CHARACTERISATION OF COLLOIDS Kausar Ahmad akausar@iium.edu.my Physical Pharmacy 21

CHARACTERISATION OF COLLOIDS

Kausar Ahmadhttp://staff.iium.edu.my/akausar

akausar@iium.edu.my

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OBJECTIVES OF CHARACTERISATION

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To determine stability

To formulate emulsions or suspensions with higher stability

Take precautions

HOW TO CHARACTERISE?

Microscopy Macroscopic observations Turbidity

Rheology Particle sizing Zeta potential

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PROPERTIES OF COLLOIDAL SYSTEMS

Optical Kinetic

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OPTICAL PROPERTIES OF COLLOIDS

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LIGHT SCATTERING

caused by fine particles

Faraday Tyndall effect

a visible cone is formed

Turbidity

Molecular weight

KINETIC PROPERTIES OF COLLOIDS

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• arises from bombardment of dispersed particles by molecules of dispersion medium.

Brownian motion

• particles diffuse spontaneously from region of high to low concentration

Diffusion

• allows the calculation of molecular weight of colloidOsmotic pressure

• given by Stoke’s lawSedimentation/Creaming

• resistance to flow under applied stress.Viscosity

COLLOIDAL VS COARSE

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• Particles cannot be seen using ordinary microscope• Visible only in electron microscope

• Particles diffuse (slowly)

Colloidal

• Particles visible under microscope• Do not pass through filter paper• Particles do not diffuse

Coarse

KINETICS OF COAGULATIONRATE AT WHICH A SOL COAGULATES

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Temperature increases, thermal energy increases, kinetic energy increases.

REPULSIVE POTENTIAL ENERGY?

STABLE

COAGULATION

HIGH

LOW

RATE OF PARTICLE AGGREGATION

• The rate at which particles aggregate is given by:

-dn/dt = k2n2

n - number of particles per unit volume of sol at time t

k2 is a second-order rate constant

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PARTICLE CONCENTRATION

Directly: by visual particle counting

• microscopy

Indirectly: from turbidity

• spectrophotometric or light scattering measurements.

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MICROSCOPIC EXAMINATIONAggregation of Solids

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PHASE SEPARATION

• Phase separation occurs as a result of creaming or sedimentation.

• The volumes of the different phases that separated out are recorded as a function of time.

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0%

20%

40%

60%

80%

100%H

eigh

t of

pha

se

aqueous phase white emulsion yellow emulsion oil layer

Phase separation of emulsions in 100 ml measuring cylinder.

Oil emulsions with different emulsifiers at 10% concentration;

emulsified at 6000 rpm for 30 min. at 30C.

TURBIDITY

1 2 3 40

10

20

30

40

50

60

70

80

90

Na+

Al3+

% T

rans

mit

tanc

e

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Time

END OF LECTURE 1/2

PARTICLE SIZE ANALYSIS

Common method to determine overall stability of colloid

The particle size distribution (PSD) of a colloid can be determined using:

• Microscope• Coulter counter• Instrument based on laser diffraction technique• Instrument based on photon correlation spectroscopy

For monitoring stability the PSD is taken as a function of time

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PARTICLE SIZE DISTRIBUTION

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Effect of Span®20/Tween®20 concentration on size and polydispersity of oil droplet

2.46 2.37 2.36

9.23 9.699.63

3.513.52

3.20

0

5

10

15

20

5 10 15 20 25 30 35 40 45

Surfactant concentration (% w/w)

Pa

rtic

le s

ize

(m m

)

0.00

1.00

2.00

3.00

4.00

5.00

Po

lyd

isp

ers

ity

D(v,0.5) D(v,0.90) Polydispersity

VISCOSITY

An obvious change in viscosity as a function of time, is observed in gel formation.

Partially flocculated system will also show an increase in viscosity.

Depending of the type of particles, viscosity may decrease or increase following coagulation.• Gel formation - increase• Sedimentation - decrease

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MEASURING VISCOSITY

Rotational Viscometer Cone and plate U-tube

Flow cup GPC

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VISCOSITY OF EMULSIONS

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3.6

11.6

83.5

267544

298021602000

1

10

100

1000

10000

10 20 30 40 50 60 70

Oil content (% w/w)

Vis

cosi

ty (

cP)

Effect of oil content on oil emulsion with 12% E906; emulsified at 6000 rpm for 30 min. at 40C.

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ZETA POTENTIAL

ZETA POTENTIAL FACTORS

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pH Ionic strength

Type of electrolyte Concentration of additives

Zeta potential of Intralipidas a function of electrolyte concentration

and type of electrolyte

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NaCl

CaCl2

From Florence & Attwood

STAT

E O

F AG

GRE

GAT

ION

VS

ZETA

PO

TEN

TIAL

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Change in Viscosity in relation to change in zeta potential

If ZP remains constant

• viscosity of the system will also remain constant.

If ZP is lowered by cationic electrolytes or polyelectrolytes

• stability of the system will reduce progressively • from simple agglomeration to fluid gel formation or a rigid

gel.

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SEDIMENTATION

• Difference in density between particles in dispersed phase and medium

• accumulate under the influence of gravity

– at the bottom -SEDIMENTATION

– On top -CREAMING

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TYPE & VOLUME OF SEDIMENT

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particles that form bridges, give high volume BUT form loose sediment

which is more easily dispersed.

dense sediment difficult to redisperse

INTENTIONAL REDUCTION OF ZETA POTENTIAL

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add polyelectrolytes

allow ‘bridging’

result in loose aggregates

cake can redisperse easily

CONTROLLED FLOCCULATION

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ACCELERATED STABILITY TESTS

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• Phase inversion, destabilise, precipitation of SAA

Heating at elevated

temperatures

• Compression of electrical double layer

High salt or electrolyte

concentration

• Compression of electrical double layer

Severe acidity or alkalinity

REFERENCES

RJ Hunter, Foundations of Colloid Science Volume 2 Chapter 16, Clarendon Press Oxford (1989)

Lab data

The aggregation pix from internet but I have lost the link. I acknowledge the contribution of authors. Thank you.

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