Multipole moments of water molecules and the aqueous solvation of monovalent ions

Abstract

The differences in solvation of ions in water play important roles in biology and chemistry, such as the kosmotropic and chaotropic properties of different ions. However, the differences in certain properties of ions in aqueous solutions have been difficult to capture in atomistic computer simulations. Given the uniqueness of water as a liquid, this may be due to oversimplifications in empirical potential energy functions used to describe water in these simulations. Recently, a single-site multipole (SSMP) non-polarizable model of water, which uses multipoles from a quantum mechanical calculation, has been shown to model the properties of liquid water over a range of temperatures and pressures with remarkable accuracy compared to both nonpolarizable and polarizable multi-site models. Thus, it appears to capture features of the average electrostatic potential that other models lack. Here, the ability of SSMP to model the structure of the hydration shell of monovalent ions is investigated. Although only multipoles up to the octupole are needed for water-water interactions, it is shown to be necessary to include the hexadecapole for interactions with charged species, so that the multipole expansion energies are all truncated at order 1/r5. More importantly, it is shown that solvation of cations in SSMP water leads to agreement of the orientational distribution of water molecules with ab initio molecular dynamics results, unlike typical multi-site models. The additional orientation seen in SSMP water may lead to an understanding of the differences in the kosmotropic and chaotropic properties of sodium and potassium ions.