MOLECULAR-DYNAMICS INVESTIGATION OF THE SURFACE BULK EQUILIBRIUM IN AN ETHANOL-WATER SOLUTION/

Molecular dynamics simulations of the vapour/solution interface, initi ated from an ethanol-water mixture at a mole fraction of ethanol, X(e) = 0.1, have been performed. Rapid redistribution of ethanol molecules to the free surface was observed during simulations of two different sized systems. Inspection of the calculated surface structure reveals that after redistributing the ethanol molecules are preferentially ori ented such that the alkyl group points out of the solution. A 'depleti on layer' of enhanced water density beneath the ethanol surface excess was revealed by the simulation. Analysis of the structure involving m olecules dose to the surface shows that water molecules are arranged t o maximize the hydrogen bonding between the oriented ethanol and the a djacent water molecules. The enhanced water density beneath the interf ace results from this optimization. The surface tension calculated fro m the simulation is ca. 50% higher than the results of macroscopic exp erimental measurements. This overestimation is attributed to shortcomi ngs in both the potential function and the simulation methodology. The number density profiles of the ethanol surface excess were compared w ith results from neutron reflectivity measurements. The calculated pro files agree better with the experimental measurements at the calculate d surface tension (i.e. X(e) = 0.022) than those reported for X(e) = 0 .1 mixtures.