On the Kinetics of Evaporation

Abstract

The kinetics of evaporation of liquids has been reconsidered from the point of view of classical kinetic reactions and also by application of the theory of absolute reaction rates. It is shown that evaporation treated as a unimolecular rate process, with a rate proportional to the surface concentration of energetic molecules, leads to the Knudsen equation for spherical molecules provided 6 square terms contribute to the energy of activation. As was pointed out in an earlier publication, the theory of absolute reaction rates, after correction for lack of equilibrium between normal molecules and the activated complex, leads to the Knudsen equation for spherically symmetric molecules if reasonable assumptions are made concerning the nature of the activated complex. Evidence is presented in support of the idea that the equilibrium theory of absolute reaction rates is not consistent with the model of the liquid used to determine evaporation rates. The theoretical treatment is next extended to polar liquids with restricted rotation and it is shown that the evaporation coefficient should be identified with the free-angle ratio, a conclusion which has been verified quantitatively by Wyllie.