Molecular dynamics study on the evaporation part of the kinetic boundary condition at the interface between water and water vapor

Molecular dynamics simulations of vapor-liquid equilibrium states and those of evaporation from liquid phase into a virtual vacuum are performed for water. In spite of the formation of molecular clusters in the vapor phase and the presence of the preferential orientation of molecules at the interface due to uneven sharing of the bonding electron pair, essentially the same results as in our previous study for argon are obtained. That is, when the bulk liquid temperature is relatively low, the distribution function of evaporation can be expressed as the product of the equilibrium distribution of saturated vapor at the temperature in the bulk liquid phase and a well-defined evaporation coefficient, which is determined as a decreasing function of the liquid temperature, and is found to approach unity with the decrease of the temperature.