Solvent effects on the effective interactions among colloidal particles: a simple molecular model

A simple analysis of the effect of the molecular structure of a polar solvent on the effective interactions among the components of an ionic solution is presented. For this purpose we introduce a simplified rigid interaction site model (RISM) for the water molecules which only includes three site charges and neglects any other (short-ranged) interaction. The small ions in the suspension are considered of vanishingly small diameter, and the macroparticles have finite diameter. The correlations among the water components and the point ions are treated with the simplest linear approximation. The effective interactions among the ions obtained after ‘contracting’ the water have a Coulomb form modulated by a set of distance-dependent dielectric functions. The effective interactions among the macroions obtained after ‘contracting’ the supporting electrolyte (water molecules plus small ions) has in the asymptotic limit the same screened Coulomb form of the Derjaguin–Landau–Werwey–Overbeek (DLVO) model, but with different values of the renormalized charges. For a suspension of neutral macroparticles the effective interaction among them becomes weakly attractive and short-ranged.