Equilibrium properties of charged hard spheres of different diameters in the electrolyte solution regime: Monte Carlo and integral equation results

Extensive Monte Carlo (MC) and integral equation calculations of thermodynamic and structural properties for the 1-1 aqueous electrolyte regime of the primitive model are reported. The salt concentration covers the range 0.1-2 M. The ratio of the radii of the two ionic species a covers the range 0.1-0.8. The MC results are compared to the results of the HNC, MS, and EXP approximations. The excess internal energies calculated in the HNC are found to be in good agreement with the MC values. In the MSA the energy turns out to be too high for large a and too small for small a. For α=0.4 the MSA value is accurate as compared to the simulation. Similar remarks hold for the osmotic coefficient. The radial distribution functions (RDF) calculated in the EXP approximation agree fairly well with the MC results at all molarities and radius ratios investigated, except for α=0.1, where the agreement is only qualitative. The MSA yields RDF which are in qualitative agreement with the MC only at high concentrations and for not too small a. Some preliminary MC results are also reported for the 2-2 aqueous electrolyte solutions at 2 M with α=0.4. An enhancement occurs in the peak of g++(r) and g --(r) with respect to the peak of the corresponding g like(r) of the 2-2 symmetrical case (α=1). This peak is commonly associated with ion clustering. It appears that this effect is more pronounced for ions of different diameters.