Molecular dynamics investigation of the surface/bulk equilibrium in an ethanol–water solution

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Abstract
Molecular dynamics simulations of the vapour/solution interface, initiated from an ethanol–water mixture at a mole fraction of ethanol, Xe= 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 oriented such that the alkyl group points out of the solution. A ‘depletion layer’ of enhanced water density beneath the ethanol surface excess was revealed by the simulation. Analysis of the structure involving molecules close to the surface shows that water molecules are arranged to maximize the hydrogen bonding between the oriented ethanol and the adjacent water molecules. The enhanced water density beneath the interface results from this optimization. The surface tension calculated from the simulation is ca. 50% higher than the results of macroscopic experimental measurements. This overestimation is attributed to shortcomings in both the potential function and the simulation methodology. The number density profiles of the ethanol surface excess were compared with results from neutron reflectivity measurements. The calculated profiles agree better with the experimental measurements at the calculated surface tension (i.e. Xe= 0.022) than those reported for Xe= 0.1 mixtures.

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