THE STRUCTURE OF THE SURFACE OF ETHANOL-WATER MIXTURES

Neutron and X-ray grazing incidence reflection have been used to study the structure of the surface of ethanol/water solutions at compositio ns 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 th e surface of small molecule mixtures. The paper describes how isotopic substitution may be used to provide reflection data which give a reas onably model independent structure of the interface. The surface exces ses determined from the neutron results are approximately consistent w ith previous thermodynamic measurements, the area per adsorbed ethanol molecule varying from 54 +/- 5 angstrom2 at the lowest concentration to 23 +/- 2 angstrom2 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 angstrom2 over the concentration range studied. This distrib ution is centred at 3.0, 2.6, and 2-3 +/- 0.3 angstrom above the plane of zero water absorption, at the three concentrations 0.022, 0.045 an d 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 e thanol molecule is 5-5 angstrom and allowing for some roughness of the surface, which has been determined approximately from X-ray reflectio n, the thickness and position of the ethanol layer suggests that the m olecule may be partially oriented with the ethyl group pointing toward s the vapour phase.