ABSTRACT
I. INTRODUCTION Many industrial formulations consist of dispersions of the solid-liquid (suspensions) or liquid-liquid (emulsions) types. The stabilization of these dispersions against flocculation and/or coalescence requires the presence of an energy barrier between the particles or droplets that prevent their close approach where the van der Waals attraction is large. Two general mechanisms of stabilization can be applied. The first, referred to as electrostatic stabilization, is based on charge separation and formation of electrical double layers [l]. The double layer is characterized by a surface charge that is compensated by unequal distribution of counter and coions. When two particles or droplets approach to a distance of separation h that is smaller than twice the double layer thickness, repulsion occurs since the double layers cannot be fully developed. The magnitude of repulsion depends on the surface
(or~) potential and electrolyte concentration and valency. At low electrolyte concentrations, the double layers are extended and the repulsive energy at intermediate distances becomes larger than the van der Waals attraction, producing an energy barrier that prevents approach of the particles or droplets. This picture forms the basis of the theory of colloid stability developed independently by Deryaguin and Landau [2] and Verwey and Overbeek [3] more than 50 years ago (DLVO theory). The production of charge on particle or droplet surfaces can be due to the presence ofionogenic groups or by adsorption of ionic surfactants. Unfortunately, this stabilization mechanism is seldom sufficient in practice since most practical industrial formulations contain high amounts of electrolyte. In addition, ionic surfactants may not be strongly adsorbed on the particle surface and on close approach desorption may take place. An alternative approach is to use nonionic surfactants of the ethoxylate type that may tolerate high electrolyte concentrations. However, as with ionic surfactants such molecules are seldom strongly adsorbed on the particle surface. The most effective procedure for
stabilization of dispersions is to use surface active polymers (to be referred as polymeric surfactants) which not only adsorb very strongly on the particle or droplet surface but also can be applied in the presence of high electrolyte concentrations and at high temperatures. This forms the second mechanism of stabilization that is usually referred to as steric stabilization [4]. The polymeric surfactant molecule can be specifically designed to have a strong "anchor" chain and a "stabilizing" chain that extends from the surface giving a layer thickness 6 that is several nanometers thick. When two particles or droplets approach to a distance of separation h that is smaller than 26, the stabilizing chains may undergo overlap and/or become compressed. The chains will also lose configurational entropy in the overlap region. When these chains are in good solvent conditions, such overlap is unfavorable and this leads to strong repulsion that increases very sharply with decrease of h when h < 26.
