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CHARACTERISATION OF COLLOIDS
Kausar Ahmadhttp://staff.iium.edu.my/akausar
Physical Pharmacy 2 1
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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