Understanding the mechanism for the mass accommodation of ethanol by a water droplet

The interaction of small gas-phase molecules with the liquid/vapor interface of aqueous droplets and their subsequent accommodation into the bulk of the droplet is an integral part of the chemistry of the troposphere. On the basis of an analysis of a kinetic mechanism for the mass accommodation process, Worsnop, Davidovits, and co-workers (J. Phys. Chem. 1996, 100, 13007) predict that most gas-phase molecules must surmount a substantial free energy of activation before becoming solvated into the aqueous phase. In this Letter, molecular dynamics computer simulations are used in conjunction with statistical mechanical perturbation theory to examine the molecular-level details of this process. Due to the availability of experimental data with which to compare our findings, the interaction of ethanol with a H2O lamella was chosen as a prototypical system for study. The calculated equilibrium free energy surface for transporting ethanol across the liquid/vapor interface and into bulk H2O exhibits a barrier to solvation that is 8.2 kcal/mol smaller than that predicted by the Worsnop/Davidovits model. This discrepancy suggests that nonequilibrium solvation or other kinetic effects may dominate the transport of small molecules across the liquid/vapor interface of water.