Transcript Slide 1

Stability of Colloids
Kausar Ahmad
Kulliyyah of Pharmacy, IIUM
http://staff.iiu.edu.my/akausar
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
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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 nonionic surfactant just balances.

If temperature >> PIT, emulsion becomes unstable

because the surfactant reaches the cloud point
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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.
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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
paraffin-in-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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Electrical theories of emulsion stability
Charges can arise from:
1.
Ionisation
2.
Adsorption

3.
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

The first observations on emulsions stabilised by solids
were made by Pickering.

Basic sulfates of iron, copper, nickel, zinc and
aluminum in moist conditions act as efficient
dispersing agents for the formation of petroleum o/w
emulsion

The DRY calcium carbonate can also promote
emulsification but emulsion not stable.
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Emulsion formation with solids


Briggs observed formation of

o/w emulsion with kerosene/benzene and ferric
hydroxide, arsenic sulfide and silica

w/o emulsions were produced with carbon black
and lanolin
Weston produced o/w and w/o emulsions with 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:
1.
Protection given by the multilayer against
coalescence due to Van der Waals forces of
attraction.
2.
Prevent thinning of the 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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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 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
spherical
Polydisperse sample
coalescence
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Non-spherical
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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
BREAKING OF EMULSION
Without
surfactant
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With 10% surfactant
Homogenisation for 30 min
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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 to mainly
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)
Aluminium magnesium silicate
Sodium alginate
Sodium starch
Polymer
Mechanism of their operation:
1) Adsoption onto the surface
of particles
2) Increasing the viscosity of
medium
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
u = 2r2(r – ro)g / 9ho
Forces acting on particles
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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.
may 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

Polyacrylamide (30% hydrolysed)


an anionic polymer which can induce flocculation in
numerous system such as silica sols and kaolinite at very low
concentrations.
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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Definition - Gel Formation

When the 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_l
ayer.html
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