| Summary: | Neutron reflection has been used to determine the coverage and distribution along the surface normal direction for the air/solution interface of solutions of phenol in water. The coverages are in reasonable agreement with surface tension determinations of the surface excess over the whole range of concentration up to the solubility limit of 8.6 wt % of phenol. At the highest coverages the layer is found to be substantially thicker than expected for a layer one molecule thick, with a full width at half-height of 17 ± 2 Å, which becomes about 14 Å after allowance for the contribution to the roughening of the surface by capillary waves. The experimental results at high coverage disagree with the only available computer simulation which predicted a full width at half-height much less than that measured in the neutron reflection experiment, for example, about 5 Å at a coverage where the neutron measurements give 14 Å after removal of an estimated capillary wave contribution, which was not included in the simulation. It is suggested that in such a diffuse layer the phenol molecules are unlikely to be inclined at a single angle to the surface normal, as suggested from the interpretation of second harmonic generation measurements. The agreement between the simulation and neutron reflection is good at about half the maximum coverage, where the phenol appears to form a single molecular layer. At the highest coverages, the width of the phenol distribution is comparable with that obtained for many surfactant layers, although surfactant molecules are intrinsically much longer, and the layer seems to have a multilayer character unlike those found for the short chain alkanols.
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