|Thousand of years ago, the Eqyptions 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.
|Stabilization of Aqueous
Concept Zhion, Spring 2006
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
To stablize the system, a repulsive interaction among the colloidal
particles must be overcome the attractive force. The simpliest way to
obtain this repulsive interaction is through "charge repulsion" or
electrostatic stablization among the particles
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 sensative to the ionic strength of the
medium; (2) It alows a high solid content dispersion with low
viscosity; (3) It is equally effective in aqueous and nonaqueous
dispersion media. Consequently, steric stablization is widely used in
various industrial products such as paints, inks, pharmaceutical and
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 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.
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.
When the free, non-adsorbed polymer concentration continues to
increase to a concentration that the colloids redisperse or restabilize