Structure of Colloidal Solution in Presence of Mixed Electrolytes: A Solvent Restricted Primitive Model Study

The structure of colloidal solution in presence of mixed electrolytes is studied using Monte Carlo simulation and density functional theory, based on a four-component model of the spherical double layer. In this model the ions and solvent molecules are treated as charged and neutral hard spheres, respectively, having equal diameter, and in addition the mixture of mono- and multivalent co-ions are also considered. The macroion is considered as a uniformly charged hard sphere surrounded by the electrolyte and the solvent. The density functional theory is based on a partially perturbative scheme, where the electrical part is calculated through perturbation with respect to uniform density and the hard sphere contribution is approximated using a weighted density approach. The theory is found to be in quantitative agreement with the Monte Carlo simulation results, for singlet density as well as the mean electrostatic potential profiles. The system is studied over a wide range of parametric conditions, viz, with different ionic valences as well as size, at varying electrolyte concentration ratio of mono- and multivalent co-ions of mixed electrolytes, at different surface charge densities, and radius of the macroion. The present work reflects that even a simple primitive model for the solvent is able to manipulate the hard-sphere and electrostatic correlations of the diffuse double layer in the ionic density as well as mean electrostatic potential profiles.