Stability of colloidal dispersions. Theory for the interaction between particles dispersed in a regular mixture.

Abstract

The interaction between particles dispersed in a binary mixture, composed of equally sized molecules, has been studied theoretically using the multilayer lattice model for adsorption from solution. The excess free energy of interaction (i.e. the deviation of the total free energy of interaction from a simple linear dependence on solution concentration) has been evaluated as a function of the separation of parallel plates (representing the surfaces of the particles) and the concentration of the mixture. The dependence of the interaction between the plates on the differential energy of adsorption, range of surface forces, and solution intermolecular interaction has been studied.

For ideal solutions or solutions showing positive deviations from Raoult's Law, the excess free energy of interaction is an attractive contribution to the total interaction. This additional interaction would tend to coagulate a dispersion. If the two plates have dissimilar adsorption properties then the excess free energy of interaction may be a repulsive contribution to the total interaction energy. This repulsive contribution would tend to prevent heterocoagulation in a mixed dispersion.