CHAPTER 2 Coconut fatty acid-sucrose esters: …shodhganga.inflibnet.ac.in/bitstream/10603/25187/6/7...Coconut fatty acid-sucrose esters: Synthesis, characterization and applications

Ph. D. Thesis, Priya S. Deshpande, NMU, Jalgaon, 2013

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CHAPTER 2

Coconut fatty acid-sucrose esters: Synthesis, characterization and applications

2.1 Background and objectives of investigations

Sucrose is one of the world’s most abundantly produced organic compounds and

the esters derived from its reaction with vegetable oils or corresponding FAME have a

wide variety of applications as presented under section 1.2.3 of Chapter 1. However, the

literature on synthesis of sucrose esters is scarce. The rheological and surface active

properties of these derivatives still need to be evaluated. Chemical engineering principles

also need to be applied for scale up and purification problems associated with their

manufacture. It is with this intention, the investigations on synthesis and explorations of

diverse applications of sucrose esters have been reported in present Chapter while

applications of reaction engineering principles in reference to their synthesis have been

presented under Chapter 4.

2.1.1 Synthesis of coconut fatty acid-sucrose esters

In industry, sucrose, which is available in large amounts and at attractive prices, is

used as the preferred starting raw materials for the selective functionalisation with

vegetable oils/ FAME for the construction of a perfect amphiphilic structure which could

be realized through simple technical process. The literature review presented under

Chapter 1 implies that transesterification of fatty esters or triglycerides with sucrose using

a basic catalyst is the only commercially feasible option for synthesis of sucrose ester. The

problem in synthesising sucrose esters is related to the high functionality of the sucrose

molecule with eight hydroxyl groups, which compete during the derivatization step.

Although sucrose ester is an expensive specialty chemical with greater potential utility, the

literature on its synthesis, in general, and kinetics of its synthesis, in particular, is scant. In

fact, the patent literature is abundant in this rapidly growing field which highlights the

secrecy associated with its synthesis and utilization1-37

. This study therefore aims at

elucidating the effects of molar ratio, catalyst loading, reaction temperature and fatty acid

chain length on formation of sucrose esters.

The physicochemical properties of mixtures of regioisomers, as well as mono-, di-

and polyesters depend on the average degree of substitution and fatty acid chain length.

The transesterification reaction may not allow regioselectivity and the degree of hydroxyl

group substitution on the sugar is uncontrolled38

. Nevertheless, the reaction may be


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oriented towards a specific substitution pattern by carefully choosing the catalyst, as well

as the reaction conditions.

Alkaline metal alkoxides (such as CH3ONa) as catalysts give very high yields

(> 98%) in short reaction times (30 min), even at low molar concentrations (0.5 %).

However, these most active catalysts require the absence of water (substantially anhydrous

reactants) and AV (acid value) < 0.5 making them difficult to use industrially. Alkaline

metal hydroxides/ carbonates (KOH, NaOH and K2CO3) are less active than metal

alkoxides and allow for equally high yields by increasing the catalyst concentration to 1 or

2 %. However the reaction of the hydroxide with the alcohol will introduce some water in

the system, which will hydrolyze some of the produced ester, with consequent soap

formation. This reduces the final conversion. The use of potassium carbonate, in a

concentration of 2 or 3 % reduces the soap formation and permits increased yields. In fact,

the addition of potassium carbonate allows for the formation of bicarbonate instead of

water, and the esters are not hydrolyzed. Hence it was selected as catalyst in present

investigations.

Since the medium molecular weight fatty acids of coconut oil are used in a wide

range of food, surfactant and cosmetic products, the oil/ its mixed methyl esters were

chosen as source of fatty materials in present work. The influences of choice of coconut

fatty acid to sucrose molar ratio (M), catalyst loading, reaction period (t) and temperature

(T) on the synthesis of sucrose ester were used to control the rate of formation in specific

substitution mode- mono/ di/ polyesters. In order to understand the influence of molecular

weight and chain length (C12-C18) of coconut esters on the synthesis of sucrose ester, the

transesterification was performed with methyl esters of principle coconut fatty acids

(lauric and myristic) and coconut oil. The synthesis data has been utilized in Chapter 4

for establishing the selective and nonselective kinetic model of transesterification.

2.1.2 Characterization of coconut fatty acid-sucrose esters

The viscosity of sucrose ester governs the flow characteristics which hold

significance in many applications including low calorie fat. Hence the rheological

characterizations of sucrose ester solutions in DMF were performed and interpreted on the

basis of molecular interactions and degree of substitution.

The development of surfactants based on carbohydrates and oils is the result of a

product concept that is based on the exclusive use of renewable resources. Sucrose esters

of fatty acids having 12 or more carbon atoms and carrying degree of substitution < 3 have

very broad tensio-active properties due to the wide arbitrary hydrophilicity (free hydroxyl


52

groups)-lipophilicity (alkyl chain) and hold major applications in food, pharmaceutical,

cosmetics and nanomaterials industries as surfactant. Hence the evaluations of coconut

fatty acid-sucrose esters as surfactant, on the basis of determinations of adsorption density

and critical micelle concentration (CMC), were conducted in reference to the precursor

and precipitant used in the synthesis of lead chrome nanopigments.

2.1.3 Utilization of sucrose ester as surfactant in the polymorph selective synthesis of

lead chrome nanorods

Motivated by the size dependent electrical, magnetic, thermal, optical and catalytic

properties demonstrated on the nanoscale, the field of nanotechnology has been the subject

of intensive investigations in recent years for obtaining highly functional advanced

materials carrying applications in many fields such as catalysis, medicine, electronics,

ceramics, pigments, cosmetics, etc. Regulation of the architecture and morphology at the

nanoscale through control of particle size, phase, polymorhism is essential for the

fabrication of nanocrystals such as nanoparticles, nanotubes, nanowires, nanofibers and

nanorods for development of collective optical, magnetic and electronic properties. The

preparation of nanomaterials is, thus, a very challenging task.

Currently, sustainability initiatives that use green chemistry to improve and/or

protect our global environment are becoming focal issues in synthesis of nanomaterials.

Amongst all the methods of producing nanomaterials, the wet chemosynthesis governed

by surfactant templates is the most efficient, ecofriendly, safer and widely used one that

traditionally proceeds in batch reactors. Surfactants are known to form monolayers or

micelles in solution, provide microenvironment/ sites for reactive events (in addition to

bulk medium reactions) with high specificity and selectivity and facilitate nucleation and

oriented crystallization of nanoparticle products. Sucrose esters, representing excellent

scaffolds for stabilised synthesis of nanomaterials, has hydroxyl groups, a hemiacetal

reducing end, and other functionalities that can play important roles in both the reduction

and the stabilisation of metallic compound nanoparticles. Surfactants derived from sucrose

fatty acid esters are attractive because of their ready biodegradability, low toxicity, low

irritation to eyes and skin, and the renewable nature of the sugar and fatty acid starting

materials. However, literature review implies that only limited works have been reported

on preparing nanomaterials in sugar-ester based templates.

The compound lead chromate (PbCrO4) is an important brilliant yellow pigment,

photosensitizer and humidity sensing resistor and can be obtained as nanorods and

nanoparticles. The traditional methodologies have limited influence on the crystal


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morphology and structure of lead chrome (monoclinic vs. orthorhombic). The objective

was thus to conduct sucrose ester stabilized solution spray synthesis of PbCrO4 nanorods

with well defined morphology and crystal structure.

2.2 Raw materials and chemicals

The renewable feedstock for synthesis of sucrose ester by transesterification

reaction in present investigation includes coconut oil and the fatty acids derived from

them. Refined coconut oil was purchased from local market. Table 2.1 presents GC

analysis and other characteristics of coconut oil. The physical characteristics and sources

of fatty acids, sucrose and chemicals/ reagents used in present experimental investigations

are given in Table 2.2. All the chemicals and reagents were analytical grade and used as

received.

Table 2.1 % fatty acid composition and other characteristics of coconut oil

Oil

Fatty acid composition by GC Other

characteristics

C6 C8 C10 C12 C14 C16 C18 C18:1 C18:2 SV IV ρ

(200C)

Coconut 0.01 5.4 5.6 48.4 21.2 10 2.2 6 0 250 8 0.918

SV: saponification value, IV: iodine value, ρ: specific gravity at 200C

Table 2.2 Sources and characteristics of raw materials for the synthesis of sucrose

ester

Sr.

No.

Name of the

chemical

Mol.

Wt.

Density

(g/ml,

250C)

B.P.,

(0C,760

mm of Hg)

M.P.

(0C)

Minimum

assay (%) Supplier

1 Sucrose 342 0.889-

0.885

182/

20mmHg 20 99.00 S. D. Fine

2 Lauric acid 200.32 -- -- 44 99.00 S. D. Fine

3 Myristic acid 228.38 -- -- 52 98.00 S. D. Fine

4 Dimethyl formamide 73.10 0.95 153 -- 99.50 Merck

5 Lead nitrate 331.2 4.53 270 -- 95 S. D. Fine

6 Potassium chromate 194.19 2.73 968 1000 95 S. D. Fine


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2.3 Experimental methodology

2.3.1 Synthesis of coconut fatty acids methyl esters (FAME) and methyl laurate/

myristate

Preparation of coconut fatty acids

Refined coconut oil was saponified using 30% NaOH solution at 900C followed by

acidulation using conc. HCl. The resulting fatty acid layer, after separation, was washed

repeatedly with brine to eliminate traces of mineral acids and dried under IR lamp.

Preparation of fatty acid methyl esters (FAME)

The FAME were prepared through acid catalysed esterification of following fatty acids:

coconut, lauric and myristic. FAME were obtained by p-toluene sulphonic acid (PTSA)

(1%) catalysed esterification of fatty acids with methanol (20% molar excess) under reflux

conditions for 1 hr followed by distillation of excess methanol and byproduct water. The

esterification reaction was continued in presence of additional methanol to reduce acid

value (AV) below 0.5. The crude esters were purified by performing vacuum distillation

under reduced pressure of 10 mm Hg.

2.3.2 Synthesis of sucrose ester by transesterification of refined coconut oil, distilled

FAME/ methyl laurate/ myristate-sucrose

Six station reaction assembly (Carousel 6 plus model, Radleys Tech., US) equipped with

magnetic stirring system (RPM=236), refluxing condenser and electrical heating system

with energy regulator capable of maintaining reaction temperature within ±0.50C was used

to carry different transesterification runs based on variation of molar ratio of FAME to

sucrose (M=0.36 to 6)/ reaction period (t=30-150 min) under identical experimental

conditions (temperature (T), catalyst % and stirring rate). A mixture of sucrose powder,

dimethyl formamide (DMF), distilled FAME of coconut/ oil or methyl laurate/ myristate

(prepared as per procedure described under section 2.3.1) and K2CO3 catalyst was refluxed

at T=120-1400C for t=30-120 min (Fig. 2.1). The transesterification reaction was

monitored by determining the initial and final hydroxyl value (HV) and saponification

value (SV). For TLC analysis, the reaction mixture was dissolved in a mixture of

chloroform, methanol and water (volume ratio 1:1:0.2). The plate with spots was dipped in

a mixture of toluene, ethyl acetate, methanol and water (volume ratio 9:8:4.5:0.2).

Purification of sucrose ester was accomplished through neutralisation of catalyst

using oxalic/ lactic acid, precipitation of unreacted sucrose from the reaction mixture by

adding toluene, repeated brine washing for removal of water soluble impurities and

subsequent evaporation of solvent through use of rotary vacuum evaporator.


55

OH

OH

HO

O

HO

O

OH

OH

HO

HOO OH

OH

HO

O

HO

O

OH

OH

HO

RCOOO

+ RCOOMeK2CO3

DMF, 110-1500 C

MeOH+

Fig. 2.1 Base catalyzed transesterification of sugar polyols with FAME for synthesis

of sucrose ester

2.3.3 Sucrose ester stabilized impinging solution spray synthesis of lead chrome

nanorods

Lead chrome nanoparticles were synthesized by undertaking coprecipitation reaction (Fig.

2.2) in impinging solution spray reactor patented by Mishra and co-workers39

. The use of

this reactor, incorporating external mixing two fluid nozzle for atomization of precursor as

well as precipitant, has been reported for the synthesis of nano iron oxide and nano

prussian blue pigments40

. The atomised streams of precursor and precipitant, after exit

from spray device, were arranged to impinge or collide at the upper-middle part of the

reactor and mix with each other as flowing thin film zone. In present synthesis, coconut

fatty acid-sucrose ester (Batch SE3, refer Table 2.3 for further details) was used as

surfactant. Solutions of Pb(NO3)2 (50 ml of 0.1 M containing 0.2% sucrose ester wt/vol.)

and K2CrO4 (50 ml of 0.1 M containing 0.2% sucrose ester wt/vol.) were sprayed at

controlled flow rate (air feed pressure- 20 kgf/cm2) independently and simultaneously for

10 minutes at 300C through nozzles to impinge and form thin film flowing reaction zone

which subsequently was allowed to fall and mix in 1 lit of 0.2% sucrose ester aqueous

solution maintained at the reactor base. The sprays were carefully monitored for the

intimate contact of the reactants at the degree of atomization of 5-10μ. After completion of

the addition, the reaction mixture was kept under stirring for additional 30 minutes to

encourage the growth and stabilization of PbCrO4 nanocrystals. The resulting dilute

suspension was centrifuged in refrigerated centrifuge (Remi C- 30BL) run at 5000 rpm

and -80C for 15 minutes. The supernatant solution was decanted and the solid nanoproduct

was redispersed in deionised water. The process was repeated three times for the removal

of final traces of byproduct, unreacted materials and excess surfactant. Finally the

precipitates were carefully dried at 800C for five hours. Two additional batches were

conducted by loading sucrose ester at 0.5% and 1.0%.

Pb(NO3)2 + K2CrO4 PbCrO4 ↓ + 2KNO3

Fig. 2.2 Synthesis of lead chrome by double decomposition process


56

2.4 Characterization techniques

The physical and chemical characterizations of raw materials, intermediates and

sucrose esters as well as the monitoring of transesterification were conducted by using

various analytical methods. The functional group tracking, polymorphism, optical

properties etc. of the intermediates and the final target molecules were carried out by using

sophisticated instrumental techniques.

2.4.1 Analytical techniques

2.4.1.1 Acid value (AV)

For base catalysed transesterification of sucrose with fatty acid methyl ester (FAME)/ oil,

the ester must carry AV below 0.5. AV also provides information on development of free

acidity during transesterification. To determine the acid value, 1 g of oil/ fatty acid/

sucrose ester is dissolved in 25 ml methanol and titrated with (0.5 N for fatty acids/ 0.1or

0.05 N for oils) KOH (in MeOH) using phenolphthalein as indicator, until the faint pink

colour stays for 30 seconds. The AV is defined as the number of milligrams of KOH

required to neutralize free carboxylic groups in 1 g of reaction mixture (DIN EN ISO

3682) and is calculated using Eq. 2.1.

KOH56.1X N X volume of KOH solutionAcid Value = Eq. 2.1

Weight of sample in g

Where N is the exact normality of alc. KOH (in MeOH).

2.4.1.2 Hydroxyl value (HV)

The HV characterization was used to monitor the extent of transesterification (or degree of

substitution) of sucrose with FAME/ oil. It is determined by the titration of residual

unreacted acetic anhydride left from the reaction of the free OH groups of the sample with

an excess of acetic anhydride (DIN EN ISO 4629). About 1.5 g of the OH-containing

compound are weighted into a 250 ml Erlenmeyer flask and dissolved in 10 ml pyridine.

To this solution, 10 ml of a freshly prepared acetylation mixture (1:7 v/v ratio of acetic

anhydride and dry pyridine) is added and the mixture is refluxed for 60 min at 1100C. The

cooling of the reaction mixture was followed by addition of 10 ml distilled water through

condenser and refluxing for additional 15 min. The condenser and the walls of the flask

were rinsed using 25ml of n-butanol and the contents of the flask were titrated with 0.5/1N

KOH (in MeOH) using phenolphthalein as an indicator. Similarly, the blank value was

also determined (in absence of the sample) under identical conditions. The HV is defined

as the mg KOH equivalent to hydroxyl content of 1 g of the compound and is calculated

using Eq.2.2.


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KOH56.1X N X (Blank-Sample)Hydroxyl Value = + AV Eq. 2.2


Where N is the exact normality of the alco. KOH.

2.4.1.3 Iodine value (IV)

This method is used to determine the ethylenic unsaturation of oils. The reaction of iodine

monochloride with the double bond results in the addition of chlorine to the double bond

and the liberation of iodine (DIN 53241-1). Excess iodine monochloride is then reacted

with iodide to form also iodine, which is then back titrated with thiosulphate. The IV is

related to grams of iodine equivalent to the double bond content of 100 gm of sample and

calculated using Eq. 2.3.

2 2 3Na S O12.69 X N X (Blank-Sample)Iodine value = Eq. 2.3


Where N is the exact normality of thiosulphate solution.

2.4.1.4 Saponification value (SV)

The saponification value (SV) is the number of mg of potassium hydroxide required to

saponify 1 gm of sample. 1.5 to 2 gm of sample was weighed in 250 ml of flask. Excess

alcoholic potassium hydroxide, added for saponification of sample under reflux for 1 hr,

was back titrated with standard 0.5N hydrochloric acid using phenolphthalein indicator.

The SV was calculated using Eq. 2.4.

56.1X Blank-Sample X NSaponification value = Eq. 2.4


Where N is the exact normality of hydrochloric acid.

2.4.1.5 Surface tension of sucrose ester solution

Surface tension of sucrose ester solution in demineralised water was determined at

different dilutions by Stalagnometer drop counting method41

. Surface tension of precursor

and precipitant solutions, at different loadings of sucrose ester, used in lead chrome

synthesis, were also determined.

2.4.1.6 Density of nanopigments

The density of lead chrome nanoparticles, synthesised using sucrose polyester promoted

solution spray process, was determined by specific gravity bottle method using xylene as

solvent.

2.4.1.7 Determination of pH of precursor and precipitant solution

The pH of precursor and precipitant solutions were determined by using Milwaukee pH

Tester (pH-600 model).


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2.4.2 Instrumental analysis

2.4.2.1 FTIR spectroscopy

The functional group analysis of the sucrose ester and lead chrome was carried using

Fourier-Transformed Infrared (FTIR) spectroscopy. The FTIR spectra were recorded on a

Shimadzu FTIR-8400 equipped with KBr beam splitter. Diffuse reflectance system (DRS)

was used for powder samples and NaCl plate for liquid samples by thin film deposition

technique. A regular scanning range of 400-4000 cm-1

was used for 20 repeated scans at a

spectral resolution of 4 cm-1

. All the spectra were recorded and processed using IR

solution software.

2.4.2.2 Gas Chromatography (GC)

GC analysis was performed with Shimadzu GC 14 B. Sample analysis was carried out on

packed column- Restek RTX wax (2 m × 0.53 mm × 0.31 μm). Samples (5 μL) were

injected by a sampler injector at an oven temperature of 1800C. After an isothermal period

of 5 min at 1800C, the GC oven was heated at the rate of 2

0C min

-1 to 200

0C, and then at

70C min

-1 to 250

0C (hold for 2 min at 200, 230 and 250

0C for a total run time of

34.14 min). Flow rates of the gases were: Nitrogen- 2.0 ml/min, Hydrogen- 45.0 ml/min,

Air- 450.0 ml/min. The injector and detector temperatures were 2300C and 270

0C

respectively. The RTD data, obtained using FID detector and processed using GC solution

software, were used to obtain fatty acid composition of oils. FAME, monoglycerides,

diglycerides, and triglyceride standards were used for the purpose of calibration.

2.4.2.3 1HNMR spectroscopy

The 1HNMR spectra were recorded on Bruker Avance 400 spectrometer (Bruker,

Rheinstetten, Germany) operated at 400 MHz using CDCl3 as solvent.

2.4.2.4 Viscometric analysis

The viscosities of the sucrose ester solutions were determined using Brookfield Cone and

Plate Rheometer- R/S Plus (Spindle No.: C 25-02, Speed: 100 RPM, Time: 120 Sec,

Temperature: 280C, Point Reading: 10). To determine the viscosity, the solutions of

purified samples of sucrose esters were prepared in DMF at three concentrations: 9, 40 and

80%. Small amount of test sample (approximately 0.25 g) was filled into the cone plate of

the Rheometer. Test data was gathered automatically by the computer program.

2.4.2.5 X-Ray diffraction (XRD) spectrometer

Evaluation of the phase and morphology of nanoparticles was conducted using D-8

Avance XRD of Brucker, Germany at 40 kV and a current of 30 mA with CuKa radiation

(1.54060 -1.54439). The samples were placed on a sample holder made up of silicon wafer


59

and the measurements were taken continuously from 200 to 80

0 angles. The resultant

intensity data was processed by using in-built diffraction software Diffraceva to monitor

the peak position and its corresponding data was further matched with ICDD data.

2.4.2.6 Field Emission Scanning Electron Microscopy (FESEM) and Energy

Dispersive X-Ray (EDX) spectrometer

Size and morphological analysis of lead chrome nanoparticles were performed using high

vacuum cold Field Emission Scanning Electron Microscope (FESEM, Hitachi S-4800 II

Model) capable of providing accelerating voltage of 0.5 to 30 kV (variable at 100V/step)

and magnification of X20 to X800,000. The samples were prepared using carbon coating

attachment (Sputter Coater E-1010) and also characterized for surface elemental

composition using Bruker Energy Dispersive X-ray spectrometer (EDX, QUANTAX 200

model) with XFlash®

5030 detector for fast and high resolution real time spectrometry

(LN2-free detector with integrated Peltier cooler).

2.4.2.7 Colour matching spectrophotometer (Gretag Macbath, ColorEye XTS)

It was used to determine CIE (internationally agreed system of color specification) values

for lead chrome through comparison with corresponding reference standard of commercial

pigment in micron size using D65 illuminant. The assessment of color of nanopigment

was conducted to understand the change in hue (H), chroma (C), lightness (L), (a) (a+:

redness; a-: greenness), and (b) (b+: yellowness; b-: blueness) with a reduction in the

particle size from micron to nanodimensions.

2.4.2.8 Bomb colorimeter

It was used to determine calorific value of sucrose, coconut FAME and coconut fatty acid-

sucrose esters.

2.4.2.9 UV-Visible spectrophotometer

The UV- visible absorption characteristics of lead chrome nanopigment were recorded on

UV- Visible Spectrophotometer (Shimadzu UV-1800 Model) in the wavelength range

330–1100 nm using a 10mm cuvette. Samples were prepared as dispersion of 0.0025 g of

sample in 100 ml of deionised water obtained through ultrasonication.

2.5 Results and discussion

2.5.1 Synthesis of coconut fatty acid-sucrose esters

The K2CO3 catalysed transesterification process and the corresponding

characterization techniques for the synthesis of coconut fatty acid-sucrose esters have been

presented under sections 2.3.2 and 2.4, respectively. Table 2.3 reports the results of

experimental investigations of 24 runs of synthesis of sucrose esters based on variation of


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initial FAME/ sucrose molar ratio (M), holding period (t, min), reaction temperature (T,

0C), % potassium carbonate catalyst and raw materials for medium molecular weight fatty

esters (mixed methyl esters derived from coconut oil, methyl laurate, methyl myristate and

coconut oil). The objective in choosing different raw materials of medium molecular

weight fatty acids- mixed FAME derived from coconut oil, methyl laurate, methyl

myristate and coconut oil triglycerides was to understand the influence of molecular

weight and chain length on kinetics of transesterification, other parameters being

maintained constant. In all experiments, TLC analysis was conducted to confirm the

formation of sucrose ester.

The determination of the HV of the reaction mixture which represents sucrose

hydroxyl concentration available for the transesterification and SV of product sucrose

ester which is governed by extent of mono/ di/ tri/ poly transesterification formed the basis

of monitoring of the progress of the reaction. It was observed that the sucrose hydroxyl

concentration decreases immediately and rapidly during base catalyzed transesterification

of coconut oil/ methyl esters with sucrose. The HV, as shown in Table 2.3, exhibited

maximum drop within first 30 min (reaction period) followed by retardation in reaction

progress or sluggish reaction in terminal period. This represents the essential feature of

homogeneous kinetics for all 2nd

order reactions. In heterogeneous kinetics (for e.g.

synthesis in absence of DMF solvent), the experimental concentrations remain

approximately constant during the initial period (within 1st hour) and then gradually

decreases. The transesterification conversion XA was calculated as follows:

A

Final HV- Initial HVX =

Initial HV

The conversion versus reaction period data, corresponding to different reaction

temperatures and initial reactant molar ratios M, was used in the calculations of 2nd

order

rate constant as presented in Chapter 4. The magnitude of SV of product sucrose ester

increases with rise in extent of transesterification. Thus, SV of sucrose diester would be

higher than that of monoester, the triester will in turn carry SV higher than that of diester

and so on. The SV of particular product sucrose ester, carrying mixture of monoester,

diester and multiple esters, will be decided by its ester distribution. For example, a product

richer in monoester will have SV lower than that carrying diester as major component. In

general, with increase in FAME/ sucrose molar ratio M which permits more degree of

substitution, one observes drop in HV and rise in SV with holding period t.


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Although transesterification is not regioselective, by choosing the specific molar

ratio ‘M’, the reaction can be made selective as presented below. In all 24 runs, the

reactant molar ratios (M=NB0/NA0; B= FAME, A=sucrose) were chosen to provide diverse

transesterification opportunities for eight hydroxyl groups present in sucrose. Sucrose is

non-reducing disaccharide of unique structure containing nine chiral centers. The eight

hydroxyl groups (Fig. 2.1) include three primary hydroxyls (at carbons 6, 1’ and 6’) and

five secondary hydroxyls (at carbons 2, 3, 4, 3’, and 4’). The increase in M from 0.73 to

6.0 augments the opportunity for participation of more hydroxyls in transesterification and

thus the activity contribution of each hydroxyl towards average activity per hydroxyl

group is raised with increase in M. When fatty esters are supplied at less than the

stoichiometric proportions (M< 1, excess sucrose), there is increased chance of

substitution of the three primary hydroxyl groups of sucrose. On the other hand, as the

molar ratio M of FAME: sucrose increases (M> 1.5, excess FAME), the more highly

substituted derivatives (degree of substitution DS: 4-8) of sucrose are preferentially

synthesized. Hence the results and discussion on 24 runs is primarily divided for three

ranges of molar ratios: M< 1, M=1.5 and M=3/6.

I. M< 1: Experiments were conducted at following molar ratios: M= 0.364 for FAME (run

1 to 3), 0.73 for FAME (run 4 to 6) and 0.8 for coconut oil (run 23). Use of molar ratio M<

1 implies that FAME is available at quantity even less than that required for

transesterification of one out of 8 hydroxyl of sucrose.

At M< 1, the competition between different hydroxyls in attacking the carbonyl

group of fatty ester is created. Due to the abundant availability of hydroxyl group in

relation to the ester, the substitution of at the most one hydroxyl in majority of the sucrose

molecules would be anticipated. Thus, out of 8 hydroxyl groups in sucrose, the three

hydroxyl groups viz. 6 OH of glucose unit and 1’ OH and 6’ OH of fructose unit of

sucrose molecule, being primary, are favoured as the most reactive sites. Nevertheless, due

to steric hindrance, the 1’ OH position of fructose is substituted less readily. Similarly 6’

OH may be a relatively lower activity position. The fructose moiety of sucrose appears to

be less reactive and the reaction was rather selective on the glucose moiety. Thus, York

et. al.42

have reported that about 80% of the fatty acid of the monoester is found on the

glucose portion of the sucrose molecule, with the ratio of the substitution of the glucose to

fructose moiety being 4:1 and the reactivity order being 6 OH >6’ OH >>1’ OH. Thus

when one initiates synthesis by providing M=fatty ester/ sucrose molar ratio ≤ 1, the

mono-substitution in sucrose takes place preferentially at C-6 in the glucose unit.


62

In batch SE1, to keep excess sucrose well dispersed, higher amount of solvent

(62%) was used. Also lower catalyst dosing (1.1%) was used. The underlying assumption

was that the surplus hydroxyl group in excess sucrose may provide catalytic effect.

However this was not the case as evident from the rate calculations presented under

Chapter 4. It was associated with lowest specific reaction rate. In remaining batches

(excluding those corresponding to coconut oil), around 2.0% catalyst loading and 50%

solvent were utilized which provided appreciable reaction rates and sufficiently rapid drop

in HV. However the transesterification of coconut oil was slow under these conditions.

Hence the batches SE8 and SE9 were conducted at higher catalyst loading and reduced

solvent usage (i.e. higher reactant concentration). Even with use of high catalyst and

reactant concentration, the transesterification performance of coconut oil, as shown in

Table 2.3, was just comparable or slightly better in reference to that of batch SE1 (a batch

conducted at lowest catalyst loading of 1.1%).

A higher reaction temperature (1400C) accelerates transesterification and permits

additional drop in HV (e.g. run 1 vs run 3 for batch SE1, run 4 vs 6 for batch SE2).

However, the increase in reaction temperature from 120 to 1400C does not bring

appreciable rise in extent of transesterification. Apparently this appears to contradict with

Arrhenius rule which states that with rise in reaction temperature by 100C, the reaction

speeds up or the rate almost doubles up. Transesterification, in present case, includes

series reactions: mono-transesterification⟶ di-transesterification⟶tri-transesterification

and so on. The rate of mono-transesterification would be higher than that of di-

transesterification which in turn proceeds faster than tri-transesterification. Moreover the

transesterification is an equilibrium reaction. Increase in temperature also brings rise in

reverse reaction rate. The rate constant calculations presented under Chapter 4 also

support these observations.

II M=1.5: Experiments were conducted for FAME (run 7-9), methyl laurate (run 15-18),

methyl myristate (run 19-22) and coconut oil (run 24). At M=1.5, fatty esters are provided

at 50% excess for reaction with one hydroxyl of sucrose.

With increase in molar ratio of fatty acid: sucrose above 1, the yield of sucrose di-

and tri-esters is expected to increase along with that of monoesters i.e. with more

availability of fatty esters (M just > 1), besides 6 OH of glucose unit, 1’ OH and 6’ OH of

fructose unit would also be substituted to yield mono, di- and tri- esters as presented by

Chung et al43

and Queneau et al44

. The further drop in HV and increase in SV for batch


63

Table 2.3 Base catalysed synthesis of coconut fatty acid-sucrose ester

Batch

code

Run

No.

Reaction Parameters Characteristics Fractional

conversion

XA

CA0,

gmole/

lit

M Cat % (by wt on

A & B)

%

DMF

T, 0C

t,

min HV SV

Synthesis of sucrose ester from FAME and sucrose

SE1

1.86

0.364

1.1 62.1

0 1347.5 -- --

1 120

30 439.6 63.7 0.674

2 120 203.6 76.7 0.849

3 140 30 401.3 -- 0.702

SE2

1.155 0.73 2.0 48.1

0 848.4 -- --

4 120

30 319.1 -- 0.624

5 120 135.7 89.5 0.840

6 140 30 304.9 -- 0.641

SE3

0.816 1.5 2.1 49.1

0 682.6 -- --

7

120

30 379.8 -- 0.444

8 60 145.9 94.3 0.786

9 120 102.7 106.0 0.8495

SE4

0.518 3.0 2.1 49.0

0 458.6 -- --

10 120 30 266.6 119.9 0.419

11 140

30 233.3 -- 0.491

12 60 133.6 -- 0.709

SE5

0.298 6.0 1.9 48.6

0 277.1 -- --

13 120

30 147.7 -- 0.467

14 120 18.3 205.8 0.934

Synthesis of sucrose ester from methyl laurate and sucrose

SE6

0.816 1.5 2.0 50.0

0 660.4 --

15

120

60 170.3 0.742

16 90 120.8 0.817

17 120 102.1 0.845

18 150 98.7 0.851

Synthesis of sucrose ester from methyl myristate and sucrose

SE7

0.816 1.5 2.0 50.0

0 642.2 --

19

120

60 210.3 0.673

20 90 140.3 0.782

21 120 112.3 0.825

22 150 105.2 0.836

Synthesis of sucrose ester from coconut oil and sucrose

SE8

0.7 0.8 3.0 32.0 0 554.8 --

23 120 120 306.8 0.447

SE9

0.58 1.58 4.0 38.4 0 503.0 --

24 120 120 358.4 0.288


64

SE3 (HV: 102.7, SV: 106.0, run 9) in comparison to those for SE2 (HV: 135.7, SV: 89.5,

run 5) and SE1 (HV: 203.6, SV: 76.7, run 2) provides the confirmation of additional

transesterification. The SV of batch SE3 is close to the theoretical SV of pure sucrose

monolaurate-103. Thus transesterification may lead to the formation of at least sucrose

monoesters. Diesters may be present partly due to the substitution of second hydroxyl by

surplus fatty ester and partly due to the disproportionation of monoester into saccharose

and dieseters. When one compares the extent of HV drop for run 7, 8 and 9 for FAME-

sucrose esters, the reaction was observed to retard after 1 hr. Similar retardations were

observed for batch SE6 and SE7.

The comparison between FAME (batch SE3, run 9), methyl laurate (batch SE6, run

17), methyl myristate (batch SE7, run 21) and coconut oil (batch SE9, run 24) based on

HV drop/ fraction conversion XA indicates the influence of molecular weight and chain

length on extent of transesterification. Due to lower molecular weight (C12, MW 214),

methyl laurate transesterification conversion was higher over those of methyl myristate

(C14, MW 242) and coconut oil (triglyceride, MW 673.2) transesterification for identical

reaction conditions. Coconut FAME carries mixture of fatty acids ranging from C6 to C18:

1, the principle fatty acids being C12 (lauric) and C14 (myristic) (refer Table 2.1 for fatty

acid composition). The presence of low molecular fatty acids (C6, C8 and C10) in Coconut

FAME apparently provided HV drop higher over that for methyl myristate and

comparable with that for methyl laurate. Under similar conditions, coconut oil, being

triester, presents slowest rate of transesterification.

Sucrose ester product of run 9 of batch SE3 was chosen for evaluation of surface

activity (section 2.5.3) and as surfactant for stabilized solution spray synthesis of lead

chrome nanorods (section 2.5.4).

III. M=3/6: Run 10-12 (batch SE4) and run 12-14 (batch SE4) were conducted for higher

molar usage of FAME.

At M=3/6, fatty esters were provided in sufficient excess to permit the

transesterification of more than one hydroxyl present in sucrose. Under these conditions,

the base catalysed transesterification may not allow regioselectivity and the degree of

hydroxyl substitution on the sugar is uncontrolled. The product sucrose ester would then

carry mixtures of mono-, di- and even higher esters. The lowest hydroxyl value (18.3) and

highest SV (205.8) observed in run 14 of SE5 batch confirms the formation of

polysubstituted sucrose ester which is normally used as a noncaloric substitute for fat.


65

Fig. 2.3 presents FTIR spectra of sucrose, FAME, and sucrose ester derived from

mixed FAME (SE1-SE5). In the FTIR spectra, the characteristic functional group

absorption bands to look for were: 3420-3500 cm-1

(O-H stretch of free hydroxyl in

sucrose), 1740-1750 cm-1

(ester C=O), 1056, 1107 cm-1

(C-O stretch of C-O-C), 995 cm-1

(glycosidic bond stretch of sucrose) and 2847-2860, 2904-2945, 1460-1470 cm-1

(C-H

stretch in CH3 and/or CH2). Apparently, the bands at 1747 and 995 cm-1

showed that the

products were sucrose ester. The reduction in intensity of hydroxyl band and formation of

strong bands corresponding to ester carbonyl and C-H stretch in CH3 and/or CH2 (1700-

1750 cm-1

) were noticed in FTIR spectra of products from different run. The spectrum of

polysubstituted sucrose ester (SE5) exhibited maximum carbonyl band intensity in

comparison to those in spectra of SE1 to SE4.

Fig. 2.3 FTIR spectra of sucrose, FAME and sucrose ester (SE1-SE5 batches)


66

2.5.2 Rheological characterization of coconut fatty acid-sucrose ester

Since there are eight hydroxyl groups on sucrose, with increase in M (e.g. M=3/6),

transesterification yielded numerous sucrose esters of higher molecular weights. The

viscosity of sucrose esters could be increased with multiple substitutions/ rise in molecular

weights. Thus, the sucrose to FAME molar ratios (M) varying between 0.364 and 6, which

control the degree of substitution implicitly, could be correlated to the viscosity of the

mixture. Hence the objective was to investigate the structure-viscosity (η) relationships for

sucrose esters derived from mixed FAME and coconut oil. In general, viscosity

measurements were carried out in three concentration regions: dilute (9%), semi-dilute

(40%), and concentrated (80%) regions. While the viscosity of sucrose esters in dilute

regime is same as that of DMF (< 5-6 mPa.s) and doesn’t differ much with change in

concentration, the viscosity measurements in semi-dilute and concentrated region

highlight the specific rheological behaviour of sucrose ester solutions. The Cone and Plate

viscosities (280C) of sucrose esters in DMF, recorded at two concentrations, 40 % and

80%, have been reported in Table 2.4 and plotted in Fig. 2.4 as a function of degree of

substitution through initial molar ratio.

Table 2.4 Cone and Plate viscosity of coconut fatty acid-sucrose ester solutions

as function of concentrations and initial molar ratio

While the rise in viscosity in mPa.s of sucrose ester solutions from dilute (5-6) to

semi-dilute range (316-438) was very high and that from semi-dilute to high concentration

range (347-452) was marginal. The explanations for this unusual rheological behaviour

could be furnished on the basis of free volume theory and molecular interactions as given

below.

Batch

Code Run No. M

Viscosity (mPa.s) of sucrose ester

solutions in DMF, 280C

@40% by wt @80% by wt

SE1 02 0.364 367.8 388.4

SE2 05 0.73 406.2 405.7

SE3 08 1.5 316.7 369.1

SE4 12 3.0 331.6 -

SE5 14 6.0 438.7 452.0

SE8 23 0.8 - 406.0

SE9 24 1.58 317.3 347.3


67

The solute molecule (sucrose ester), in present case, is much greater in size than

the solvent (DMF) molecule, hence the relation between the viscosity of the sucrose ester

solution and the concentration of sucrose ester cannot be expressed as some sort of

mixture law, using mole or volume fractions of the components of the mixture. Rather it

could be derived only from depicting the sucrose ester molecules as loose, randomly

arranged coils and then considering the balance of the three types of intermolecular forces,

i.e. those between the sucrose ester solute molecules, between the DMF solvent molecules,

and between the sucrose ester solute and the DMF solvent molecules. Since DMF is a

thermodynamically ‘good’ solvent for the sucrose ester, solute-solvent intermolecular

forces are strongest allowing DMF molecules to penetrate the sucrose ester coils. In the

dilute region (around 10%), the sucrose ester coils and their associated solvent molecules

are widely isolated from each other and not able to interact hydrodynamically. Due to the

numerous presences, DMF-DMF intermolecular forces are also intense. Consequently, the

viscosity of the dilute sucrose ester solution is similar to that of DMF. In semi-dilute

region (40%), due to the increased concentration, the sucrose ester coils are more closely

spaced and the coils now interact with each other through hydrogen bonding between

unsubstituted hydroxyl groups and Vander Wall forces between the coils. In fact, the coils

compete for the space due to reduction in free volume. DMF, due to amide group, is both a

hydrogen-bond donor and acceptor solvent. Possibly, DMF can bridge sucrose ester

molecules by functioning as a hydrogen-bond donor with one sucrose ester molecule and a

hydrogen-bond acceptor with the other; such bridging would counteract the effectiveness

of viscosity reduction by substantial solvent presence (60%) in semi-dilute region. Hence

in the semi-dilute region, the viscosity starts to change more rapidly with concentration. In

the concentrated region (around 80%), further contraction of effective volume is obtained

by overlap of the coil volumes. This results in marginal increase in viscosity with

concentration. It should be emphasized that the coils cannot be thought of as discrete

entities with definite boundaries, unlike dispersion or emulsion particles. The coil volume

has dimensions determined by the statistical average positions of the individual sucrose

ester molecular segments in space, averaged over a sufficient length of time. This explains

why sucrose ester coil volumes can overlap or interpenetrate in the concentrated region,

although they have apparently reached their maximum dimensions at a lower

concentration (semi-dilute range). The range of each regime (dilute/ semi-dilute/

concentrated) may depend on a number of factors e.g. the molecular weight and molecular


68

weight distribution of sucrose ester/ degree of substitution and distributional individual

coil dimensions which requires additional investigations.

The variations in viscosity (η) of sucrose ester solutions as a function of initial

reactant molar ratio (M) (in fact as indirect function of the degree of substitution) are

plotted in Fig. 2.4 at two different concentrations and were found to be governed by

following relationship.

η = 11.73 M2 - 64.83 M + 406.7----@40% conc. Eq. 2.5

η = 10.08 M2 - 56.49 M + 427.3 ----@80% conc. Eq. 2.6

Fig. 2.4 Viscosity of coconut fatty acid-sucrose ester solutions at different

concentrations as indirect function of initial reactant molar ratio

The viscosity curve, for both concentrations, passed through maximum, then

attained minimum and thereafter exhibited rising trend in relation to different initial

reactant molar ratios. Thus as one moves from sucrose ester obtained at M 0.364, the

viscosity of solutions passes through the maxima at around M 0.8 sucrose ester

corresponding to precisely monoester and highest hydrogen bond density due to the

interaction between remaining seven hydroxyl groups promoted by DMF. For M< 1, the

FAME is available at quantity even less than that required for transesterification of one out

of 8 hydroxyl of sucrose and the transesterification is highly selective. The minimum was

observed for sucrose ester obtained using initial molar ratio of 1.5. For M> 1, SV

(saponification value) of this ester is close to theoretical SV of monoester. However,

transesterification in all probability yields mixture of monoesters and higher esters. Hence

diversity of esters leads to weak sucrose ester molecular interactions and this may be


69

responsible for drop in viscosity. It should be noted that for similar degree of substitution

(i.e. for similar initial molar ratios of M=1.5), sucrose esters obtained from coconut oil and

coconut FAME will have same viscosity (Table 2.4).

At higher molar ratios (M=3/6), the rise in degree of substitution gives increase in

molecular weight as indicated by increase in SV. Hence the viscosity curve now exhibits

rising trend as a square function of M (Eq. 2.5 and 2.6). Thus for sucrose esters obtained at

M=3/6, the viscosity is directly related to the degree of substitution and with due

calibrations, former could be taken as direct measure of the later.

Analysis of sucrose polyester as low calorie substitute for dietary fat

Coconut fatty acid-sucrose esters obtained at higher M are polyesters and possess

flow characteristics (viscosity) similar to that of vegetable oil (Table 2.4). At moderate to

high degree of substitution, these compounds are lipophilic, nondigestible and

nonabsorbable molecules with physical (e.g. viscosity, density) and chemical properties

(eg. SV) similar to those of natural fats and oils. The calorific values of these polyesters

(SE5 and SE12), determined using Bomb Calorimeter, were found to be in the range of 4-

7 kcal/g. Thus, they can be used as low calorie substitute for dietary fat. Recently, the food

industry has focused attention on the possible use of sucrose polyesters as fat substitutes in

formulating food products. It has been approved by the FDA as a food additive used in

preparing low-fat deep-frying foods such as savoury snacks. These can be used to reduce

the total fat content of certain food products such as salad dressings, ice cream-type

products, and reduced-fat ground beef products. In addition, these products often improve

the texture of the finished food because they contribute to thickening or bulking and

because they maintain the moisture content. When mixed with gums and water or with

emulsifiers, some sucrose-based fat substitutes add flavour to a variety of foods and

improve the overall quality of the product.

2.5.3 Surfactant properties of coconut fatty acid-sucrose esters

The moderate solubility in water (neither high nor low), amphiphilic structure and

capability of major reduction in surface tension of aqueous medium were the basis for

selection of particular batch of coconut fatty acid-sucrose ester as surfactant in impinging

solution spray synthesis of nanopigments. Batch SE4 ester (M=3, Table 2.3, Run 12) was

poorly water dispersible while that from batch SE5 (M=6, Table 2.3, Run 14) was almost

insoluble. Sucrose ester with three or lesser fatty acids is suitable as surfactants. Thus

higher degree of substitution and water insolubility/ non-dispersibility (higher

hydrophobicity) of batch SE4 and SE5 rules out their selection as surfactant for


70

nanopigment synthesis. On the other hand, sucrose esters from batch SE1 (M=0.364,

Table 2.3, Run 2) and batch SE2 (M=0.73, Table 2.3, Run 5) were highly water soluble

(highly hydrophilic) and lacking in amphiphilic behaviour. In comparison to these sucrose

esters, batch SE3 ester (M=1.5, Table 2.3, Run 9) provided clear aqueous solutions. The

surface tensions of aqueous solution of SE3 sucrose ester, as presented in Table 2.6, were

found to decrease from 71.4 to 18.3 dynes/cm, with increased loading of sucrose ester.

The reduction in surface tension with increase in concentration of SE3 sucrose ester

confirms their tensio-active properties. Hence in experimental investigations on nano lead

chrome synthesis, coconut fatty acid-sucrose ester of batch SE3 was chosen as surfactant.

Table 2.5 Surface tension of water as function of concentration of coconut fatty acid

sucrose ester

Conc. of SE3 sucrose

ester, g/lit.

Surface Tension,

dyne/cm

Adsorption

density

(gmol/cm2)

Area per

molecule (02A )

0.00012 56.35 1.14E-10 145.18

0.006 45.08 2.12E-10 78.19

0.02 38.64 5.02E-10 33.05

0.1 18.27 Attainment of CMC

Adsorption is an entropically driven process by which molecules diffuse

preferentially from a bulk phase to an interface. Due to the affinity that a surfactant

molecule encounters towards both polar and non-polar phases, thermodynamic stability

(i.e. minimum in free energy or maximum in entropy of the system) occurs when these

surfactants are adsorbed at a polar/ non-polar (e.g. air/ water or electrolyte solution/

nanoparticle surface) interface. The difference between surfactant concentration in the

bulk and that at the interface is the surface excess concentration (Γ1) or the adsorption

density at the surface (in units of moles per unit area). It was evaluated through its

relationship to surface tension (γS) and surfactant concentration (c) as governed by the

Gibbs Adsorption Equation45

(Eq. 2.7). The packing area at the interface or the area per

molecule of adsorbed surfactant46

(the surfactant head group area) was determined using

Eq. 2.8.

11

1 1

1

ln

S S

T T

c

RT c RT c

Eq. 2.7

Area per molecule 1

1620 10

ANA Eq. 2.8

where NA is the Avogadro’s constant (6.023 X 1023

molecules per mole).


71

Fig. 2.5 a presents the variation in surface tension S of demineralised water with

ln (natural logarithm) of sucrose ester concentration. At any point on the curve, the values

of d S /d 1ln c give the corresponding values of the surfactant adsorption density Γ1 and are

reported in Table 2.5. Surface tension of the solution rapidly decreases with surfactant

concentration and takes on constant value at critical micelle concentration (CMC): 0.1

g/lit. Correspondingly the surface concentration also increases and reaches a maximum

level at a specific bulk concentration, somewhere in between 0.02 and 0.1 g/lit i.e. just

below the CMC value. The slope d S /d 1ln c was linear in this range, which on the basis of

Gibbs equation implied that there was no further increase in the adsorption density with

increase in bulk concentration. At maximum Γ1=5.02 X 10-10

gmol/cm2

, the surface is now

fully packed with surfactant molecules, although σ still continues to fall. At CMC, a sharp

transition, as shown in Fig. 2.5 a, occurs which apparently corresponds to zero adsorption

(i.e. dγS/dlnc1= 0).

The aqueous phase synthesis of nano lead chrome uses inorganic salts as precursor

(Pb(NO3)2) and precipitant (K2CrO4). The presence of these inorganic electrolytes

modifies the solubility, hydrophobicity and ionic strength (only applicable for the ionic

surfactant) characteristics of surfactant. Thus the standard surface excess concentration

and CMC data reported in Table 2.5 or in literature for sucrose ester cannot be utilised to

understand their influence on morphosyhthesis and size stabilisation of nanoparticles.

Hence the experimental measurements of surface tensions of sucrose ester containing

Pb(NO3)2 and K2CrO4 solutions over a range of concentrations used for the nanoparticle

synthesis was conducted and have been reported in Table 2.6 and Table 2.7, respectively.

The corresponding surface excess concentrations and CMC data were determined in

reference to plots in Fig. 2.5 b and c.

Table 2.6 Surface tension and other characteristics of 0.1M solution of lead nitrate as

function of sucrose ester (SE3) concentration

Concentration of

SE3 sucrose ester,

g/lit.

Surface tension,

dyne/cm

Adsorption

density

(gmol/cm2)

Area per

molecule

(02A )

0 85.50 -- --

2 57.35 6.00E-10 27.67

5 43.50 5.61E-10 29.58

10 33.70 Attainment of CMC


72

Table 2.7 Surface tension and other characteristics of 0.1M solution of potassium

chromate as function of sucrose ester (SE3) concentration

Concentration of

SE3 sucrose ester,

g/lit.

Surface tension,

dyne/cm

Adsorption

density

(gmol/cm2)

Area per

molecule

( 02A )

0 77.50 -- --

2 51.64 2.83763E-10 58.51

5 45.09 4.36965E-10 38.00

10 37.46 Attainment of CMC

The progressive increase of the SE3 surfactant concentration from 0 to 1% (0-10

g/lit) were accompanied by a parallel marked decrease of the surface tension of lead

nitrate and potassium chromate solutions from 85.5 to 33.7 mN/m and from 77.1 to 37.46

mN/m, respectively. The results indicate requirement of ten times rise in sucrose ester

concentration to maintain surface tension reduction profile and surface excess

concentration similar to that in demineralised water. Thus the presence of electrolytes

necessitates addition of higher surfactant concentrations. Between the two electrolytes-

lead nitrate and potassium chromate, one observes formation of less sucrose ester head

group area on potassium chromate. The concentration regions corresponding to the

attainment of linearity of the slope dγS/dlnc1 were also found to be different, between 2-5

g/lit for lead nitrate and between 5-10 g/lit for potassium chromate. The reason for this

dissimilar behavior lies in different pH profile of the two electrolyte solutions (Table 2.8).

Former solution is moderately acidic while the later solution is weakly basic. The

effectiveness of sucrose ester surfactant, which carries ester as backbone functionality,

declines in alkaline medium.

a


73

b

c

Fig. 2.5 Variation in surface tension γS with sucrose ester (ln) concentration in

a) water, b) 0.1 M lead nitrate solution and c) 0.1 M potassium chrome

solution

2.5.4 Use of coconut fatty acid-sucrose ester as surfactant in stabilized synthesis of

lead chrome nanopigment by impinging solution spray process

The procedure for SE3 sucrose ester assisted impinging solution spray synthesis of

nanopigment has been presented in section 2.3.3. The influence of variation of SE3

sucrose ester surfactant concentration from 0 to 1% (w/v) to facilitate controlled

polymorph selective synthesis of nano lead chrome produced by reactive crystallization

between equimolar solutions (50 ml of 0.1 M each) of Pb(NO3)2 and K2CrO4 solutions

were investigated by FTIR (Fig. 2.6), XRD (Fig. 2.7), FESEM (Fig. 2.8) and UV- Visible


74

(Fig. 2.9) spectrophotometric analysis of product nanoparticles. All other parameters such

as air pressure, orifice size and flow rate were selected to provide finer atomization

(droplet size of 9-10 μm determined on the basis of viscosity, surface tension and density

of solutions) and adequate thin film contact between reactants. In general, the yields of

products were observed to be in the range of 84-87%. Table 2.8 reports the consolidated

results of these experimental investigations.

2.5.4.1 Interpretation of FTIR spectra

The scrutiny of IR spectra of sucrose ester mediated nanoproducts of impinging

solution spray process (Fig. 2.6 b-d) in reference to blank run spectrum (Fig. 2.6 a- bulk

product, no surfactant) and spectrum of pure sucrose ester SE3 (Fig. 2.3) confirms that the

bands observed in IR spectra of Fig. 2.6 b-d, besides those ascribed to lead chrome, are

primarily due to the characteristic vibrations of nonpolar part/ alkyl chain in sucrose ester

adsorbed on nano lead chrome surface. For example, the C-H bands at 2856-54 and 2926-

2924 cm-1

observed in spectra were ascribed to the symmetric and asymmetric vibrations

of -CH2- and -CH3 groups. The intensities of these peaks were increased with rise in

surfactant concentration. This also corresponds to the small drop in density of product

from 7.8 to 7.5 g/cc (Table 2.8). Thus FTIR analysis confirms the presence of surfactant

in the form of adsorbed layer on lead chrome surface. Since sucrose ester is non-ionic

surfactant, the size stabilisation would be based on entropic stabilizing interactions

between lead chrome surface and surfactant nonpolar chains.

One observes the the presence of minor peaks at 1340-1383 and 1627-1688 cm-1

in

FTIR spectrum of the bulk lead chrome (Fig. 2.6 a) as well as in the spectra of lead

chrome obtained by employing different surfactant concentrations (Fig. 2.6 b-d).

However, the major absorption peaks observed due to the characteristic vibrations of

PbCrO4 noticeably differ in intensity. The main peak at 852 cm-1

for bulk lead chrome

particles splits with formation of shoulder at 823 cm-1

. On the other hand, in presence of

surfactant (Fig. 2.6 b-d), same peak broadens with increase in intensity with rise in

surfactant concentration. At 1% sucrose ester usage, the band split was completely

eliminated. The change in lead chrome peak position in spectra for sucrose ester mediated

products (Fig. 2.6 b-d) in comparison to that for bulk product imply the possibility of

different crystal orientation of products obtained by surfactant mediated synthesis. Thus

the FTIR analysis reveals some insight into the likely role of sucrose ester surfactant in the

facilitation of polymorph selective synthesis of lead chrome nanopigments.


75

Table 2.8 Coconut fatty acid-sucrose ester assisted synthesis of PbCrO4 nanoparticles by impinging solution spray process

Precursor

Pb(NO3)2

Reaction parameters for all

batches with respect to precursor

Feed pressure = 20 kg/cm2; Spray nozzle dimension= 1mm; Molarity of

solution= 0.1 M; addition time= 10 min; Flow rate= 5 ml/min

Conc. of SE3, gm/100 ml 0.2 0.5 1.0

Density, g/cc 1.0289 1.0322 1.0338

Surface tension, dyne/cm

(85.5(@0% SE3) 57.35 43.50 33.70

pH (3.84@0% SE3) 4.8 4.1 3.9

Precipitant

K2CrO4

Reaction parameters for all

batches with respect to precipitant

Feed pressure = 20 kg/cm2; Spray nozzle dimension= 0.3mm; Molarity of

solution= 0.1 M; addition time= 10 min; Flow rate= 5 ml/min

Conc. of SE3, g/100 ml 0.2 0.5 1.0

Density, g/cc 1.0162 1.0178 1.0191

Surface tension, dyne/cm

(77.5@0% SE3) 51.64 45.09 37.46

pH(8.7@0% SE3) 8.5 8.7 8.4

Reaction

chamber

Conc. of SE3, gm/100ml 0.2 0.5 1.0

parameters for all batches pH =7.3, Reaction temp.=300C, Reaction time =30 min

Yield (%) 84.1 87.3 86.7

Particle size by FESEM (Length x Dia in nm) 701-1100 X 44.4-64.0 576-807 x 35.2-59.6 373-869 x 25.1-51.5

Morphology by FESEM Well separated

nanorods

Well separated

nanorods

Well separated nanorods

FTIR Presence of sucrose

ester as adsorbed

layer

Presence of sucrose

ester as adsorbed layer

Presence of sucrose ester

as adsorbed layer

Polymorph analysis by XRD Monoclinic Monoclinic Monoclinic & uniform

phase

λmax in nm and UV absorbance 506, 427 & 0.102 504, 427 & 0.084 507, 427 & 0.074

Density of nanolead chrome, g/cc 7.80 7.79 7.50


76

Fig. 2.6 FTIR Spectra of PbCrO4 synthesized by (a) bulk addition, no surfactant and

impinging solution spray process at(b) 0.2%, (c) 0.5% and (d) 1% sucrose

ester additions.

2.5.4.2 Interpretation of XRD spectra

Fig. 2.7A presents the XRD spectrum of yellow PbCrO4 precipitates, obtained in

the blank bulk coprecipitation run (without surfactant). Processing of resultant intensity

data using in-built diffraction software Diffraceva and corresponding data matching using

ICDD data showed clearly that the obtained precipitates were orthorhombic polymorph

with lattice parameters a, b, and c as given under Table 2.9. The XRD spectrum presents

(210), (102), (211), (221), (122) and (303) peaks corresponding to 2θ of 25.9900, 27.021

0,

28.8920, 40.395

0, 42.360

0 and 49.623

0 respectively, which represent orthorhombic crystal

orientation. Thus bulk mixing in absence of surfactant facilitates the formation of the

metastable phase of greenish-yellow orthorhombic polymorph at room temperature. On


77

the other hand, Fig. 2.7 B-D exhibited intense Bragg reflections characteristic of

monoclinic polymorph of nano PbCrO4 particles synthesized by impinging solution spray

process in stabilised presence of sucrose ester with (101), (200), (012), (-131) and (132)

peaks corresponding to 2θ of 20.300, 25.7, 29.6

0, 40.0

0, and 49.3

0, respectively. The

monoclinic polymorph formed at all surfactant concentrations have identical lattice

Fig. 2.7 XRD patterns of PbCrO4 synthesized by (A) bulk mixing, no surfactant and

impinging solution spray process at (B) 0.2%, (C) 0.5% and (D) 1% sucrose

ester addition.


78

parameters- a, b, c and angle β (Table 2.9). The results, thus, indicated the influence of

sucrose ester in the formation of specific polymorph. The long alkyls chains of coconut

fatty acid esters of sucrose, whose presence on lead chrome surface has already been

proved on the basis of IR spectra (Fig. 2.6 b-d, subsection 2.5.4.1), align along the major

axis of needle shaped crystals and caused their stabilization. One observes some less

pronounced peaks (110), and (320), (011) and (002) formed at 0.2 and 0.5% sucrose ester

which were eliminated at higher sucrose ester concentration of 1%. Thus increased

surfactant concentration favoured the enhancement of monodispersity of the monoclinic

peaks and in other words, the formation of uniform phase of lead chrome crystals.

The role of sucrose ester in influencing phase and polymorph selective synthesis of lead

chrome was, thus, well justified on the basis of XRD observations. The observations

related to the surfactant mediated size stabilization as well as the confirmation of

monoclinic polymorph were carried on the basis of FESEM morphological analysis of

nano lead chrome crystals.

Table 2.9 XRD data of synthesized lead chrome pigments

XRD data (PDF 01-074-0812) of lead chrome, bulk mixing, no surfactant

Lattice parameters Orthorhombic, a= 8.67000, b= 5.59000, c= 7.13000, a/b= 1.55098, c/b= 1.27549

2θ 20.471 25.990 27.021 28.892 40.395 42.360 49.623

hkl 200 210 102 211 221 122 303

XRD data (PDF 01-073-2059) of lead chrome nanopigments, 0.2% sucrose ester and solution spray

process

Lattice parameters Monoclinic, a=7.12000, b=7.43000, c=6.79000, a/b=0.95828 c/b=0.91386,

β=102.420

2θ 17.581 20.397 25.709 27.331 29.615 39.875 40.105 45.435 46.255 49.458

hkl 110 101 200 021 012 221 -131 103 320 132


process


β=102.420

2θ 17.925 20.357 25.755 26.938 27.189,

27.252

29.620 40.059 45.900 49.359

hkl 011 101 200 002 120 012 -131 -132 132


process


β=102.420

2θ 20.302 25.667 27.306 29.496 40.000 45.885 49.332

hkl 101 200 120 012 -131 -132 132


79

2.5.4.3 Analysis of FESEM micrograph

The FESEM images of PbCrO4 nanopigments obtained by impinging solution

spray process at varied loading of sucrose ester of 0.2 to 1.0 % have been presented in Fig.

2.8 A-C. The FESEM micrographs, obtained at 1 μm resolution, pointed to the success of

impinging solution spray process in promoting the formation of lead chrome crystals in

nano range. It highlights the impact of superior molecular contacting between atomized

precursor and precipitant sprays (9 to 10 μm droplets as stated in the beginning of section

2.5.4) in impinging thin film and subsequent segregation of product through instantaneous

dilution at reactor base for elimination of dominant aging processes such as aggregation

and coarsening.

These micrographs displayed presence of rod like morphology with dissimilar

dimensions in nano range corresponding to different concentrations of sucrose ester. Thus,

FESEM studies provided the confirmation of XRD results of formation of monoclinic

(needle/rod) polymorph. Secondly, for same impinging solution spray process, the

micrographs advocated the influence of surfactant concentration (in fact of peculiar

surfactant assemblies- multilayered/ micellar structure as discussed under subsection

2.5.4.5) on size stabilization of lead chrome nano crystals. It could be seen from Fig. 2.8 A

that the well separated, elongated rods like structure of PbCrO4, produced at 0.2 % sucrose

ester, were approximately 700-1100 nm in length and 44-64 nm in diameter. One observes

reduction in rod diameter with increased surfactant concentration. At sucrose ester

addition corresponding to 1%, major size reduction to the tune of 25-51 nm was achieved

(Fig. 2.8 C). Thus 1% concentration of sucrose ester was found to be more effective for

the stabilization of particle size in nano range.

The FESEM images, viewed at different resolutions (300 nm to 2 µm), also

presented a distinct confirmation of surfactant mediated polymorphism. The images

showed no evidence of spherical particles at all resolutions. These results, which are in

agreement with XRD analysis, substantiate the polymorph selective synthesis (nanorod

formation) promoted by sucrose ester.

2.5.4.4 Interpretation of UV-Visible spectra

Fig. 2.9 A presents UV-Visible absorption spectrum of lead chrome crystals

obtained by blank run (bulk mixing, no surfactant) while Fig. 2.9 B-D exhibit spectra of

lead chrome nanorods obtained by impinging solution spray process in sucrose ester (0.2-

1%) mediated runs. The corresponding wavelength-absorption data has been presented in

Table 2.10.


80

Fig. 2.8 FESEM images of PbCrO4 nanorods synthesized by solution spray process at

(A) 0.2%, (B) 0.5% and (C) 1% sucrose ester.


81

Fig. 2.9 UV-Visible spectra of PbCrO4 nanoparticles synthesized by (A) bulk mixing,

no surfactant and impinging solution spray process at (B) 0.2%, (C) 0.5%

and (D) 1% sucrose ester addition.

Following salient features were noticed on the basis of evaluation of these spectra:

i. The ordinary lead chrome bulk material (pigment grade) has characteristic λmax of 436

and 511 nm. The spectrum exhibited absorption pattern typical of lead chrome bulk

pigments/ microparticles.


82

ii. Sucrose ester mediated lead chrome nanorods exhibited less absorption (almost 1/3)

than that by bulk particles and the nature of the absorption spectra were very different. The

absorption is blue shifted (436⟶427 nm). The size reduction in nano range is the possible

cause of this shift in λmax.

iii. All sucrose mediated run products exhibited similar λmax and UV-Visible absorption.

Since there was simultaneous decrease in length and diameter of naorods with rise in

surfactant concentration, as indicated by FESEM analysis (subsection 2.5.4.3), the aspect

ratio of nanorods remained more or less unaltered. Hence the absorption patterns were

similar for products obtained in all sucrose mediated runs.

Table 2.10 UV-Visible absorbance data of synthesized lead chrome pigments

Sample λmax, nm Absorbance

A 511.0 0.332

436.6 0.327

B 506.40 0.101

427.80 0.102

C 504.80 0.084

427.60 0.084

D 507.40 0.074

427.40 0.072

Thus UV-Visible analysis of nano lead chrome crystals was consistent with

FESEM and XRD analysis.

Colour Matching Analysis

Lead chrome is well known as brilliant yellow pigment in paint industries. With

decrease in particle size, particularly in nano range, the tint gets modified as a result of

change in magnitude of selective absorption and reflection in visible range of

electromagnetic spectrum. Sucrose ester mediated products obtained through impinging

solution spray process were darker greener, less yellower and less saturated in comparison

to the standard, micron sized pigment particles. The colour matching analysis indicated

that the modification in tint was due to the nano size and the crystal orientation of

nanorods.

2.5.4.5 Inter-Langmuir-Blodgett/ micelle reactant exchange mechanism and

formation of monoclinic polymorph of lead chrome nanopigment

The FTIR, XRD, FESEM and UV-Visible analysis of sucrose ester mediated

synthesis of lead chrome nanorods in impinging solution spray reactor absolutely


83

established the influence of surfactant on size, phase and polymorph stability of lead

chrome nanocrystals. Surfactant mediated growth mechanism governing the formation of

nanorods and nanoparticles has been proposed47,48

. However the mechanism does not

correlate size stabilization capability or polymorph selective synthesis mode of surfactants

with their aggregation formation capabilities. The morphology of surfactant self assembly

depends on the magnitude of its concentration. At different concentrations, the surfactant

molecules can form various molecular aggregations, e.g. multilayers, micelle, liquid

crystal and vesicle which are used as perfect templates to prepare nanostructure materials.

We propose ‘inter-multilayer/ micellar reactant exchange mechanism’ for providing

the illustration of influence of sucrose ester on size, phase and polymorph stability of lead

chrome nanocrystals as shown in Fig. 2.10.

On the basis of continuous drop in surface tension (e.g. 85.50→57.35→43.50 in

lead nitrate solution and 77.10→51.64→45.09 in potassium chromate solution) at 0.2 and

0.5% sucrose ester surfactant usage and subsequent calculations presented in Table 2.6

and 2.7, one can conclude that the surfactant concentrations in general and surface excess

concentrations/ adsorption density in particular were still lower than that required for large

scale aggregation of surfactant as 3- Dimensional micelle. At 0.2 and 0.5%, therefore,

oriented surfactant molecules were organizing in the form of self-assembled bi/

multilayers at water-air interface called Langmuir-Blodgett (LB) layers (Fig. 2.10 A). LB

films constituted by n layers are obtained by repeating this procedure n time. Upon

addition of lead nitrate and potassium chromate to water to obtain 0.1 M solution, these

2D- multilayers transfer from water-air interface to water-solid interface and host

precursor/ precipitant molecules. The solubilization of finite amount of water leads to the

formation of an aqueous domain interposed between surfactant layers.

When sucrose ester concentration was raised to 1%, lowest attainable surface

tension (33.70 and 37.46 of lead nitrate and potassium chromate solution, respectively)

was recorded which corresponds to the attainment of CMC and formations of micelle.

Beyond CMC, individual surfactant monomers begin to aggregate with their hydrophilic

heads pointing outwards towards the solution and the hydrophobic tails pointing inwards

away from the water in order to minimize the free energy (i.e. maximize the entropy) of

the system (Fig. 2.10 B). Upon addition of lead nitrate and potassium chromate to water to

obtain 0.1 M solution, the free spaces created in the centre of micelles are expected to

accommodate ionic precursors and precipitants. However the reactants are hydrophilic.

Hence the micelles are required to reorient to form bi-aggregates which maintain


84

hydrophilic heads on exterior (i.e. pointing towards continuous aqueous phase) side as

well as at centre (Fig. 2.10 B). In general, micelles are spherical aggregates of surfactant

molecules (for sucrose monolaurate49

, the aggregation number is 52) of about 4-10 nm in

diameter that are in equilibrium with single surfactant monomers in the bulk aqueous

solution and can only form when the surfactant solubility is equal to or greater than the

CMC50

. Given the smallness of the micelles, the micelle/electrolyte interface has a huge

value. With a typical value of 30-38 02A of the surfactant head group area (Table 2.6 and

2.7) and based on aggregation number (which is 52 in present case49

), about 1019

nearly

identical self-assembled aggregates, which can also be viewed as nanoreactors,

nanocarriers, or nanocontainers, are dispersed in a liter of a 0.1 molar surfactant solution50

.

Reactants in molecular form come from the two contacting aggregates (LB/

Micelles) and the impinging sprays of atomised droplets (9-10 μm in diameter as stated in

the beginning of section 2.5.4) are directed towards redistribution of reactants by hopping

or spontaneous diffusion through the aqueous medium. The inter-LB/ micellar material

exchange process allows the transport of hydrophilic reactants so that they can come in

contact and react51

as a result of occurrence of following elementary steps50,51

:

i. diffusion controlled motion of aggregates in the aqueous medium,

ii. inter-LB/ micellar collisions and possible coalescence between LB film/ micelles

iii. diffusion of solubilizates molecules in the confined space of the transient micelle dimer

iv. coprecipitation reaction and monolayer/ micelle binding of nanomaterials.

Since the surface of the lead chrome is liphophilic and the nano particle is

dispersed in water, reorientation of surfactant aggregates takes place. Thus in LB film

mediated process; the monolayers of sucrose ester molecules bind the lead chrome

nanorods (Fig. 2.10 Aa). The micelle bi-aggregates transform back to mono-aggregates to

take care of lipophilic affinity of nanorods (Fig. 2.10 B). It has been demonstrated under

subsection 2.5.4.3 that at sucrose ester addition corresponding to 1%, major size reduction

to the tune of 25-51 nm was achieved (Fig. 2.8 C) providing evidence in the support of

inter-micelle reactant exchange mechanism. It also proves that the inter-micelle reactant

exchange mechanism provides superior control on size, phase and polymorph stability of

lead chrome nanocrystals in comparison to inter-LB reactant exchange mechanism

prevailing at lower surfactant concentrations. Sucrose ester is a nonionic surfactant

compound, which hydrates completely in water. The long alkyl chains of fully hydrated

bulky nonionic surfactant are advantageous to the oriented growth of PbCrO4 rods as


85

shown in Fig. 2.11. Thus the inter-LB/ micelle reactant exchange mechanism illustrates

well the influence of sucrose ester surfactant on formation of specific monoclinic

polymorph of lead chrome nanopigment.

A.

B.

Fig. 2.10 A. Inter-LB reactant exchange mechanism, B. Inter-micelle reactant

exchange mechanism


86

Fig. 2.11 Binding of nanorods by oriented sucrose ester aggregates.

In short, the LB/ micelle system offered by sucrose ester surfactant for reactive

events and nucleation, the thin film spray reaction zone promoting faster precipitation and

instantaneous dilution facilitating segregation at reactor base were primarily responsible

for stabilization of nano size and specific crystal morphology of nano lead chromate. It is

therefore very much feasible to accomplish the polymorph selective synthesis of

nanocrystals through the use of specific type and amount of surfactants.

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CHAPTER 2 Coconut fatty acid-sucrose esters: …shodhganga.inflibnet.ac.in/bitstream/10603/25187/6/7...Coconut fatty acid-sucrose esters: Synthesis, characterization and applications