The structure of the surface of ethanol/water mixtures

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Abstract
Neutron and X-ray grazing incidence reflection have been used to study the structure of the surface of ethanol/water solutions at compositions with a mole fraction (x e) of ethanol of 0·022, 0·045, 0·10, and 0·24. This is the first time reflection has been used to investigate the surface of small molecule mixtures. The paper describes how isotopic substitution may be used to provide reflection data which give a reasonably model independent structure of the interface. The surface excesses determined from the neutron results are approximately consistent with previous thermodynamic measurements, the area per adsorbed ethanol molecule varying from 54 ± 5 Å2 at the lowest concentration to 23 ± 2 Å2 at the highest concentration studied. The width of the ethanol excess (full width at half height) was found to be 6–6·5 ± 1·5 Å2 over the concentration range studied. This distribution is centred at 3·0, 2·6, and 2·3 ± 0·3 Å above the plane of zero water absorption, at the three concentrations 0·022, 0·045 and 0·10 (x e). This means that there is significant projection of the ethanol molecule into the vapour region. Given that the length of an ethanol molecule is 5·5 Å and allowing for some roughness of the surface, which has been determined approximately from X-ray reflection, the thickness and position of the ethanol layer suggests that the molecule may be partially oriented with the ethyl group pointing towards the vapour phase.

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