Quadrupole-mediated dielectric response and the charge-asymmetric solvation of ions in water

Treating water as a linearly responding dielectric continuum on molecular length scales allows very simple estimates of solvation structure and thermodynamics for charged polar solutes. While this approach can successfully account basic energy ion solvation, computer simulations indicate not only its quantitative inaccuracies but also inability to capture some important aspects microscopic polarization response. Here we consider one such shortcoming, failure distinguish the cations from that otherwise-identical anions, pursue simple, physically inspired modification model address it. The adaptation is motivated by analyzing orientational response an isolated molecule whose dipole rigidly constrained. Its free suggests Hamiltonian fluctuations accounts implicitly influence higher-order multipole moments, while respecting constraints geometry. We propose field theory with suggested form, nonlinear breaks charge symmetry solvation. An approximate variational solution theory, single adjustable parameter, yields energies agree closely simulation results over considerable range solute size charge.