Thermodynamics of ion solvation in dipolar solvent using Monte Carlo mean reaction field simulation

Abstract

The mean reaction field approximation has been developed for Monte Carlo simulation of ion solvation thermodynamics and structure in ion–dipolar systems. Calculations have been carried out for systems having one hard-sphere ion and N-1 hard-sphere dipoles of equal diameter, 256⩽N⩽1372, where N is the number of particles. The reduced charge on the ion, q*, was varied from 0 to 14.15, and two values of the reduced dipole moment, µ*, (1.14 and 1.50) were used for the simulations. The mean reaction field corrections were introduced to take into account the changes in long-range dipole–dipole interactions due to the presence of the ion. The corrections were performed for molecules both inside and outside the truncation sphere, which has an ion at the center, resulting in a small dependence of solvation energy on simulation cell size. Expressions have been developed within the framework of thermodynamic integration and perturbation techniques for calculating the free-energy change on ion charging. The applicability of this approach for simulating the solvation internal energy has been investigated and compared with other methods. The influence of system size and cut-off radius on the thermodynamic solvation parameters and the radial distribution functions were studied. It has been shown that for N500, variation of the simulated system size and cut-off radius does not drastically affect the calculated thermodynamic quantities and radial distribution functions when the mean reaction field corrections were applied. The simulated thermodynamic solvation properties were compared with results found in the literature for various molecular statistical approaches [mean spherical approximation (MSA), linearized hypernetted-chain (LHNC), etc.].