A perturbation theory and simulations of the dipole solvation thermodynamics: Dipolar hard spheres

Padi truncation of the thermodynamic perturbation theory is used to calcula te the solvation chemical potential of a dipolar solute in a model fluid of dipolar hard spheres. Monte Carlo simulations of the solvation thermodynam ics are carried out over a wide range of solute and solvent dipoles in orde r to address the following major issues: (i) testing the performance of the Padi perturbation theory against simulations, (ii) understanding the mecha nism of nonlinear solvation, and (iii) elucidating the fundamental limitati ons of the dielectric continuum picture of dipole solvation. The Pade form of the solvation chemical potential constructed in the paper agrees with th e whole body of simulation results within an accuracy of 3%. Internal energ y and entropy of solvation are also accurately described by the perturbatio n treatment. Simulations show a complex nonlinear solvation mechanism in di polar liquids: At low solvent polarities the solvation nonlinearity is due to orientational saturation that switches to the electrostriction mechanism at higher dipolar strengths of the solvent. We find that the optimum cavit y radius of the Onsager reaction-field theory of solvation depends substant ially on solvent polarity. A general method of testing the performance of l inear solvation theories is proposed. It shows that the fundamental failure of continuum theories consists in their inaccurate description of the inte rnal energy and entropy of solvation. (C) 1999 American Institute of Physic s. [S0021-9606(99)04301-9].