Evaporation through monolayers-theoretical treatment of the effect of chain length

A model has been developed to explain the observed exponential dependence of evaporation resistance on chain length in monolayers of long-chain compounds on a water surface. The model postulates that the permeant molecule passes through a series of potential energy wells as it proceeds through the monolayer. Application of the steady-state approach leads to an expression for the dependence of evaporation resistance on chain length in terms of the rate constants for upward (k) and downward (k) movement of permeant molecules between potential energy wells. By adjusting the relative magnitudes of k and k, either a linear (Fick's law) or exponential dependence of evaporation resistance on chain length can be derived. Literature data for longchain alcohols and acids show the exponential dependence and imply that k = 1.6 k