NONLINEAR RESPONSE IN IONIC SOLVATION - A THEORETICAL INVESTIGATION

The Born model of ionic solvation assumes that the solvent is a contin uum which has a linear response and does not explicitly include nonlin ear effects such as dielectric saturation and electrostriction. Noneth eless, the Born model has been surprisingly successful in estimating t he solvation energies of ions in solution. Recently, we developed a si mple quasicontinuum theory of ionic solvation, referred to here as the HBI model [J.-K. Hyun, C. S. Babu, and T. Ichiye, J. Phys. Chem. 99, 5187 (1995)]. The HBI model exhibits dielectric saturation, which is d emonstrated in the expressions for the average orientation of the solv ent molecules and the solvation energy, but does not incorporate the e ffects of electrostriction. In this article, new models of ionic solva tion are developed that exhibit not only dielectric saturation but als o electrostriction. Expressions for the radial and orientational distr ibution functions of the solvent molecules, and the solvation energy a re derived and compared with those from the HBI and Born models to exa mine the characteristics of different models of ionic solvation. Compa risons are also made to molecular dynamics simulations of ions in aque ous solution, where the complex molecular effects make separation of d ifferent nonlinear effects difficult. In particular, the systematic di screpancies in the solvation energies from the different models are us ed to understand how different nonlinear effects contribute to the res ponse when water is the solvent. For small, monovalent ions, the succe ss of the Born model is attributed not to the lack of dielectric satur ation or electrostriction, but rather to the cancellation of one effec t by the other. (C) 1998 American Institute of Physics.