On the role of ion size in coagulation

Abstract

It is a well-established fact that the coagulation of colloidal particles suspended in an aqueous medium is affected by the size of the dissolved ions. In general the larger the ions, the more salt is needed to coagulate a suspension. The explanation most commonly given is that the finite size of ions somehow increases the double layer thickness, resulting in a larger critical coagulation concentration (CCC), or more convincingly that the ions adsorb onto colloidal particles and affect the ζ-potential. Without invoking these effects it can be proven that the CCC will increase with ion size because the diffusion and friction coefficients of charged particles are affected by the presence of electrical double layers around them. The reduction in the diffusion coefficient and the increase in the friction coefficient depends explicitly on the size of the ions in the double layer. Provided the ζ-potential is not affected by the size of the ion it is predicted that the CCC increases linearly with the scaled friction coefficient of the ions, in agreement with literature data.