Electrostatic interactions in colloidal solutions: Comparison between primitive and one-component models

Abstract

In the framework of the primitive model, colloidal solutions are considered as mixtures of charged hard spheres interacting through unscreened Coulomb potentials. The structure factors are calculated using the mean-spherical approximation (MSA) with a rescaling method (R) which improves the results in the low concentration regime. With a view to study such solutions as one-component systems, different expressions for the polyion–polyion effective pair potential veff(r) are derived which depend on the volume fraction of the colloidal particles and on the finite size of the small ions. With these expressions for veff(r), the conventional RMSA formalism of the one-component model leads to polyion–polyion structure factors which are nearly identical to those obtained in the primitive model. The effective potential presents a larger electrostatic repulsion than the classical DLVO potential. As a consequence, the use of the DLVO expression induces an overestimation of the colloidal charge. Due to its exponential form, the HNC integral equation in the primitive model, compared to the RMSA equation, produces a larger accumulation of counterions on the colloidal surface and, then, a lower effective repulsion between macroions.