Dynamics near a liquid surface: Mechanisms of evaporation and condensation

The rates of evaporation and condensation under the vapor-liquid equilibrium condition are investigated with a molecular dynamics computer simulation for argon at 80 K, methanol at 300 K, and water at 400 K. The molecular reflection at the surface, which is related to the condensation coefficient α, is divided into two parts, self reflection and molecular exchange. There is no significant difference among the three substances concerning the ratio of self reflection to collision. The total ratio of reflection is estimated as α ≅0.8 for argon, 0.2 for methanol, and 0.4 for water. The ratio of molecular exchange is much larger for methanol and water than for argon, which suggests that the conventional assumption of condensation as a unimolecular process fails for associating fluids.