Thousand of years ago, the Eqyptians and Chinese used naturally occurring polymers to stabilize colloidal dispersions. They mixed carbon black and a polymeric stabilizer (e.g. gum Arabic or egg albumin) and then dried the mixture into a pen-like shape. When they needed ink, they dipped the "pen" into water. The carbon black was redispersed immediately to form the ink. In fact, the polymer formed a protective sheath around the colloidal particles to prevent their aggregation. Today, stabilization of colloidal dispersions is widely applied in the paint, electronics, ceramics, oil recovery, food, waste treatment and pharmaceutical science.
In pharmaceutical industry, aqueous colloidal dispersions are used as drug carriers and film-coating materials. In the film coating, the water-insoluble polymer is dispersed in an aqueous medium and sprayed onto tablets or beads. For a uniform film coat, the colloidal dispersion must be stable. In
addition, aggregated particles may clot the spray nozzles causing trouble to the coating process or even interrupt the film formation process.
DISPERSION
When a solid is dispersed into colloidal-sized particles in a continuous medium, the total interfacial area as well as the "interfacial free energy" are increased. This system is very thermodynamically unstable. Consequently, the colloidal particles aggregate rapidly.
ELECTROSTATIC STABILIZATION
To stabilize the system, a repulsive interaction among the colloidal particles must be overcome the attractive force. A simple way is to obtain this repulsive interaction is through "charge repulsion" or electrostatic stabilization among the particles
STERIC STABILIZATION
Steric stabilization involves the adsorption of polymers onto the particle surface to prevent particle aggregation.
Steric stabilization has several advantages over electrostatic stabilization: (1) It is less sensitive to the ionic strength of the medium; (2) It allows a high solid content dispersion with low viscosity; (3) It is equally effective in aqueous and nonaqueous dispersion media. Consequently, steric stabilization is widely used in various industrial products such as paints, inks, pharmaceutical and food dispersions.
CATASTROPHIC FLOCCULATION
A minimum thickness of the adsorbed layer is needed for steric stabilization. Its thickness is strongly affected by the medium solvency. A decrease in solvency for the stabilizing moieties will cause a "catastrophic flocculation".
BRIDGING FLOCCULATION
Bridging flocculation is related to the adsorption of polymers to particle surface, as what happens in steric stabilization. However, the polymer here is so "big" that the polymer attaches to a few particles simultaneously. In this situation, the polymer is a flocculant instead of a stabilizer.
DEPLETION FLOCCULATION
When free, non-adsorbed polymer concentration increases and reaches the critical flocculation concentration, a sudden change such as an increase in viscosity in dispersion behavior occurs.
DEPLETION STABILIZATION
When the free, non-adsorbed polymer concentration continues to increase to a concentration that the colloids redisperse or restabilize again.
