Specific ion effects on double layer forces through ion hydration

An attempt is made to explain the dependence of the strength of repulsion between two charged parallel plates immersed into an electrolyte solution on the nature of the electrolyte. The theory provides a sequence of ions which almost coincides with the Hofmeister series for both anions and cations. The repulsive force is calculated by taking into account that the ions are hydrated; this hydration generates a negative volume exclusion entropy, which is considered to be responsible for ion specificity. In contrast to the traditional theory in which the electrolyte solution is considered to be ideal, the present one includes a nonideal contribution caused by ion hydration. It will be also shown that at high salts concentrations the anions of the same counterion provide strong specificities (large differences between the repulsive interactions of successive ions) for positively charged surfaces, and the cations of the same counterion strong specificities for negatively charged surfaces.