ADSORPTION AND SOLVATION OF ETHANOL AT THE WATER LIQUID-VAPOR INTERFACE - A MOLECULAR-DYNAMICS STUDY

The free energy profiles of methanol and ethanol at the water liquid-v apor interface at 310 K were calculated using molecular dynamics compu ter simulations. Both alcohols exhibit a pronounced free energy minimu m at the interface and, therefore, have positive adsorption at this in terface. The surface excess was computed from the Gibbs adsorption iso therm and was found to be in good agreement with experimental results, Neither compound exhibits a foe energy barrier between the bulk and t he surface adsorbed stale, Scattering calculations of ethanol molecule s from a gas phase thermal distribution indicate that the mass accommo dation coefficient is 0.98, and the molecules become thermalized withi n 10 ps of striking the interface. II was determined that the formatio n of the solvation structure around the ethanol molecule al the inter- face is not the rate-determining step in its uptake into water droplet s. The motion of an ethanol molecule in a water lamella was followed f or 30 ns., The time evolution of the probability distribution of findi ng an ethanol molecule that was initially located at the interface is very well described by the diffusion equation on the free energy surfa ce.