Stability of colloids

By: Maryam kazemi Pharm.D . Ph.D student of pharmaceutics

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.

Colloidal stability• To maintain stability through Brownian motion

we need to prevent particles sticking when they collide.

The forces between colloidal particles1.vanderWaals forces or electromagnetic forces (attraction)

2. electrostatic forces (repulsion)

3. steric forces due to adsorbed molecules at the particle in-terface (repulsive)

4. solvation forces (repulsive)

Criteria of stability

Salt concentration Counter-ion valencyζ-potentialParticle size

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 in-creases (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.

ζ-potential

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

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

Flocculating Agents

Flocculating agents decreases zeta potential of the sus-pended 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 agentsSulfate, citrates, phosphates salts

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.

surfactant

Both ionic and non-ionic surfactants can be used to bring about flocculation of suspended particles.

Optimum concentration is necessary because these com-pounds 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.

polymers

Polymers possess long chain in their structures. Starch, alginates, cellulose derivatives, carbomers, traga-

canth 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 for-mation of flocs.

Particle size• Both the attractive and repulsive contributions are pro-

portional to the particle radius. At small sizes the value of VT is directly proportional to the particle size. How-ever, at large sizes the value of VT has a more compli-cated variation.

• a larger particle radius leads to a higher energy barrier; in other words, electrostatic stability increases with in-creasing particle radius (all other factors remaining con-stant). For small particle sizes (<100 nm radius) the pri-mary 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

• We can make a distinction here between two types of ag-gregation:

• Coagulation is the rapid aggregation that happens in the absence of a primary maximum and leads to a strong ir-reversible aggregated structure.

• Flocculation is a reversible aggregation that occurs in a secondary minimum as described. Flocculation is re-versible on the addition of energy to the system, usually the application of a shear field by shaking, stirring or other mechanical processes.

THANKS FOR YOUR ATTEN-TION