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Physical Pharmacy 21 Stability of Colloids Kausar Ahmad Kulliyyah of Pharmacy, IIUM

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Presentation on theme: "Physical Pharmacy 21 Stability of Colloids Kausar Ahmad Kulliyyah of Pharmacy, IIUM"— Presentation transcript:

1 Physical Pharmacy 21 Stability of Colloids Kausar Ahmad Kulliyyah of Pharmacy, IIUM

2 Physical Pharmacy 22 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

3 Physical Pharmacy 23 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

4 Physical Pharmacy 24 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.

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 25

6 6 PIT Factor – Type of oil the more soluble the oil for a non-ionic emulsifier, the lower the PIT. e.g. at 20 o C, POE nonylphenylether (HLB=9.6) dissolves well in benzene, but not in hexadecane or liquid paraffin. The PIT was ca. 110 o C compared to only 20 o C for benzene with 10% w/w of the emulsifier. -

7 Physical Pharmacy 27 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

8 Physical Pharmacy 28 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.

9 Physical Pharmacy 29 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)

10 Physical Pharmacy 210 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)

11 Physical Pharmacy 211 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.

12 Physical Pharmacy 212 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

13 Physical Pharmacy 213 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 25 o C and 1 atm, the d.c. or relative permittivity for water is 78.5; for benzene ca. 2.5.

14 Physical Pharmacy 214 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

15 Physical Pharmacy 215 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.

16 Physical Pharmacy 216 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.

17 Physical Pharmacy 217 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 90 o. A concentration of solids at the interface represents an interfacial film of considerable strength and stability (compare with liquid crystal!)

18 Physical Pharmacy 2 End of lecture 1/2 18 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.

19 Physical Pharmacy 219 Destabilisation of Colloids Emulsions Suspensions Hydrophilic colloid? Creaming Phase separation Demulsification Ostwald ripening Heterocoagulation Flocculation Coalescence Caking Aggregation

20 Physical Pharmacy 220 Demulsification By physico-chemical method Compression of double layer Add polyelectrolytes, multivalent cations. add emulsion/dispersion with particles of opposite charge - HETEROCOAGULATION

21 Physical Pharmacy 221 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).

22 Physical Pharmacy 222 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.

23 Physical Pharmacy 223 Mechanism of Ostwald Ripening Oil molecule absorbed by big droplet Oil molecule diffused out of small droplet

24 Physical Pharmacy 224 Oil droplets in aqueous medium coalescence Polydisperse sample Non-spherical spherical

25 Physical Pharmacy 225 Destabilisation scheme From Florence & Attwood Rupture of interfacial film Interfacial film intact Bridging flocculation

26 Physical Pharmacy 226 Separation of phases in o/w emulsions Without homogenisation Without surfactant With 10% surfactant Homogenisation for 30 min BREAKING OF EMULSION

27 Physical Pharmacy 227 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.

28 Physical Pharmacy 228 Destabilisation of hydrophilic colloid Due to mainly Depletion of water molecules when the colloid is contaminated with alcohol Evaporation of water Addition of anion

29 Physical Pharmacy 229 Destabilisation of Hydrophilic Sols by Anions Hofmeister (or lyotropic series): in decreasing order of precipitating power citrate tartrate sulfate acetate chloride nitrate bromide iodide.

30 Physical Pharmacy 230 Destabilisation of suspensions as a result of sedimentation difficult to re-disperse. Caking cluster of particles held together in loose open structure (flocs) Presence of flocs increases the rate of sedimentation. BUT re-disperse easily. Flocculation through dissolution and crystallisation. Particle growth

31 Physical Pharmacy 231 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

32 Effect of viscosity Stoke’s Law The velocity  of sedimentation of spherical particles of radius r having a density  in a medium of density  o & a viscosity  o & influenced by gravity g is  = 2r 2 (  –  o )g / 9  o Forces acting on particles Physical Pharmacy 232 Gravity Brownian movement 2-5 μm

33 Physical Pharmacy 233 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.

34 Physical Pharmacy 234 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 mediumminimise sedimentation

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 235

36 Physical Pharmacy 236 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.

37 Physical Pharmacy 237 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.

38 Physical Pharmacy 238 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) ayer.html


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