Hydration of the tetramethylammonium ion: From water structure to the free energy of hydration

We focus on prediction of structural organization of water in the vicinity of molecular ions and relating this water structure quantitatively to the ion hydration free energy. Analogous to results for clusters comprising covalently bonded nonpolar sites, we found that water structure near tetramethylammonium (TMA) ions in various charge states can be predicted sufficiently accurately by using proximity approximation. In particular, we show that a modified two-site proximity approximation that uses pair and triplet correlations with three nearest ion sites is able to capture details of water structure including asymmetry of hydration of positive and negative ions. We integrate the predicted charge densities along with a background charge applied uniformly within the integration sphere to calculate electrostatic potentials at the charge sites. The background charge density is chosen such that Stillinger-Lovett zeroth moment sum rule is satisfied within the integration sphere. Hydration free energies of TMA ion in seven different charge states (-3e,-2e,+3e) calculated using electrostatic potentials are in excellent agreement with molecular simulation results using Ewald summation technique and reproduce accurately the favorable hydration of negative ions with respect to positive ions.

@article{garde1999hydration,
  title   = {Hydration of the tetramethylammonium ion: From water structure to the free energy of hydration},
  author  = {Garde, Shekhar and Hummer, Gerhard and Paulaitis, Michael E.},
  journal = {AIP Conf. Proc.},
  volume  = {492},
  pages   = {202--224},
  year    = {1999},
  doi     = {10.1063/1.1301529}
}