Determining the solvation free energies of single ions in water and the distribution of ions at the air-water interface are two of the most important problems in physical chemistry and yet both remain unresolved. A key reason for this is that the contribution from the surface potential of the air-water interface is poorly understood and very difficult to determine experimentally. Here, we provide rigorous definitions for four different types of single ion solvation free energies based on different treatments of the surface potential. We discuss the motivation behind each of the definitions and show how to convert between them. We then calculate the four solvation free energies for positive and negative charged hard spheres using ab initio molecular dynamics (AIMD). In contrast with classical molecular dynamics (CMD) simulation, we show that Ewald summation leads to highly unphysical values for the solvation free energy due to the large positive surface potential of real water. We also show that water responds linearly to cations but that there is a small non-linearity for small anions and that water's preference for solvating anions is much larger than for CMD models. The implications for the distribution of ions at the air-water interface is also discussed. These results have important implications for coarse grained models of electrolyte solution.

Surface potential of water with <i>ab initio</i> molecular dynamics

253rd National Meeting (2017)

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Surface potential of water with ab initio molecular dynamics