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A theoretical derivation of condensation coefficient based on transition state theory is presented in this paper by considering the three-dimensional movement of condensing molecules in the liquid–vapor interface region. The theoretical expression is a function of free volume ratio of liquid to vapor and activation energy for condensation. We have developed an evaluation of the activated state conditions in the interface region with the use of molecular dynamics (MD) simulations for argon and water. From the molecular scale consideration, it is found that a characteristic length ratio 3Vl/Vg has an important role in evaluating the condensation coefficient because the restricted translational motion is dominant in the condensation process compared with the rotational motion. Present theoretical values agree well with MD results in both monatomic and polyatomic polar molecules. Finally, we conclude that the condensation coefficient is an inherent physical property of a given pure liquid–vapor interface and the interface structure plays a primary role in condensation.

Molecular dynamics (MD) simulations are carried out for water using two kinds of intermolecular potentials, the Carravetta−Clementi (C−C) model and the extended simple point charge (SPC/E) model, to understand the mechanism of interface mass transfer between liquid and vapor. Effects of different interface structures on the condensation process are investigated, and computational data on the condensation coefficient are presented. By changing incident conditions such as the translational and rotational energies of the incident molecules on the liquid surface, we find that the condensation coefficient of water primarily depends on the translational energy and the surface temperature, as is the case for a simple gas such as argon. The molecular exchange phenomenon caused by incident molecules has no marked influence on the condensation coefficient. A formula for the condensation coefficient is summarized as a function of the surface-normal component of the translational energy and the surface temperature. Also, relations between the surface structure and the condensation coefficient are discussed based on the transition state theory developed in our previous study. The paper demonstrates that the theory can explain the MD data very well, and it is concluded that the translational motion is important compared with the rotational motion, even for polyatomic molecules.

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