Adsorption and Solvation of Ethanol at the Water Liquid-Vapor Interface: A Molecular Dynamics Study

Michael A. Wilson and Andrew Pohorille

Exobiology Branch
NASA -- Ames Research Center
MS 239-4
Moffett Field, California 94035-1000

Department of Pharmaceutical Chemistry,
University of California, San Francisco,
San Francisco, California 94143

Abstract:

The free energy profiles of methanol and ethanol at the water liquid-vapor interface at 310 K were calculated using molecular dynamics computer simulations. Both alcohols exhibit a pronounced free energy minimum at the interface, and, therefore, have positive adsorption at this interface. The surface excess was computed from the Gibbs adsorption isotherm, and found to be in good agreement with experimental results. Neither compound exhibits a free energy barrier between the bulk and the surface adsorbed state. Scattering calculations of ethanol molecules from a gas phase thermal distribution indicate that the mass accommodation coefficient is 0.98, and the molecules become thermalized within 10 ps of striking the interface. It was determined that the formation of the solvation structure around the ethanol molecule at the interface is not the rate-determining step in its uptake into water droplets. The motion of an ethanol molecule in a water lamella was followed for 30 ns. The time evolution of the probability distribution of finding an ethanol molecule that was initially located at the interface is very well described by the diffusion equation on the free energy surface.