Study on liquid–vapor interface of water. I. Simulational results of thermodynamic properties and orientational structure

Abstract

Molecular dynamics simulations have been carried out for liquid–vapor interface of water and also of Lennard-Jones system. Surface tension and surface excess energy are calculated, from which surface excess entropy is evaluated. These thermodynamic quantities suggest the existence of some liquid-structural change near the surface of water, which is not seen in Lennard-Jones system. For water, orientational structuring near the surface is studied and two types of orientation are found. In the vapor side, a water molecule has a tendency of projecting one hydrogen atom toward the vapor, and in the liquid side, a molecule prefers to lie down on the surface with both hydrogen atoms slightly directed to the liquid. From these results, surface potential χ can be evaluated to be about +0.16 V at T=300 K, which confirms recent experimental results. The ellipticity coefficient is also discussed and the assumption often used in analysis of experimental results of ellipsometry is found to be inadequate for water due to the structural change. These observed orientational orderings are qualitatively consistent with results of molecular theory of surface recently developed and also with simulational results of water near hydrophobic walls.