STABILITY OF COLLOIDS
Kausar Ahmad
http://staff.iium.edu.my/akausar
akausar@iium.edu.my
Physical Pharmacy 2
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CONTENTS
Lecture 1
1) Non-ionic SAA and Phase Inversion Temperature
2) Stabilisation factors
– Electrical stabilisation
– Steric stabilisation
• Finely divided solids
• Liquid crystalline phases
Lecture 2
3) Destabilisation factors
– Compression of electrical double layer
• Addition of electrolytes
• Addition of oppositely charged particles
• Addition of anions
4) Effect of viscosity
Physical Pharmacy 2
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PHASE INVERSION TEMPERATURE
• PIT, or Emulsion Inversion Point (EIP), is a characteristic property of
an emulsion (not surfactant molecule in isolation).
• At PIT, the hydrophile-lipophile property of non-ionic surfactant
just balances.
• If temperature >> PIT, emulsion becomes unstable
– because the surfactant reaches the cloud point
Physical Pharmacy 2
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CLOUD POINT
• Definition - The temperature at which the SAA
precipitates.
• Common for non-ionic SAA.
As temperature increases, solubility of the POE chain
decreases i.e. hydration of the ether linkage is
destroyed.
• Hydration of POE is most favourable at low temperature.
• For the same type of SAA, cloud point depends on
length of POE.
Physical Pharmacy 2
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PIT FACTOR
Cloud point
of SAA
Salt, acid,
alkali,
additives
Type of oil
Length of
oxyethylene
chain
Surfactant
system
Physical Pharmacy 2
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PIT FACTOR – CLOUD POINT
• the higher the cloud point in aqueous
surfactant solution, the higher the PIT.
• This coincides with Bancroft’s rule that the
phase in which the emulsifier is more soluble
will be the external phase at a definite
temperature.
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PIT FACTOR – TYPE OF OIL
• The more soluble the oil for a non-ionic
emulsifier, the lower the PIT.
•
e.g. at 20oC, POE nonylphenylether (HLB=9.6) dissolves well in
benzene, but not in hexadecane or liquid paraffin.
The PIT was ca. 110oC compared to only 20oC for benzene with
10% w/w of the emulsifier.
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PIT FACTOR - LENGTH OF OXYETHYLENE CHAIN
• the longer the chain length, the higher
the PIT
•
e.g. in benzene-in-water emulsions, the PIT increased as the
chain length increased
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PIT FACTOR - SURFACTANT MIXTURES
• when stabilised by a mixture of
surfactants, the PIT increased
compared to the expected PIT from
single surfactant.
•
e.g. in heptane-in-water emulsion, blending POE
nonylphenyl ether having HLB of 15.8 and 7.4 resulted in a
higher PIT.
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PIT FACTOR - SALTS, ACIDS AND ALKALIS
• Increase in concentration of salt will
decrease PIT of o/w emulsion.
•
e.g. PIT of cyclohexane-in-water emulsion
NaCl (N)
PIT of o/w (C)
0
75
1.2
50
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PIT FACTOR - ADDITIVES IN OIL
•
in the presence of fatty acids or alcohols, the PIT of both
o/w & w/o emulsions decreases as the concentration of
these additives increases, regardless of the chain length of
the additives.
•
e.g. lauric/myristic/palmitic/stearic acids in liquid paraffinin-water emulsion
Acid (mol/kg)
PIT (C)
0
100
0.25
30
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FORCES OF INTERACTION
BETWEEN COLLOIDAL PARTICLES
Electrostatic forces of repulsion
Van der waals forces of attraction
Born forces – short-range, repulsive force
Steric forces – depends on geometry of molecules adsorbed at particle
interface
Solvation forces – due to change in quantities of adsorbed solvent for
close particles.
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Charges can arise from:
ELECTRICAL THEORIES OF EMULSION STABILITY
Ionisation
Adsorption
The electrical charge on
a droplet arises from the
adsorbed surfactant at
the interface.
Frictional contact
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CHARGES ARISING FROM FRICTIONAL CONTACT
• For a charge that arises from frictional contact, the empirical
rule of Coehn states that:
substance having a high dielectric constant (d.c.) is positively
charged when in contact with another substance having a
lower dielectric constant.
• E.g. most o/w emulsions stabilised by non-ionic surfactants
are negatively charged – because water has a higher d.c. than
oil droplets. At 25oC and 1 atm, the d.c. or relative permittivity
for water is 78.5; for benzene ca. 2.5.
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ELECTRICAL STABILISATION
The presence of the charges on the
droplets/particle causes mutual repulsion of
the charged particles.
This prevents close approach i.e. coalescence,
followed by coagulation, which leads to
• breaking of an emulsion
• Aggregation of solids
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STABILISATION OF EMULSIONS BY SOLIDS
Emulsion Type
Petroleum
(Pickering)
Kerosene/benzene
(Briggs)
0/W
Hydrated sulfates
of iron, copper,
nickel, zinc &
aluminum
ferric hydroxide,
arsenic sulfide &
silica
Oil phase?
(Briggs)
Oil phase?
(Weston)
W/O
carbon black &
lanolin
Clay
Physical Pharmacy 2
Clay
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ADSORPTION OF SOLIDS AT INTERFACE
• The ability of solids to concentrate at the boundary is a
result of:
wo > sw + so
• The most stable emulsions are obtained when the contact
angle with the solid at the interface is near 90o.
• A concentration of solids at the interface represents an
interfacial film of considerable strength and stability
(compare with liquid crystal!)
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STABILISATION BY
LIQUID CRYSTALLINE PHASES
•
•
Emulsion stability increases as a result of:
– Protection given by the multilayer liquid crystalline phase
against coalescence (coalescence due to Van der Waals
forces of attraction).
• The multilayer adsorbed at the interface prevents
thinning of the interfacial films of approaching
droplets.
These are achieved due to the high viscosity of the liquid
crystalline phases compared to that of the continuous
phase.
End of lecture 1/2
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Physical Pharmacy 2
DESTABILISATION OF COLLOIDS
• Emulsions
• Suspensions
• Hydrophilic colloid?
•
•
•
•
•
•
•
•
•
Creaming
Phase separation
Demulsification
Ostwald ripening
Heterocoagulation
Flocculation
Coalescence
Caking
Aggregation
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DEMULSIFICATION
•
By physico-chemical method - Compression of
Electrical Double Layer
•
Add polyelectrolytes, multivalent cations.
•
Add emulsion/dispersion with particles of
opposite charge - HETEROCOAGULATION
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EFFECT OF POLYELECTROLYTE
• Schulze-Hardy Rule states that
The valence of the ions having a charge opposite to that of
the dispersed particles determines the effectiveness of the
electrolytes in coagulating the colloids: suspensions or
emulsions.
• Thus, presence of divalent or trivalent ions should be
avoided.
• Preparation should use distilled water, double distilled
water, reverse osmosis or ion-exchange water (soft water).
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Ostwald Ripening
• IF oil droplets have some solubility in water.
• The extent of Ostwald ripening depends on the difference in
the size of the oil droplets.
– The larger the particle size distribution, the greater the
possibility of Ostwald ripening.
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MECHANISM OF OSTWALD RIPENING
Oil molecule diffused out of small droplet
Oil molecule absorbed by big droplet
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OIL DROPLETS IN AQUEOUS MEDIUM
POLYDISPERSE SAMPLES
coalescence
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DESTABILISATION SCHEME
Rupture of
interfacial film
Interfacial film
intact
Bridging
flocculation
From Florence & Attwood
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SEPARATION OF PHASES IN O/W EMULSIONS
Without
homogenisation
Without
surfactant
With 10% surfactant &
Homogenisation for 30 min
BREAKING OF EMULSION
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DESTABILISATION OF MULTIPLE EMULSION
For w/o/w: Coalescence of internal water droplets.
Coalescence of oil droplets.
Rupture of oil film separating internal and
external aqueous phases.
Diffusion of internal water droplets through the oil
phase to the external aqueous phase resulting in
shrinkage.
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DESTABILISATION OF HYDROPHILIC COLLOID
Due mainly to depletion of water molecules
– when the colloid is contaminated with alcohol
– Evaporation of water
– Addition of anion
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Destabilisation of Hydrophilic Sols by Anions
• Hofmeister (or lyotropic series): in decreasing order of
precipitating power
citrate
tartrate
sulfate
acetate
chloride
nitrate
bromide
iodide
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DESTABILISATION OF SUSPENSIONS
Caking
• as a result of sedimentation
• difficult to re-disperse.
Flocculation
• cluster of particles held together in loose open structure (flocs)
• Presence of flocs increases the rate of sedimentation.
• BUT re-disperse easily.
Particle growth
• through dissolution and crystallisation.
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MINIMISING CREAMING/SEDIMENTATION/CAKING
Addition of viscosity modifiers
Carboxymethylcellulose (CMC)
Mechanism of their operation:
Aluminium magnesium silicate 1) Adsoption onto the surface
of particles
Sodium alginate
2) Increasing the viscosity of
Sodium starch
medium
Polymer
3) Bridging
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EFFECT OF VISCOSITY
Stoke’s Law
The velocity u of
sedimentation of spherical
particles of radius r
having a density r in a medium
of density ro &
a viscosity ho
& influenced by gravity g is
Force acting on particles
u = 2r2(r – ro)g / 9ho
Physical Pharmacy 2
Gravity
Brownian movement
2-5 μm
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VISCOSITY MODIFIER FOR
NON-AQUEOUS SUSPENSION
E.g. amorphous silica for ointments
• Aerosil at 8-10% to give a paste.
The increase in viscosity resulted from hydrogen bonding
between the silica particles and oils: peanut oil, isopropyl
myristate.
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ROLE OF POLYMERS IN THE STABILISATION OF DISPERSIONS
Addition of polymeric surfactant
adsorption of the polymer onto the
particle surface
provides steric stabilization.
increase viscosity of medium
minimise sedimentation
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FLOCCULATION
• Because of the ability to adsorb, polymers are used as flocculating agent by
– promoting inter-particle bridging
– BUT, at high concentration of polymers, the polymers will coat the
particles (and increase the stability). No floc!
 With agitation the flocs are destroyed.
 Thus caking may result.
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FLOCCULATING AGENT
E.g. Polyacrylamide (30% hydrolysed)
• an anionic polymer
E.g. Application
• only 5 ppm of polyacrylamide is required to
flocculate 3% w/w silica sol.
• Restabilisation of the colloid occurs when the
dosage of polymer exceeds the requirement.
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GEL FORMATION
• When particles aggregate to form a continuous network
structure which extends throughout the available volume
and immobilise the dispersion medium………
• The resulting semi-solid system is called a gel.
• The rigidity of a gel depends on the number and the
strength of the inter-particle links in this continuous
structure.
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REFERENCES
PC Hiemenz & Raj Rajagopalan, Principles of Colloid and Surface
Chemistry, Marcel Dekker, New York (1997)
HA Lieberman, MM Rieger & GS Banker, Pharmaceutical Dosage Forms:
Disperse Systems Volume 1, Marcel Dekker, New York (1996)
F Nielloud & G Marti-Mestres, Pharmaceutical Emulsions and
Suspensions, Marcel Dekker, New York (2000)
J Kreuter (ed.), Colloidal Drug Delivery Systems, Marcel Dekker, New York
(1994)
http://www.chemistry.nmsu.edu/studntres/chem435/Lab14/double_laye
r.html
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