1. Kulliyyah of Pharmacy, IIUM
Physical Pharmacy 2
4/15/2017
Stability of Colloids
Kausar Ahmad
Kulliyyah of Pharmacy, IIUM
Physical Pharmacy 2
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2. 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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3. Phase Inversion Temperature
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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
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4. 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.
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5. PIT Factor – Cloud point
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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.
The higher the cloud point, the higher the solubility of the SAA molecule in the aqueous phase.
Thus, only at high temperature will the molecule become insoluble.
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6. Physical Pharmacy 2
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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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7. PIT Factor - Length of oxyethylene chain
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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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8. PIT Factor - Surfactant mixtures
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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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9. PIT Factor - Salts, acids and alkalis
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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) 75
1.2 50
in general, addition of salts will destabilise colloidal system
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10. PIT Factor - Additives in oil
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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)
100
0.25 30
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11. FORCES OF INTERACTION between colloidal particles
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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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12. Electrical theories of emulsion stability
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Electrical theories of emulsion stability
Charges can arise from:
Ionisation
Adsorption
The electrical charge on a droplet arises from the adsorbed surfactant at the interface.
Frictional contact
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13. Charges arising from 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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14. Electrical stabilisation
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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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15. Stabilisation of emulsions by 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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16. Emulsion formation with solids
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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.
Moist precipitates are better emulsifiers
Highly gelatinous or highly dispersed precipitates are more efficient than granular.
Aluminum hydroxide precipitates improved on ageing (why?)
 Exercise: predict HLB of silica, carbon black, lanolin
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17. Adsorption of solids at interface
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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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18. Stabilisation by Liquid Crystalline Phases
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Stabilisation by Liquid Crystalline Phases
Emulsion stability increases as a result of:
Protection given by the multilayer against coalescence due to Van der Waals forces of attraction.
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.
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19. Destabilisation of Colloids
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Destabilisation of Colloids
Emulsions
Suspensions
Hydrophilic colloid?
Creaming
Phase separation
Demulsification
Ostwald ripening
Heterocoagulation
Flocculation
Coalescence
Caking
Aggregation
Define all the terms above.
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20. Demulsification By physico-chemical method Compression of double layer
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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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21. Effect of polyelectrolyte
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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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22. Ostwald Ripening If oil droplets have some solubility in 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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23. Mechanism of Ostwald Ripening
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Mechanism of Ostwald Ripening
The small oil droplets, due to the high SSA, are thermodynamically unstable.
The small oil droplets undergo degradation and thus diffuse into the aqueous phase.
The diffused molecules are then absorbed by the big oil droplets.
The big oil droplets will get bigger and the number of small droplets diminish.
Oil molecule diffused out of small droplet
Oil molecule absorbed by big droplet
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24. Oil droplets in aqueous medium
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Oil droplets in aqueous medium
spherical
Polydisperse sample
coalescence
Non-spherical
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25. Destabilisation scheme
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Destabilisation scheme
Rupture of interfacial film
Interfacial film intact
Bridging flocculation
From Florence & Attwood
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26. Separation of phases in o/w emulsions
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Separation of phases in o/w emulsions
With 10% surfactant
Homogenisation for 30 min
Without
homogenisation
Without
surfactant
BREAKING OF EMULSION
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27. Destabilisation of Multiple 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.
One of the main DRIVING FORCES for destabilisation is to reduce the interfacial energy brought about by the small particulates.
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28. Destabilisation of hydrophilic colloid
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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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29. Destabilisation of Hydrophilic Sols by Anions
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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.
the hydrophilic sols destabilised as a result of the higher affinity for hydration of the ions resulting in the separation of water molecules from the colloidal particles. Thus destabilised.
Exercise: what is the effect of salt on starch solution?
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30. Destabilisation of suspensions
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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.
Exercise: any particle growth in emulsion?
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31. Minimising Creaming/Sedimentation/Caking
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Minimising Creaming/Sedimentation/Caking
4/15/2017
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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32. Effect of viscosity Stoke’s Law Forces acting on particles
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Effect of viscosity
Stoke’s Law
Forces acting on particles
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
Gravity
What forces are responsible for the following:
destabilisation of hydrophilic sols by anions
Heterocoagulation
Stabilisation by liquid crystalline phases
Stabilisation by solids
Brownian movement
2-5 μm
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33. Viscosity modifier for non-aqueous suspension
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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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34. Role of polymers in the stabilisation of dispersions
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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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35. 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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36. Flocculating agent Polyacrylamide (30% hydrolysed) Application
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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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37. Definition - Gel Formation
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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.
Peptisation is a process in which dispersion is achieved with little or no agitation, by changing the composition of the dispersion medium.
This may be achieved by:
addition of polyvalent co-ions (e.g. polyphosphate ions to a negatively charged coagulated dispersion)
addition of surfactants
dilution of the dispersion medium
dialysis
In each case, VR is modified so as to create a potential energy maximum to act as a barrier against recoagulation.
The sensitisation of a hydrophobic colloid occurs when a hydrophilic or hydrophobic colloid of opposite charge is added.
Due to compression of the electrical double layer.
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38. Physical Pharmacy 2
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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)
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