Why monovalent salt reduces charge inversion of macroion by trivalent counterions

Abstract

A macroion with a strongly charged negative surface in 3:1 salt water solution becomes net positive due to adsorption of an excessive number of positive 3-ions. The widely accepted theory of such charge inversion is based on the idea that adsorbed at the macroion surface 3-ions form a two-dimensional strongly correlated liquid which attracts excessive 3-ions by their negative images (correlation holes). In the absence of 1:1 salt this theory reasonably agrees with experiments and numerical simulations. However, the same theory predicts that adding a large concentration of 1:1 salt increases the net positive charge, while experiments and simulations show that the charge decreases. In this paper we argue that the theory was aiming at the case of a very large macroion surface charge and ignored the effect of the competing attraction of 3-ion to its screening atmosphere in the bulk of the solution. We show that for parameters of experiments and simulations this effect makes charge inversion weak and extremely sensitive to screening by large concentration of 1:1 salt. As a result charge inversion decreases with addition of 1:1 salt in agreement with experiments and simulations.