Molecular Dynamics Simulation of Na(+)-Cl(-) Ion-Pair in Water-Methanol Mixtures under Supercritical and Ambient Conditions.

Abstract

Constrained molecular dynamics simulations have been performed to investigate the structure and thermodynamics of Na(+)-Cl(-) ion-pair association in water-methanol mixtures under supercritical and ambient conditions in dilute solutions. From the computed potentials of mean force (PMFs) we find that contact ion pairs (CIPs) are more stable than all other associated states of the ion pairs in both ambient and supercritical conditions. Stabilities of CIPs increase with increase in the mole fraction of methanol. In supercritical conditions, major changes in PMFs occur as we go from x(methanol) = 0.00 to x(methanol) = 0.50. The stable solvent shared ion pair (SShIP) which occurs in x(methanol) = 0.00 and 0.25, vanishes when x(methanol) is 0.50 or greater. The stabilities of these ion pairs increase with increasing temperature. Local structures around the ions are studied using the radial distribution functions, density profiles, angular distribution functions, running coordination numbers and excess coordination numbers. Preferential solvation analysis shows that both Na(+) and Cl(-) ions are preferentially solvated by water. From the calculation of enthalpies and entropies, we find that Na(+)-Cl(-) ion-pair association in water-methanol binary mixtures is endothermic and driven by entropy both in ambient as well as under supercritical conditions.

DOI: 10.1021/acs.jpcb.5b05401

Cite this paper

@article{Keshri2015MolecularDS, title={Molecular Dynamics Simulation of Na(+)-Cl(-) Ion-Pair in Water-Methanol Mixtures under Supercritical and Ambient Conditions.}, author={Sonanki Keshri and Atanu Sarkar and B. L. Tembe}, journal={The journal of physical chemistry. B}, year={2015}, volume={119 50}, pages={15471-84} }