Understanding electrofreezing in water simulations.

Molecular dynamics simulations are used to investigate why external electric fields promote the freezing of liquid water models. It is shown that the melting point of water at a pressure of 1 bar increases significantly when water is polarized by a uniform field. Fields of 1 V/nm and 2 V/nm increase the melting point by 24 K and 44 K, respectively. The increased melting point is mainly due to the favorable interaction of near perfectly polarized cubic ice with the applied field. For a fixed temperature, we demonstrate that the size of the critical ice nucleus decreases with field strength, mostly because the melting point, and hence the true degree of supercooling, is increasing with field. On simulation timescales, ice nucleation is observed at ∼40 K below the field-dependent melting point, independent of the particular value of the field applied. Indeed, we find that even quite highly polarized liquid water retains the characteristic local structures, and the related anomalous properties of water. Our results are obviously relevant to the mechanism of heterogeneous ice nucleation by local surface fields. Local fields will effectively increase the degree of supercooling of locally polarized liquid, decreasing the size of the critical nucleus in the region influenced by the field, hence facilitating ice nucleation.