Viewing the born model for ion hydration through a microscope

The hydration-free energy, energy, and entropy of monovalent ions are calculated using the extended RISM integral equation theory and computer simulations. Plots of these thermodynamic quantities against 1/R, where R is the Lennard–Jones radius, lie on two distinct curves corresponding to cations and anions. This result is attributed to differences in the microscopic structure of solvent surrounding the ions. Charge distribution functions are used to analyze solvent structure. It is found that the modified Born formula proposed by the Latimer et al. gives good agreement with the RISM results for the energy and the free energy, but not for the entropy. A microscopic interpretation of Latimer formula is attempted in light of the statistical mechanical theory.