller one under gravity.

2 2 21 2 2 1 1 2

3 31 1 2

( )( )[ ]8G

g r r r rFr r

ρφ φπη

Δ − += (13)

g is the gravitational force, Δρ is the difference in density between the droplets and the

surrounding liquid, iφ is the disperse volume fraction with droplet radius ir .

On the other hand, as for higher oil concentration emulsions (≥ 70% oil), the

droplets were closely packed (Figure 3.7). The collision efficiency is also lower than


________________________________________________________________________ - 59 -

that in the dilute emulsions due to limitation of motion [81, 84]. In such close distance,

repulsive forces between the droplets are dominant and the droplets keep repelling each

other. This helps the droplets keeping a distance with each other. Although there was

some increase of the mean droplet size for these concentrated emulsions (Figure 3.2),

but the increase was negligible. Therefore, a conclusion can be made that the

concentrated emulsions were stable against the growing of droplet size in the studied

period.

As discussed above, the increase of oil concentration enhances the emulsion

stability. Consequently, the emulsion stability can be controlled by altering the droplet

size and concentration. However, this method is not favorable in formulation of

industrial products, because the increase of the oil concentration may decrease flavor

perceptions, texture of the products, and consumer acceptability [182-185].


________________________________________________________________________ - 60 -

Fig. 3.7(a)

Fig. 3.7(b)


________________________________________________________________________ - 61 -

Fig. 3.7(c)

Fig. 3.7(d)

Figure 3.7: The micrographs for emulsions with (a) 50%, (b) 60%, (c) 70%, and (d)

80% of oil and stabilized with 2 wt% of surfactant. These micrographs were obtained

after one day of storage under 45oC.


________________________________________________________________________ - 62 -

3.3 Zeta potential

The Zeta potential is a common technique being used to study the emulsions

stability. The magnitude of the Zeta potential carries information on emulsion stability

and tendency for droplet flocculation. According to the literatures’ [164, 186], rule of

thumb that is usually being practiced the droplet flocculation is when the Zeta potential

is less than the absolute value of ± 25 mV and the greater the magnitude implies the

higher the stability of the emulsion system. In our case, experimental results of the Zeta

potential for the emulsions over 30 days of storage in 45oC oven condition were

presented in Figure 3.8.

In the presence of 0.01 M of NaCl electrolyte, the Zeta potential for the

emulsions with 50% and 60% of oil were found to be in the range of -12 mV to -24 mV

and -12 mV to -18 mV for emulsions with 70% and 80% oil. The results indicated that

the droplet stability in the emulsions with 70% and 80% oil was lower than the

emulsions with 50% and 60% oil which contradicts with the other stability test results

that have shown greater emulsion stability with increase in the oil concentration (Figure

3.1). However, the Zeta potential of all emulsions increased in magnitude as the

surfactant concentration was increased indicating increase of emulsion stability. The

Zeta potential of all emulsions was also found to decrease with the emulsion storage

time. These results agreed with other stability tests performed on the emulsions.


________________________________________________________________________ - 63 -

Fig. 3.8(a)

1 2 3 4 5 6 7 8 9 10 11

-24

-22

-20

-18

-16

-14

-12

-10


Zeta

Pot

entia

l, ζ

(mV

)

1st day 7th days 30th days

Fig. 3.8(b)

1 2 3 4 5 6 7 8 9 10 11

-24

-22

-20

-18

-16

-14

-12

-10


Zet

a Po

tent

ial,

ζ (m

V)



________________________________________________________________________ - 64 -

Fig. 3.8(c)

1 2 3 4 5 6 7 8 9 10 11

-24

-22

-20

-18

-16

-14

-12

-10

Zet

a Po

tent

ial,

ζ (m

V)



Fig. 3.8(d)

1 2 3 4 5 6 7 8 9 10 11

-24

-22

-20

-18

-16

-14

-12

-10


Zet

a P

oten

tial,

ζ (m

V)


Figure 3.8: The Zeta potential of emulsions measured in the presence of 0.01 M NaCl

solutions for (a) 50%, (b) 60%, (c) 70%, and (d) 80% of oil concentration.

The surface charge of all emulsions in this study was negative although the

emulsions were stabilized with nonionic surfactant, exhibiting similarity with the naked

oil droplet as reported by a number of researchers [187-190]. Several suggestions have

been given to explain this phenomenon; first is the spontaneous adsorption of hydroxyl


________________________________________________________________________ - 65 -

ion from the aqueous phase to the surfactant head group through hydrogen bonding or

desorption of the hydrogen ions from the droplet surface to the aqueous phase to form

hydronium ion [188, 191, 192]. Nevertheless, the concentration of these ions (hydroxyl

and hydronium ions) is relatively low (~ 10-7 M), therefore one may consider the role of

the water dipoles [193]. This has the influence on the negative surface charge on the

droplet. Chibowski and Waksmundzki [194] had also proposed that the water dipoles

may have some contribution to the electrical double layer structure [191, 194]. However,

there was strong experimental evidence to prove the hypothesis of hydroxyl ion

adsorption as the origin of the droplet potential [188-191]. Thus, this hypothesis

plausibly is the best explanation on the negative Zeta potential obtained in this study.

The decrease of Zeta potential when oil concentration increased may be due to

the decrease of the droplet mobility. As has previously been discussed, the increase of

oil concentration is accompanied with an increase of droplet concentration. Thus, once

the droplet concentration increases, the available space will become crowded. The

separation distance between droplets will also decrease, thus limit the mobility of the

droplets. Besides that, in such crowded environment, there was also an increase of the

possibility for the electrical double layers between the adjacent droplets to overlap. As

the overlapping occur, the osmotic pressure which arises from the difference between

the ionic concentration in the overlapping region and the bulk phase will act and force

the droplets apart from each other [195]. That may cause a delay in the mobility of the

droplets which leads to a decrease in Zeta potential.


________________________________________________________________________ - 66 -

3.4 Rheological Properties of Emulsion System

3.4.1 Steady State Behavior

Steady state behavior on emulsions is relatively important in order to monitor

the texture of the formulation, and also the consumer acceptability of the final products.

Besides that, the flow properties of emulsions also provide information on product

processing, handling, storage and mechanical behavior [196]. However, the viscosity is

always the favored parameter for the chemists especially those with cosmetic, foods,

and pharmaceutical industries to evaluate the emulsion stability [197]. Since the

viscosity of the emulsion arises from the friction between droplets, the interdroplet

forces are now an important factor influencing the viscosity of the emulsions. Therefore,

it is reasonable to say that the factors that contributed to the interdroplet interactions

such as the mean droplet size, polydispersity of droplets, and the presence of additives

such as thickener [198] and polysaccharides [199, 200], the surfactant concentration ,

oil concentration and the age of the emulsions can influence the viscosity of emulsion.

3.4.1.1 Effect of emulsion compositions on the flow properties

The shear viscosity versus shear rate profile (flow curve) for the emulsions is

presented in Figure 3.9. The flow curves show four clear different regions, at very low

shear rate (< 0.005 s-1), the viscosity increased with the shear rate showing shear

thickening behavior. The second region starts to show independence of viscosity to the

shear rate, which is the characteristic Newtonian plateau. However, the plateau is short

and the range is getting shorter when the emulsion became concentrated (Figure 3.9)

[201]. The noticed short plateau is the typical flow behavior of emulsions without


________________________________________________________________________ - 67 -

addition of any viscosity modifier [201]. A zero shear viscosity (ηo) of the emulsions

was determined by extrapolating the Newtonian plateau [202]. The third region, which

is the shear thinning region, starts after exceeding the yield value. In this region, the

viscosity decreased smoothly (up ≈ 0.1 s-1). Finally, the viscosity decreased dramatically

and continuously with high slope. The dependence of the viscosity to the shear rate as

discussed above indicated that the emulsions were typical non-Newtonian in behavior.

Fig. 3.9(a)

1E-5 1E-4 1E-3 0.01 0.1 1 10 100 10000.01

0.1

1

10

100

1000

10000

2 wt% 3 wt% 4 wt% 5 wt% 6 wt% 7 wt% 8 wt% 9 wt% 10 wt%

Vis

cosi

ty, η

(Pas

)

Shear Rate (s-1)


________________________________________________________________________ - 68 -

Fig. 3.9(b)

1E-5 1E-4 1E-3 0.01 0.1 1 10 100 10000.01

0.1

1

10

100

1000

10000

Shear Rate (s-1)

Vis

cosi

ty, η

(Pas

)


Fig. 3.9(c)

1E-5 1E-4 1E-3 0.01 0.1 1 10 100 10000.01

0.1

1

10

100

1000

10000

Shear Rate (s-1)

Vis

cosi

ty, η

(Pas

)



________________________________________________________________________ - 69 -

Fig. 3.9(d)

1E-5 1E-4 1E-3 0.01 0.1 1 10 100 10000.01

0.1

1

10

100

1000

10000

Shear Rate (s-1)

Vis

cosi

ty, η

(Pas

)


Figure 3.9: The shear rate dependence viscosity of emulsions with a series of surfactant

concentration for (a) 50%, (b) 60%, (c) 70%, and (d) 80% of oil.


________________________________________________________________________ - 70 -

Fig. 3.10(a)

1 2 3 4 5 6 7 8 9 10 110

2000

4000

6000

8000

10000

12000

14000

Zero

She

ar V

isco

sity

, η(P

as)


50% oil 60% oil 70% oil 80% oil

Fig. 3.10(b)

1 2 3 4 5 6 7 8 9 10 110

2000

4000

6000

8000

10000

12000

14000

Zero

She

ar V

isco

sity

, η (P

as)


50% oil 60% oil 70% oil 80% oil


________________________________________________________________________ - 71 -

Fig. 3.10(c)

1 2 3 4 5 6 7 8 9 10 110

2000

4000

6000

8000

10000

12000

14000


Zer

o Sh

ear V

isco

sity

, η (P

as)

50% oil 60% oil 70% oil 80% oil

Figure 3.10: The effect of oil concentration to the zero shear viscosity of emulsion after

(a) 1 day, (b) 7 days, and (c) 30 days of storage under an accelerated condition of 45oC.

The zero shear viscosity and yield stress of the emulsions increased with

surfactant and oil concentration as shown in Figures 3.10 and 3.11, indicating

structural integrity arising from the strong colloidal interaction between the droplets.

The yield stress is the stress that has to be overcome before the emulsion starts to flow

[203]. As discussed before, the system with higher surfactant concentration tends to

form a denser interfacial layer which is incompressible [204, 205]. Hence, the droplets

in such sterically stabilized system is usually characterized as “hard sphere” [123]. The

strength of interaction forces (mainly the attractive and repulsive interactions) between

the droplets for the hard sphere system (high surfactant concentration emulsion) was

relatively greater than the one with lower surfactant concentration. In the absence of the

strong sterically repulsive effect, the droplets in the emulsions with lower surfactant

concentration were able to pack more efficiently even at low shear. Therefore, the

droplets were easily aligning themselves with the shear field to initiate flow.


________________________________________________________________________ - 72 -

Oil concentration in an emulsion is another factor affecting the flow behavior of

the emulsion [168, 206]. When the oil concentration increases, the droplet size is getting

smaller and resulting in higher droplet concentration, lower polydispersity and smaller

mean separation distance between the droplets when compared to the emulsions with

lower oil concentration (Figures 3.6 and 3.7) [168]. Since the flow property of fluid

arises from friction between the liquid layers as they slip pass each other [207], when

the droplets are at close distance, the hydrodynamic repulsive force becomes dominant

leading to an increase in the friction between the droplets as external shear is applied

[74, 208].

In addition, the attractive force which is also one of the colloidal interactions

plays an important role to the increase in viscosity and yield stress. The magnitude of

viscosity and yield stress depend on the strength of the attractive force between the

droplets [209]. The decrease of droplet size resulting from the increase in the disperse

phase volume fraction leads to the increase in the total droplet surface area. When the

total surface area of the droplet increase, the strength of the attractive force will also

increase. Thus, greater stress is required to initiate flow when high attractive force is

holding the droplets resulting in high viscosity and yield stress (Figure 3.12) [209].


________________________________________________________________________ - 73 -

Fig. 3.11(a)

1 2 3 4 5 6 7 8 9 10 110

10

20

30

40

Yiel

d St

ress

, σY (P

a)


0

15

30

45

60

75

90

Mea

n D

ropl

et S

ize,

d (n

m)

50% Oil 60% Oil 70% Oil 80% Oil

Fig. 3.11(b)

1 2 3 4 5 6 7 8 9 10 110

10

20

30

40


Yie

ld S

tress

, σY

(Pa)

50% Oil 60% Oil 70% Oil 80% Oil

0

15

30

45

60

75

90

Mea

n D

ropl

et S

ize,

d (μ

m)


________________________________________________________________________ - 74 -

Fig. 3.11(c)

1 2 3 4 5 6 7 8 9 10 110

10

20

30

40

Yie

ld S

tress

, σY (P

a)


0

15

30

45

60

75

90

Mea

n D

ropl

et S

ize,

d (μ

m) 50% Oil

60% Oil 70% Oil 80% Oil

Figure 3.11: The yield stress of the emulsions determined using the Herschel-Bulkely

model from the Bohlin rheometer software, showing the correlation between the mean

droplet size and the yield stress of the emulsion as a function of surfactant concentration

for (a) 1st day, (b) 7th day, and (c) 30th day.

The theoretical model that relates the viscosity (η ) and the yield stress ( Yσ ) is

given by the following equation [81],

1 ( )o

Y

η ηη η σσ

∞∞

−= +

+ (14)

where σ is a particular shear stress, oη is the zero viscosity (first Newtonian plateau),

and η∞ is the infinity viscosity (second Newtonian plateau). The relationship between

the Yσ and droplet radius, r is shown in the following equation,


________________________________________________________________________ - 75 -

3YkTr

σβ

= (15)

k is the Boltzmann constant, T is the temperature and β is a dimensionless constant

with a value of 0.431 [81, 210]. The equation shows that the Yσ is inversely

proportional to the droplet size which implies that the increase in the droplet size will

decrease the Yσ of the emulsions. This theoretical prediction shows a strong agreement

with the experimental results shown in Figure 3.9.

The flow behavior of the emulsions was also investigated using the Power-Law

Index, n (Table 3.1). The Power-Law Index is decreasing towards zero when the oil

concentration is increased. The decrease in the Power-Law Index as a function of

surfactant concentration is profound for the emulsions with lower oil concentration (<

70% oil) after 1 day of storage under 45oC. The experimental result shows that the

Power-Law Index for the emulsions with lower oil concentration decreased after 30

days of storage. This indicates the increase in the degree of thinning for the emulsions

after the storage period. However, the effects of storage time and surfactant

concentration were insignificant in the emulsions with higher oil concentration (≥ 70%

oil).


________________________________________________________________________ - 76 -

Table 3.1: The Power Law index, n of the emulsions for over 30 days of storage time.

All n were smaller than 1 indicating that the emulsions exhibits shear thinning behavior.

Age Surfactant

ConcentrationPower Law index, n

(wt%) 50% Oil 60% Oil 70% Oil 80% Oil

1st day 2 0.75 0.67 0.22 0.21

3 0.86 0.66 0.12 0.23

4 0.73 0.55 0.04 0.17

5 0.83 0.60 0.04 0.19

6 0.64 0.54 0.11 0.21

7 0.24 0.17 0.18 0.17

8 0.23 0.19 0.20 0.21

9 0.22 0.22 0.19 0.24

10 0.23 0.21 0.23 0.23

7th days 2 0.65 0.19 0.15 0.18

3 0.69 0.30 0.12 0.20

4 0.70 0.16 0.14 0.23

5 0.62 0.16 0.17 0.23

6 0.64 0.21 0.19 0.21

7 0.59 0.17 0.16 0.21

8 0.51 0.15 0.23 0.23

9 0.44 0.17 0.22 0.22

10 0.35 0.15 0.19 0.21

30th days 2 0.21 0.08 0.19 0.21

3 0.11 0.14 0.18 0.26

4 0.17 0.11 0.20 0.25

5 0.15 0.12 0.21 0.26

6 0.12 0.22 0.17 0.22

7 0.27 0.19 0.19 0.20

8 0.29 0.15 0.23 0.18

9 0.35 0.18 0.22 0.22

10 0.30 0.17 0.22 0.22


________________________________________________________________________ - 77 -

Figure 3.12: Schematic representation of structural change when shear applied. (a) The

shear thickening region, (b) the First Newtonian region, (c) the shear thinning region,

and (d) the Second Newtonian region.

Fig. 3.13(a)

-10 0 10 20 30 40 50

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

Droplet Size, d (μm)

Pop

ulat

ion

50% Oil 60% Oil 70% Oil 80% Oil


________________________________________________________________________ - 78 -

Fig. 3.13(b)

-10 0 10 20 30 40 50

0

200

400

600

800

1000

1200


Pop

ulat

ion

50% Oil 60% Oil 70% Oil 80% Oil

Fig. 3.13(c)

-10 0 10 20 30 40 50-200

0

200

400

600

800

1000

1200


Pop

ulat

ion

50% Oil 60% Oil 70% Oil 80% Oil

Figure 3.13: The droplet size distribution of the emulsions stabilized with 2 wt% of

SFAE for (a) 1st day, (b) 7th day, and (c) 30th day of storage time.


________________________________________________________________________ - 79 -

The Power-Law equation (Eq. 7) is used to illustrate the shear thinning effect of

an emulsion, while the Power-Law Index, n is an indication of the degree of thinning.

The shear thinning region begins right after the yield stress of the emulsions is exceeded

where the viscosity of the emulsion decreased with shear rate is observed. Shear

thinning behavior is initiated from the perturbation of the original structure of the

emulsion and followed by rearrangement into an ordered layer when shear is applied

(Figure 3.12) [211]. Thus, the droplet structural orientation and the polydispersity are

important factors because they affect the packing efficiency of the droplets especially

for a densely packed system.

The width of the droplet size distribution curves for those emulsions which are

shown in Figure 3.13 increased with storage time and decreased with surfactant and oil

concentration. The distribution of droplets for the emulsions clearly shows that the

frequency of larger droplet size increased after 30 days of storage (Figure 3.13(c)). This

implies that the droplets in the aged emulsions can pack more efficiently than the

freshly prepared emulsions. According to the literature [212, 213], the maximum

disperse phase volume fraction, mφ (oil concentration) for the droplets to be randomly

close packed in concentrated emulsion is 61% for monodisperse and 71% for

polydisperse system, further addition of the φ will cause droplets to deform leading to

hexagonal close packing. However, the magnitude of the mφ is polydispersity

dependence, higher polydisperse droplets tend to pack more efficiently at higher volume

fraction when shear is applied and vice versa (Figure 3.14). In this case, the

polydisperse droplets can easily slip pass each other and can easily align themselves

along the flow line induced by the movement of the geometry at lower shear stress ,

thus reducing the viscosity [214].


________________________________________________________________________ - 80 -

(a)

(b)

Figure 3.14: The packing of emulsion droplets in the (a) polydisperse system and (b)

monodisperse system showing the droplet packing efficiency.

The shear thinning behavior of the emulsions can also be predicted using

Equations 14 and 15. This shear thinning effect is an evidence of the competition

between the thermal and hydrodynamic forces [215]. At the first Newtonian region

( σ ‹ Yσ ), no flow is being observed and the droplets experience thermal force

(Brownian). When the emulsions started to flow (σ › Yσ ), the hydrodynamic force

came into consideration and dominated over the thermal force [215]. At high shear

stress (σ » Yσ ), the droplets become organized into layers along the line of shear field

and closer to each other, hence, the hydrodynamic shear force dominates (second

Newtonian plateau) [81]. For large droplets, the Yσ is often so low (Figure 3.11) that the

shear thinning effects is hardly observed as compared to the emulsions with smaller

droplets [81]. These were supported by the experimental results where the Power-Law

index, n for the emulsions with larger droplet size was greater and closer to 1 (Table

3.1). Therefore, a conclusion can be drawn where the degree of thinning of the

emulsion decreased with the increase of mean droplet size [216].


________________________________________________________________________ - 81 -

3.4.1.2 Aging effect to the emulsions flow properties

Aging effect has led to the decrease of the emulsions stability resulting from an

increase of droplet size. The increase of droplet size had weaked the interdroplet

interactions, thus lowering the viscosity, yield stress and degree of thinning of the

emulsions [135, 217]. Among the emulsions examined in this study, the emulsions with

50% oil were the most unstable, which show phase separation for all range of surfactant

concentration after 30 days of storage. The mean droplet size was increased and the

flow properties show significant changes after 30 days of storage time. On the other

hand, the aging effect was negligible in the very stable emulsions which are the one

with 80% oil. According to the stability test, there was no phase separation observed

form these emulsions throughout the 30 days of storage time. There were also no

significant increase in the droplet size of these emulsions over time (Figure 3.2) which

implied that the emulsions with 80% oil were relatively stable against destabilization

processes.

Upon aging, the viscosity of emulsions especially the dilute emulsions (< 70%)

decreased. The reduction in viscosity of these emulsions was due to the increase of the

droplet size [74]. The droplet concentration in dilute emulsions was low as compared to

the concentrated emulsions (Figures 3.4 and 3.7). As a result, the droplets were more

mobile and free to move, thus enhancing droplets collision leading to the coarsening of

droplets. The appearance of large droplets coexisting with the smaller ones

(polydisperse system) influences the close packing of the droplets by locating

themselves (the smaller droplets) in between the larger droplets. Therefore, the droplets

can pack more efficiently when they were forced to be close to each other as stress is

applied (Figure 3.22 (b)). When the applied stress exceeded the yield stress of the

emulsion system, the droplets started to flow by aligning themselves into the shear line.


________________________________________________________________________ - 82 -

Many polydiserse systems started to flow at lower shear field due to low interaction

between the fine and coarse droplets thus lowering the yield stress as well as the shear

thinning effect of the emulsions. In addition, the larger droplet can easily slip pass over

the smaller ones when they were forced to move forward. As a result, the flow

properties of the coalescenced emulsions decreased.

3.5 Dynamic Characteristic

The viscoelastic properties of the emulsions were investigated using the strain

and frequency sweep measurements. In the strain sweep measurement, the oscillation

frequency is fixed at 1 Hz in order to obtain the linear viscoelastic region; any higher

frequency will rupture the internal structure of the emulsion. The purpose to perform

this measurement is to obtain the limits of linearity of the emulsions [218]. In this linear

region, all rheological parameters were remaining constant and independent of the strain

amplitude. The parameters start to change with the applied strain when it exceeded the

critical strain (γc) of the emulsions [219]. This γc is identified as the minimum strain

where emulsion shows departure from linearity.


________________________________________________________________________ - 83 -

Fig. 3.15(a)

1 2 3 4 5 6 7 8 9 10 110.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

0.050

0.055

Crit

ical

Stra

in, γ

c


50% Oil 60% Oil 70% Oil 80% Oil

Fig. 3.15(b)

1 2 3 4 5 6 7 8 9 10 110.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

0.050

0.055


Crit

ical

Stra

in, γ

c

50% Oil 60% Oil 70% Oil 80% Oil


________________________________________________________________________ - 84 -

Fig. 3.15(c)

1 2 3 4 5 6 7 8 9 10 110.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

0.050

0.055


Crit

ical

Stra

in, γ

c

50% Oil 60% Oil 70% Oil 80% Oil

Figure 3.15: The γc of the emulsions for (a) 1st day, (b) 7th day, and (c) 30th day.

The critical strain, γc of emulsions in different oil concentration are shown in

Figure 3.15. The γc of the emulsions increased with the oil volume fraction and

surfactant concentration, but decreased with the emulsions’ age. It can be observed from

the findings that, surfactant and oil concentration affected γc the most compared to the

emulsion age. The ageing effect is only significantly affected the γc in more dilute

emulsions which is the emulsions with 50% and 60% oil. The γc is increased more than

100% as the surfactant concentration increased from 2 wt% to 10 wt%; and almost 80%

when the oil concentration increased from 50% to 80%. These implied that the

emulsions with high surfactant and oil concentration were able to recover even after

being subjected to large deformation strain.


________________________________________________________________________ - 85 -

Fig. 3.16(a)

Fig. 3.16(b)


________________________________________________________________________ - 86 -

Fig. 3.16(c)

Fig. 3.16(d)

Figure 3.16: The morphology of emulsions with 80% oil which stabilized by (a) 7 wt%,

(b) 8 wt%, (c) 9 wt% and (d) 10wt% of SFAE.


________________________________________________________________________ - 87 -

The rapid increase of critical strain of emulsions with 80% oil when the SFAE

concentration increased above 7 wt% (Figure 3.15) implied that the highly packed

droplets (Figure 3.16) have develop a strong structural due to the great interdroplet

interaction between the droplets which correspond to the droplet size and droplet

concentration of the emulsions system. According to the morphology of the emulsions

shown in Figure 3.16, the droplet size was fine and highly packed as compared to those

emulsions prepared with lower oil and surfactant concentration (Figure 3.17). Since the

strength of the interdroplet interactions corresponds to the mean separation distance

between the droplets, the highly packed emulsion system will therefore has greater

interdroplet interaction forces. This high interdroplet interaction strength was able to

hold the droplets and withstand the large deformation forces applied during the strain

sweep test.

Fig. 3.17(a)


________________________________________________________________________ - 88 -

Fig. 3.17(b)

Fig. 3.17(c)


________________________________________________________________________ - 89 -

Fig. 3.17(d)

Figure 3.17: The morphology for the emulsions with 70% oil stabilized with (a) 7 wt%,

(b) 8 wt%, (c) 9 wt%, and (d) 10 wt% of SFAE.


________________________________________________________________________ - 90 -

Fig. 3.18(a)

1E-4 1E-3 0.01 0.1 1

10

100

1000

Ela

stic

Mod

ulus

, G' (

Pa)

Strain,γ (%)

2wt% 3wt% 4wt% 5wt% 6wt% 7wt% 8wt% 9wt% 10wt%

Fig. 3.18(b)

1E-4 1E-3 0.01 0.1 11

10

100

1000


Elas

tic M

odul

us, G

' (Pa

)

Strain,γ (%)


________________________________________________________________________ - 91 -

Fig. 3.18(c)

1E-4 1E-3 0.01 0.1 11

10

100

1000

Ela

stic

Mod

ulus

, G' (

Pa)

Strain, γ (%)


Fig. 3.18(d)

1E-4 1E-3 0.01 0.1 11

10

100

1000

Strain, γ(%)

Ela

stic

Mod

ulus

, G' (

Pa)


Figure 3.18: The elastic modulus of emulsions at the first day of age obtained from the

strain sweep to establish the linear viscoelastic range. (a) 50%, (b) 60%, (c) 70%, and (d)

80% oil.


________________________________________________________________________ - 92 -

The strain sweep profiles not only give information on the critical strain but also

provide useful information about the elastic component of the emulsions. Figures 3.18

and 3.19 show increasing trend in the elastic modulus of the emulsions with surfactant

and oil concentration which means that the interactions between droplets are relatively

strong. Comparing the results shown in Figures 3.15 and 3.19, a trend of increasing

elastic modulus accompanying with the increase of γc is observed. This further

supported the fact that increasing of the interdroplet interaction strength as the

surfactant and oil concentration increased.

21 '2c cE G γ= (16)

These interdroplet interactions can be represented as the cohesive force, Ec

which can be to estimated using the elastic modulus (G’) and γc of the emulsions (Eq.

16). The results show that the cohesive energy was increasing when the surfactant and

oil concentration increased (Figure 3.20). According to Tadros, T. [121], the cohesive

energy is related to the structure of the emulsion system which correlated to the droplet

size and number of contact area between the droplets. In other word, the droplet

concentration and the packing of the droplets directly influence the strength of the

cohesive force.

By observing and comparing the morphology of the emulsions with 70% and

80% oil (Figures 3.16 and 3.17), the number of droplets was increasing as the oil

concentration increased from 70% to 80% at fix surfactant concentration. As a result,

the number of contacts within the droplets increases. That causes the increase in the

cohesive energy of the emulsion system (Figure 3.20).


________________________________________________________________________ - 93 -

Beside the oil concentration, the surfactant concentration is also an important

factor affecting the energy of an emulsion system. According to the results shown in

Figure 3.20, the cohesive energy increased significantly with surfactant concentration

especially for the emulsions with 80% oil stabilized with 7 wt% of SFAE and above.

The dramatic increase of the cohesive force was due to the highly packed systems

related to the interdroplet interactions that had previously been mentioned.

Fig. 3.19(a)

1 2 3 4 5 6 7 8 9 10 110

200

400

600

800

1000

1200

1400

1600

1800

2000

2200 50% Oil 60% Oil 70% Oil 80% Oil

Elas

tic M

odul

us, G

' (P

a)



________________________________________________________________________ - 94 -

Fig. 3.19(b)

1 2 3 4 5 6 7 8 9 10 110

200

400

600

800

1000

1200

1400

1600

1800

2000

2200


Ela

stic

Mod

ulus

, G' (

Pa)

50% Oil 60% Oil 70% Oil 80% Oil

Fig. 3.19(c)

1 2 3 4 5 6 7 8 9 10 110

200

400

600

800

1000

1200

1400

1600

1800

2000

2200


Ela

stic

Mod

ulus

, G' (

Pa)

50% Oil 60% Oil 70% Oil 80% Oil

Figure 3.19: The magnitude of elastic modulus of the emulsions obtained from the strain

sweep measurement for (a) 1st day, (b) 7th day, and (c) 30th day.


________________________________________________________________________ - 95 -

Fig. 3.20(a)

1 2 3 4 5 6 7 8 9 10 110.0

0.5

1.0

1.5

2.0

2.5

3.0

E c (Jm

-3)

S u rfactan C oncentra tion (w t% )

50% oil 60% oil 70% oil 80% oil

Fig. 3.20(b)

1 2 3 4 5 6 7 8 9 10 110.0

0.5

1.0

1.5

2.0

2.5

3.0

Ec (J

m-3)

Surfactan Concentration (w t% )

50% oil 60% oil 70% oil 80% oil


________________________________________________________________________ - 96 -

Fig. 3.20(c)

1 2 3 4 5 6 7 8 9 10 110.0

0.5

1.0

1.5

2.0

2.5

3.0

E c (Jm

-3)

Surfactant Concentration (w t% )

50% oil 60% oil 70% oil 80% oil

Figure 3.20: The cohesive energy of the emulsions obtained at (a) 1, (b) 7, and (c) 30

days of storage periods.

All frequency sweep tests for the emulsions were performed in the linear

viscoelastic region based on the amplitude sweep profile to reduce the possibility of

introducing great damage to the microstructures of emulsion. After considering the

amplitude sweep profile of all emulsions, fix strain amplitude of 0.4% was chosen as the

applied strain in order to obtain a comparable data. The frequency sweep profile is used

to illustrate the response of the emulsions’ modulus with frequency. As shown in

Figure 3.21, the elastic modulus (G’) and the viscous modulus (G”) of the emulsions

showing the domination of G’ over G”. Also, as the frequency increased, the

magnitudes of G’ and G” increased, the distance between the two curves also getting

larger. The magnitude of G’ and G” increased with surfactant concentration as well.

These emphasize the elastic character of the emulsions.


________________________________________________________________________ - 97 -

Fig. 3.21(a)

0.01 0.1 1 1010

100

1000

10000 G' G"

Frequency (Hz)

Ela

stic

Mod

ulus

, G' (

Pa)

10

100

1000

10000 2wt% 3wt% 4wt% 5wt% 6wt% 7wt% 8wt% 9wt% 10wt% V

iscous Modulus, G

" (Pa)

Fig. 3.21(b)

0.01 0.1 1 1010

100

1000

10000

Ela

stic

Mod

ulus

, G' (

Pa)

Frequency (Hz)

G' G"

10

100

1000

10000

Vis

cous

Mod

ulus

, G" (

Pa)



________________________________________________________________________ - 98 -

Fig. 3.21(c)

0.01 0.1 1 1010

100

1000

10000

Ela

stic

Mod

ulus

, G' (

Pa)

Frequency (Hz)

G' G"

10

100

1000

10000

Vis

cous

Mod

ulus

, G" (

Pa)


Fig. 3.21(d)

1E-3 0.01 0.1 1 1010

100

1000

10000

G' G"

10

100

1000

10000


Elas

tic M

odul

us, G

' (P

a)

Frequency, f (Hz)

Viscous Modulus, G

" (Pa)

Figure 3.21: Frequency sweep profile of the emulsions with (a) 50%, (b) 60%, (c) 70%,

and (d) 80% of oil.


________________________________________________________________________ - 99 -

However, there was a slight difference in the frequency sweep profiles between

the dilute emulsions (< 70% oil) and the concentrated one (≥ 70% oil). As previously

discussed, the frequency sweep can be divided into five regions (Figure 1.12).

According to the experimental results shown in Figures 3.21(a) and (b), the frequency

sweep profiles for dilute emulsions show the transition and plateau zone of the

mechanical spectrum (Figure 1.12). But, the frequency profiles for concentrated

emulsions show only the plateau zone (Figures 3.21(c) and (d)).

In the case of dilute emulsions, the G” is found to dominate over the G’ in the

low frequency region. The Tan δ of the emulsions helps to clarify this point by giving

magnitude greater than 1 (Figure 3.22), which implies that the emulsions flow at low

frequency showing liquid like behavior. The magnitude of G’ and G” continuously

increased when the frequency increased and the rate of increase for G’ is greater than

G”. Finally, the two curves crossover at a frequency point at which G’=G”. This

crossover frequency is the characteristic frequency ( *ω ) which the inverse of this *ω

representing the relaxation time ( *t ) of the system (Eq. 17) [121]. The *ω is also a

transition point where the emulsion changes its behavior from liquid like to solid like.

After the *ω , the magnitude of G’ and G” continuously increased and this time the G’

dominated over the G” which indicated that the emulsions started to behave like solid.

On the other hand, there was no crossover point observed at the same studied

frequency range for concentrated emulsions. The G’ for these concentrated emulsions

was always dominating over the G” in the studied frequency range. In other word, the

crossover frequency had shifted to lower frequency region. This is due to high

relaxation time ( *t ) of the emulsions. The *t of an emulsion system is the time required

for the deformed system to reform under condition of low stress or rest [133].

According to the experimental results, the *t is increasing with the oil concentration. In

other word, the concentrated emulsions need longer time for it to recover once the


________________________________________________________________________ - 100 -

emulsion structure breakdowns as compared to more dilute emulsion system. The Tan δ

of these concentrated emulsions is lower than 1 which is well-proven that these

emulsions exhibit solid like behavior.

**

1tω

= (17)

In a dilute emulsion system, owing to the weak interdroplet interactions, the

systems were able to behave like a liquid after being subjected to low stress for long

time (low frequency). In the case of concentrated emulsions, no domination of the G”

over the G’ was observed when the test was carried out under same condition as applied

to the dilute emulsion. That means the applied stress is able to be stored in the elastic

component although at low stress condition. This indicated the presence of strong

interactions among the emulsion droplets. According to Tadros, T. [219], G’ is a

measure of the energy stored elastically in the system, while G” is the measure of the

energy dissipated as heat during viscous flow. The input energy which comes from the

shear stress is stored elastically and represented as G’. Therefore, a decrease in the

magnitude of the imaginary viscous component (G”) is observed. When the frequency

further decreases, most of the input energy is no longer able to be stored, but will

dissipated through viscous flow [220].

The frequency sweep profile also shows decrease of G’ slope and the flattening

of G” happened when the oil concentration increased [173]. That is illustrated by the

results shown in Figure 3.21, which shows G’ >> G” and the modulus is almost

independence of frequency. This development of plateau region maybe attributed to an

increase of interactions among the droplets, as a consequence of the decreased of

droplet size and polydispersity [221]. Besides, this is also an indication of the ability of


________________________________________________________________________ - 101 -

the emulsions to resist any structural changes under stress (as the frequency increased)

[222].

In this study, however, the effect of surfactant concentration on the viscoelastic

property of emulsion is negligible compared to the effect of oil concentration (Figures

3.22 and 3.23). Those were clearly reflected in the Tan δ of the emulsions. The Tan δ of

the emulsion with 50% oil is 2 times greater than that of the 80% emulsion system. The

magnitude of Tan δ decreased significantly with increase of the oil concentration

indicating the great elasticity of the emulsions with 80% oil [222]. Tan δ (Eq. 18) is a

dimensionless loss factor measuring the amount of energy loss during the test cycle

[223]. The emulsion with Tan δ greater than 1 behaves like a liquid because the G” is

larger than G’; while a solid like emulsion gives Tan δ lower than 1 (G’>G”) [224].

"'

GTanG

δ = (18)

The lesser influence of the surfactant concentration to the elastic property of

emulsion is illustrated in Figure 3.22. That figure indicated that there was no

complicated network formed among the adsorbed layers between droplets. This also

implied the low contribution of surfactant to the dynamic rigidity of the emulsions.


________________________________________________________________________ - 102 -

Fig. 3.22(a)

1E-3 0.01 0.1 1 100.1

1

Frequency, f (Hz)

Tan

δ


Fig. 3.22(b)

1E-3 0.01 0.1 1 100.1

1

Frequency, f (Hz)

Tan

δ



________________________________________________________________________ - 103 -

Fig. 3.22(c)

1E-3 0.01 0.1 1 100.1

1

Frequency, f (Hz)

Tan

δ


Fig. 3.22(d)

1E-3 0.01 0.1 1 100.1

1

Frequency, f (Hz)

Tan

δ


Figure 3.22: The Tan δ of emulsions with (a) 50%, (b) 60%, (c) 70%, and (d) 80% of oil

obtained after one day of storage.


________________________________________________________________________ - 104 -

As mentioned before, G’ is a measure of the energy stored elastically in the

system, while G” is the measure of the energy dissipated as heat during viscous flow. In

general, more and more energy will be stored elastically in the system when the

frequency increases and the G” tends to decrease to zero. Based on the experimental

results (Figure 3.22), the elasticity of emulsion is said to be almost independent of the

surfactant concentration. But, the magnitude of G’ was increasing in both strain sweep

and frequency sweep with the increase of surfactant concentration. This is due to the

decrease of droplet size and polydispersity of the droplets. As the droplet size and

polydispersity decreased, the interfacial area increased; thus, increased the surface-to-

surface interactions. As a result, the magnitude of G’ increased. However, the increase

of G’ is accompanied with the increasing of G” that specifies the energy dissipation

from the system. In other word, the amount of energy stored in the system when the

surfactant concentration increases will then be dissipated from the system. That

explained why the increase of surfactant concentration does not increase the interdroplet

interaction between the droplets.


________________________________________________________________________ - 105 -

Fig. 3.23(a)

1E-3 0.01 0.1 1 100.1

1

Tan

δ

Frequency, f (Hz)

50% Oil 60% Oil 70% Oil 80% Oil

Fig. 3.23(b)

1E-3 0.01 0.1 1 100.1

1

50% Oil 60% Oil 70% Oil 80% Oil

Tan

δ

Frequency, f (Hz)


________________________________________________________________________ - 106 -

Fig. 3.23(c)

1E-3 0.01 0.1 1 100.1

1

50% Oil 60% Oil 70% Oil 80% Oil

Tan

δ

Frequency, f (Hz)

Figure 3.23: The effect of oil concentration to the Tan δ of emulsions stabilized with 2

wt% of SFAE after (a) 1 day, (b) 7 days, and (c) 30 days of storage duration.

The increase of elasticity when the oil concentration increased is explained with

the effect of droplet size and droplet concentration. Unlike electrostatically stabilized

system, the sterically stabilized system involved a short range interaction. According to

Tadros, T.F. [225], the elastic modulus attributed from the hydrodynamic and surface

force is very dependent to the surface-to-surface separation distance. These interactions

between droplets become stronger when the droplets getting closer to each other. At

lower oil concentration, the droplets separation distances were large and is comparable

to droplet radius. In this case, the droplets were considered loosely packed and were still

able to diffuse with slower rate. Due to the large separation distances between the

droplets, the interaction among the droplets is relatively weak.


________________________________________________________________________ - 107 -

The increased of oil concentration resulting in the increased of droplet

concentration and decreasing the interdroplet separation distance. As the interdroplet

separation distance decreased some overlap of the interfacial layer occur. That causes

rapid increase of the repulsive forces and eventually the droplets repel each other. When

the droplets were in close distance, the hydrodynamic interaction is important to act as a

repulsive force to prevent the droplets coming into close contact. Meanwhile, the

attractive forces increase by pulling the droplets towards each other. These interdroplet

forces therefore act like a “spring” holding the droplets and the droplets end up

vibrating with a small amplitude which pronounced the elastic property [226]. As stated

by Tadros, T.F. [225], in such concentrated condition, the droplets interact with many

neighbors and the repulsive force produces a specific order among the droplets to the

extent that a highly develop structure is reached. However, the droplets will merge or

rupture at high shear stress region when the droplets are too close to each other whereby

the attractive forces are greater than the repulsive forces. When that happens, loss of

elasticity can be observed by showing a decreasing in magnitude of G’. Fortunately, the

viscoelastic profiles of the emulsions do not show this phenomenon, which indicates the

structural rigidity and high droplet stability although subjected to high stress (high

frequency).

CHAPTER 4

CONCLUSION

Chapter 4 Conclusion

________________________________________________________________________ - 108 -

4.0 Conclusion

This study has demonstrated the preparation of olive oil-in-water emulsion

stabilized with nonionic surfactant. The emulsions were prepared with different

composition of surfactant and oil and were kept for 30 days under 45oC for stability and

rheological test. The microstructure of the emulsions was studied under polarizing

microscope, while the Bohlin CVO-R rheometer was employed for characterization of

the macroscopic properties of the emulsions.

The stability of the formulated emulsions was found to be increased with the

concentration of oil and surfactant. The phase separation of the emulsions decreased

with an increase of surfactant and oil concentration indicating the greater stability of

droplets against the gravitational creaming destabilization process. However, the 80%

oil emulsions were the only emulsions that remain stable without phase separation for

30 days of storage period under accelerated condition. The other emulsions show

obvious phase separation after or within the examined period. The emulsions instability

can be explained from creaming destabilization, which causes the migration of oil

droplets to the top of the emulsions. According to the experimental results, the effect of

oil is more significant to affect the shelf life of the emulsion compared to the effect of

surfactant.

The influences of oil and surfactant concentration to the microstructure of the

emulsions were clearly shown in the micrographs. The droplet size decreased with the

increase of surfactant and oil concentration. The droplet concentration was increased

with the increase of the oil and surfactant concentration. The droplet size and droplet

concentration are the key to these destabilization processes that influence the

interdroplet interaction. This is due to the strong relationship between the droplets size

and droplet concentration with the strength of the interaction forces.


________________________________________________________________________ - 109 -

The droplet size and droplet concentration also affected the rheological behavior

of the emulsions. A very significant change of the rheological behavior of the emulsions

was the viscosity of the emulsions. The viscosity of emulsions was increased with the

increase of surfactant and oil concentration. This was due to the increase of droplet

concentration which affected the mobility of the droplets. That also increased the

interdroplet interaction which increased the yield stress of the emulsions. Besides, the

viscoelastic property of the emulsions was also affected by the changes of the

microstructure. The increase in the oil concentration had increased the droplet

concentration which enhanced the elasticity of the emulsions.

The stability and the rheological properties of the emulsions decreased with

storage time. After being kept under accelerated condition for 30 days, the droplet size

of these emulsions increased, which indicated the microstructural rearrangement. The

viscosity and the viscoelastic behavior decreased significantly after 30 days of storage

time. However, the emulsions with 80% oil were the only emulsions that are stable

against phase separation. The changes in the droplet size and rheological behavior of

these emulsions were negligible. In order word, these 80% oil emulsions have the

longest shelf life as compared to the other emulsions that had been prepared in this

study.


________________________________________________________________________ - 110 -

4.1 Future Research

It can be observed that the increase of the oil and surfactant concentration

enhanced the emulsions stability and rheological properties. But, due to the economical

concern, the increase in the surfactant concentration is not a desirable method to use as a

way to improve the emulsion stability, while the increase of oil content in the emulsion

formulations will decrease the perception of the emulsions. Therefore, the emulsions

must have lower oil content in order to create pleasant feeling when applied on the skin.

However, the emulsions formulated from low oil concentration have pronounced phase

separation within short period after preparation. Thus, the rheological properties of this

emulsion have to be modified to increase stability and the shelf life of the emulsion. In

this case, there are several factors that have to be considered.

One of the factors is to modify the viscosity of the continuous phase of the

emulsions by adding thickener into the emulsion in order to create a continuous phase

with high viscosity to control the movement of the droplets. The increase of the

continuous phase viscosity will decrease the mobility, the collision frequency as well as

the collision efficiency of the droplets. That is also able to prevent sedimentation or

creaming destabilization processes.

Thickener such as polymers were used in formulation in order to improve the

flow ability (viscosity modifier), shelf life and intergrity of an emulsion. In this study,

polysaccharides such as xantham gum is chosen as a thickener due to the factor of being

“natural”. Xantham gum is one of the examples of a high molecular weight

extracellular polysaccharide biopolymer produced by the bacterium Xanthomonas

campestris [227]. It has a broad application range including food, cosmetic, and

pharmaceutical industries [228] due to the human friendly nature. Xantham gum exists

in aqueous media with an ordered rigid chain conformation [228], thus is suitable as


________________________________________________________________________ - 111 -

stabilizer and thickener in water-based (oil-in-water) emulsion system. However, there

are also some interactions between the surfactant added into the emulsions system and

the polysaccharides. These interactions may affect the function of the polysaccharides

and the surfactants that will indirectly influence the stability of the emulsion.

In addition, the chemical composition such as pH and electrolyte concentration

as well as the viscosity of the emulsion are also important factors affecting the emulsion

stability [229]. Therefore, the investigation of the influence of thickener, pH, and

concentration of electrolytes to the emulsions system stabilized with glycoipids is

required in the future.

CHAPTER 5

REFERENCES

Chapter 5 References

________________________________________________________________________ - 112 -

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APPENDIX

Appendix

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Publication

1. Tan, H.W., Misran, M., Stability of Concentrated Olive Oil-in-Water Emulsion.

Chinese Journal of Chemistry, 2008, 26: p. 1963- 1968

Conference

1. Tan, H.W., Misran, M., Ageing Effect on Rheological Properties of Concentrated

Olive O/W Emulsion. The 2nd Penang International Conference for Young Chemist,

18-20 June 2008, Penang, Malaysia.

Documents

RHEOLOGY AND STABILITY OF OLIVE OIL CREAM …repository.um.edu.my/1181/1/Thesis- Tan Hsiao Wei.pdf · rheology and stability of olive oil cream emulsion stabilized by sucrose fatty