Thermodynamic factors of the sol—gel transition I. Reversible coagulation of organophilic and hydrophilic colloids

Abstract

The coagulation of alcoholic dispersions of organophilic silica (methylated aerosil) by water and coagulation of aqueous dispersions of hydrophilic silica (nonmodified aerosil) by the cationic surfactant cetylpyridinium bromide (CPB) is studied at low and high ionic strength of the medium. Both organophilic dispersions at small water concentrations and hydrophilic dispersions at small CPB concentrations are resistant to electrolyte coagulation. When the corresponding concentrations are sufficiently high, coagulation proceeds even in the absence of added salt. The critical water or CPB coagulation concentration increases on decreasing the ionic strength. All of the data, including the peculiar trends in coagulation rates and the reversibility tests, indicate that the stability-coagulation transition in the systems is thermodynamic rather than kinetic.