Many personal care and cosmetic formulations consist of suspensions or emulsions. Their stabilization against flocculation and/or coalescence requires the presence of an energy barrier between the particles or droplets to 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 the formation of an electrical double layer (e.g., by the use of ionic surfactants). When two particles or droplets approach to a distance of separation where the double layers begin to overlap, strong repulsion occurs provided the surface or zeta potential is sufficiently high and the electrolyte concentration and valency of the ions is low. This stabilization mechanism is seldom used in practice because it is difficult in most cases to maintain low electrolyte concentrations.
An alternative and more effective stabilization is produced when using nonionic surfactants or polymers, usually referred to as steric stabilization. When two particles or droplets approach each other to a separation distance such that the adsorbed layers begin to overlap, repulsion occurs as a result of two mechanisms: (i) Unfavorable mixing of the surfactant or polymer layers when these are in good solvent conditions; and (ii) Reduction in configurational entropy on significant overlap.
In this paper, we will describe the criteria for effective steric stabilization, particularly when using block and graft copolymers. This will be followed by a section describing a novel polymeric surfactant that was based on inulin (polyfructose). Examples will be given to illustrate the effectiveness of this polymeric surfactant in stabilization of emulsions, suspensions, nano-emulsions and multiple emulsions.
Excerpt Only This is a shortened version or summary of the article that appeared in the Apr.1, 2004 issue of Cosmetics & Toiletries magazine. The full content is not currently available online.