Dispersions of Silica Particles in Surfactant Phases

Addition of surfactants to aqueous dispersions of silica may either enhance or depress the colloidal stability of the dispersions. With a nonionic surfactant that forms micelles (Triton X-100), dilute silica dispersions have enhanced stability if the silica particles are fully covered with adsorbed surfactant micelles. Concentrated dispersions are also stable provided that the particles remain separated by at least one layer of micelles. Beyond full coverage, excess micelles may cause depletion flocculation of the silica particles. With a nonionic surfactant that forms vesicles and bilayers (polyoxyethylene alkyl ether C12E4), the effects of surface coverage are the same; however, the critical separation between particles is the thickness of two bilayers. Beyond full coverage, the surfactant forms a lamellar phase which excludes the particles.