Ion Solvation in Supercritical Water Based on an Adsorption Analogy

The solvation structure and free energy for Li +, Na +, K +, F -, Cl -, Be 2+, Mg 2+, and Ca 2+ in supercritical aqueous solutions are described by a dielectric concentric shell model incorporating solvent adsorption analogous to a Langmuir model. The ratio of solvent bulk density to the local density in the first coordination shell of alkali and halide ions obtained from molecular dynamics simulation is shown to be linear in bulk density at supercritical temperatures, justifying the model. The model utilizes two parameters: a maximum local density, which is found to be insensitive to temperature and correlated with the surface field, and a desorption constant, which we find may be approximated as zero in the free energy calculations. In comparison to the Born model, free energies of solvation predicted by the concentric shell model show far better agreement with values from full molecular simulation and with those from a compressible continuum model.