Ion Solvation in Polarizable Water: Molecular Dynamics Simulations

Abstract

We present the results of molecular dynamics simulations on gas-phase ion water clusters and ion solvation in liquid water using nonadditive many-body potential models. To our knowledge, this is the first simulation model that has led to very good agreement with experiment for the energies of water, ion clusters, and ionic solutions as well the coordination numbers for the aqueous solutions of Na+ and Cl−. We have studied the Na+ ion gas-phase complexes with one to six water molecules. In addition to obtaining good agreement with the experimental enthalpies, the calculated Na+-oxygen radial distribution function (RDF) for the Na+(H2O)6 cluster displays two distinguishable zones; integrating over the first zone yields four water molecules, and the remaining two water molecules belong the second zone. In contrast to the structure of the Na+ complex with four water molecules, the four water molecules around the Cl− ion in C−(H2O)4 are found clustered together in one hemisphere of the ion. These waters form weak hydrogen bonds with each other, resulting in an average water-water binding energy of-4.6 kcal/mol. These results indicate that the stability of the C−(H2O)4 complex arises in part from water-water binding. The coordination number of the Na+ and Cl− ions obtained from ionic solution simulations is approximately 6, in good agreement with experimental results. We have also calculated the water-water interactions in the first hydration shell of Na+ and Cl− solutions to examine the effect of these ions on the water-water interactions. We found the water-water interactions in this region of the Cl− solution are positive and ~4 kcal/mol less repulsive than the corresponding water-water interactions for the Na+ solution. Thus, the structure in the first hydration shell of full ionic solution simulation of the anion appears to have significantly different character from that of the gas-phase anion-water cluster. In addition, we found the water molecules between the first and second hydration shells are strongly mobile. Finally, we find it to be essential to include the three-body potential (ion-water-water) in the simulation of the ionic solution to obtain quantitative agreement with the experimental solvation enthalpies and coordination numbers. © 1991, American Chemical Society. All rights reserved.