SURFACE SEGREGATION OF A BINARY-LIQUID MIXTURE AS STUDIED BY METASTABLE IMPACT ELECTRON-SPECTROSCOPY

We have investigated the free liquid surface of a binary mixture of formamide and benzylalcohol using MIES (= metastable impact electron spectroscopy) and UPS. Whereas the latter samples information down to a depth of approximately 6 angstrom the method MIES is sensitive exclusively for the topmost molecular layer. The combination of both techniques allows us to determine the parameters of an exponential segregation profile. As expected from its smaller surface tension benzylalcohol is strongly enriched at the surface. The segregation depth is greatest (approximately 12 angstrom) at small molar fractions of benzylalcohol. Once the segregation profile is established it is straightforward to compute the surface excess. We find very good agreement with the prediction of the Gibbs equation.