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Stability of Colloids

Stability of Colloids. Kausar Ahmad Kulliyyah of Pharmacy, IIUM http://staff.iiu.edu.my/akausar. Contents. Lecture 1 1) Non-ionic SAA and Phase Inversion Temperature 2) Stabilisation factors Electrical stabilisation Steric stabilisation Finely divided solids Liquid crystalline phases

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Stability of Colloids

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  1. Stability of Colloids Kausar Ahmad Kulliyyah of Pharmacy, IIUM http://staff.iiu.edu.my/akausar Physical Pharmacy 2

  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 Physical Pharmacy 2

  3. 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

  4. 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

  5. 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. Physical Pharmacy 2

  6. 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.- Physical Pharmacy 2

  7. 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 Physical Pharmacy 2

  8. 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. Physical Pharmacy 2

  9. 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 Physical Pharmacy 2

  10. 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 Physical Pharmacy 2

  11. FORCES OF INTERACTIONbetween 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. Physical Pharmacy 2

  12. 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 Physical Pharmacy 2

  13. 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. Physical Pharmacy 2

  14. 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 Physical Pharmacy 2

  15. 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 aluminumin 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. Physical Pharmacy 2

  16. 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. Physical Pharmacy 2

  17. 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!) Physical Pharmacy 2

  18. Stabilisation byLiquid 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. Physical Pharmacy 2

  19. Emulsions Suspensions Hydrophilic colloid? Creaming Phase separation Demulsification Ostwald ripening Heterocoagulation Flocculation Coalescence Caking Aggregation Destabilisation of Colloids Physical Pharmacy 2

  20. Demulsification • By physico-chemical method • Compression of double layer • Add polyelectrolytes,multivalent cations. • add emulsion/dispersion with particles of opposite charge - HETEROCOAGULATION Physical Pharmacy 2

  21. 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). Physical Pharmacy 2

  22. 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. Physical Pharmacy 2

  23. Mechanism of Ostwald Ripening Oil molecule diffused out of small droplet Oil molecule absorbed by big droplet Physical Pharmacy 2

  24. Oil droplets in aqueous medium spherical Polydisperse sample coalescence Non-spherical Physical Pharmacy 2

  25. Destabilisation scheme Rupture of interfacial film Interfacial film intact Bridging flocculation From Florence & Attwood Physical Pharmacy 2

  26. Separation of phases in o/w emulsions With 10% surfactant Homogenisation for 30 min Without homogenisation Without surfactant Physical Pharmacy 2 BREAKING OF EMULSION

  27. Destabilisation of Multiple Emulsion Physical Pharmacy 2

  28. Destabilisation of hydrophilic colloid Due to mainly Depletion of water molecules • when the colloid is contaminated with alcohol • Evaporation of water • Addition of anion Physical Pharmacy 2

  29. Destabilisation of Hydrophilic Sols by Anions • Hofmeister (or lyotropic series): in decreasing order of precipitating power citrate tartrate sulfate acetate chloride nitrate bromide iodide. Physical Pharmacy 2

  30. Destabilisation of suspensions Physical Pharmacy 2

  31. Minimising Creaming/Sedimentation/Caking Physical Pharmacy 2

  32. 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 Brownian movement 2-5 μm Physical Pharmacy 2

  33. Viscosity modifier fornon-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. Physical Pharmacy 2

  34. Role of polymers in the stabilisation of dispersions Physical Pharmacy 2

  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. Physical Pharmacy 2

  36. 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. Physical Pharmacy 2

  37. 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. Physical Pharmacy 2

  38. 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_layer.html Physical Pharmacy 2

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