Stability of colloids

stability of colloids

1. Stability of colloids By: Maryam kazemi Pharm.D . Ph.D student of pharmaceutics

2. Pharmaceutical applications of colloids; 1) Colloidal silver iodide, silver chloride & silver protein are effective germicides & not cause irritation as ionic silver salts . 2) Colloidal copper used in cancer . 3) Colloidal gold used as diagnostic agent . 4) Colloidal mercury used in syphilis . 5) Association colloids (SAA) are used to increase solubility & stability of certain compounds in aqueous & oily pharmaceutical preparations.

3. Colloidal stability To maintain stability through Brownian motion we need to prevent particles sticking when they collide .

4. The forces between colloidal particles 1.vanderWaals forces or electromagnetic forces (attraction ) 2. electrostatic forces (repulsion ) 3. steric forces due to adsorbed molecules at the particle interface (repulsive ) 4. solvation forces (repulsive)

5. Criteria of stability Salt concentration Counter-ion valency ζ- potential Particle size

6. Salt concentration In each case there is a steep rise to the primary maximum. At larger distances there is a long repulsive tail, most notable at lower electrolyte concentrations . The range of the tail reduces as the electrolyte concentration increases (in line with the decrease in the Debye length ). The height of the maximum decreases with increasing electrolyte concentration . As the primary maximum falls to below zero (above 3 × 10−2 M NaCl in this case), all collisions will lead to aggregation as there is no barrier.

7. ζ- potential doubling of the ζ -potential leads to a quadrupling of the value of V max . In this example, when the ζ -potential reduces to less than −20 mV the value of V max drops below 20 kBT and significant aggregation will occur

8. Flocculating agent changes zeta-potential of the particles. It can be electrolyte , charged surfactant or charged polymer adsorbing on a surface

9. Flocculating Agents Flocculating agents decreases zeta potential of the suspended charged particle and thus cause aggregation (flock formation) of the particles. Examples of flocculating agents are: Neutral electrolytes such as KCl , NaCl . Surfactants Polymeric flocculating agents Sulfate, citrates, phosphates salts

10. Neutral electrolytes e.g. NaCl , KCl besides acting as flocculating agents, also decreases interfacial tension of the surfactant solution. If the particles are having less surface charge then monovalent ions are sufficient to cause flocculation e.g. steroidal drugs. For highly charged particles e.g. insoluble polymers and poly-electrolytes species, di or trivalent flocculating agents are used.

11. surfactant Both ionic and non-ionic surfactants can be used to bring about flocculation of suspended particles . Optimum concentration is necessary because these compounds also act as wetting agents to achieve dispersion . The particles possessing less surface free energy are attracted towards to each other by van der- waals forces and forms loose agglomerates.

12. polymers Polymers possess long chain in their structures. Starch , alginates , cellulose derivatives , carbomers , tragacanth The part of the long chain is adsorbed on the surface of the particles and remaining part projecting out into the dispersed medium. Bridging between these later portions, also leads to the formation of flocs .

13. Particle size Both the attractive and repulsive contributions are proportional to the particle radius. At small sizes the value of VT is directly proportional to the particle size . However , at large sizes the value of VT has a more complicated variation . a larger particle radius leads to a higher energy barrier; in other words, electrostatic stability increases with increasing particle radius (all other factors remaining constant ). For small particle sizes ( < 100 nm radius) the primary maximum is directly proportional to the radius. However, the relationship breaks down at larger sizes and the height of the primary maximum increases at a lower rate

14. We can make a distinction here between two types of aggregation : Coagulation is the rapid aggregation that happens in the absence of a primary maximum and leads to a strong irreversible aggregated structure. Flocculation is a reversible aggregation that occurs in a secondary minimum as described. Flocculation is reversible on the addition of energy to the system, usually the application of a shear field by shaking, stirring or other mechanical processes .

15. Thanks for your attention