METASTABILITY OF COLLOIDAL SILICA SOLUTIONS AT HIGH SUPERSATURATIONS

Abstract

The thermodynamic description of the breakdown of supersaturated solutions to form heterogeneous colloidal systems is quite a complex problem. In particular, it remains unclear what causes highly saturated solutions to break down to give stable colloidal disperse systems, such as sols and gels of amorphous silica, or oil-in-water microemulsions. Here, in terms of the formalism of the thermodynamics of small systems, we showed that the thermodynamic potential of highly supersaturated colloidal solutions of amorphous silica has a local minimum related closely to the condition that the number of colloidal particles in these solutions is maximum, which can be attained in a reversible isothermal process. It is this local minimum that may ensure the stability of these solutions. Let us consider a closed supersaturated two-component system comprising N s molecules of solvent and N t molecules of solute at temperature T and pressure P . With regard for the low solubility of silica, we suppose that the solution is diluted ( N s @ N t ), and that the volume of the system V is equal to the solvent volume. Any solution can be regarded [1] as a microheterogeneous colloidal system. Let us examine the case when the initial supersaturation of the solution is above the critical one, such that the rate of formation of primary particles of the critical radius is high, and their abundant production is not a limiting step for the processes in this disperse system. In terms of the thermodynamics of small systems [2, 3], the Gibbs potential of the disperse system, and its change in a process at constant P, T, and N s are respectively written as [3]