Combined effect of weak electric field and ions on critical water cluster: Insight from molecular dynamics simulation

Abstract

The effects of ion and electric field on the vapor–liquid transition of water, are studied by using the optimal point charge (OPC) water model in this paper. A canonical Molecular Dynamics (MD) simulation approach, which calculates equilibrium time and the number of water molecules in the clusters, is employed to study the factors affecting the nucleation of water molecules. In addition, Quantum Mechanics (QM) method is adopted to calculate the hydration free energy of ions as a supplement. The results demonstrate that the ion has a significant promoting effect on vapor–liquid nucleation below the critical nucleus size (1–3 nm). However, as the cluster grows, the ion will no longer promote cluster growth, due to electrostatic shielding effect from the aggregated water molecules. When a weak uniform electric field (less than 0.1 V/nm) is applied, the promotion of the ion reappears. According to the trajectory analysis of ion in the water droplet, we believe that the electric field changes the relative position of ions in the droplets, thus weakening the shielding of water molecules. It is further demonstrated by a proposed novel statistical method, which provides an alternative, visualized and effective approach to evaluate the promotion effect in similar analyses, compared to radial distribution function.