Ellipsometric study of undersaturated critical adsorption.

At the liquid-vapor surface of a critical AB binary liquid mixture, if the surface energies sigma(A)<<sigma(B), then component A completely saturates the surface and one is in the regime of strong critical adsorption where the local volume fraction at the surface is solely a function of z/xi where xi is the bulk correlation length and z is the depth into the liquid. If, however, sigma(A) approximately sigma(B) then the surface composition is a sensitive function of the surface energy difference Deltasigma=sigma(A)-sigma(B) and the surface is "undersaturated." We study this undersaturated critical adsorption regime using a homologous series of critical binary liquid mixtures. Component B (methyl formate) is fixed, while component A is varied from n-undecane to n-tetradecane. With increasing carbon chain length Deltasigma changes systematically from a negative to a positive value. We find that the experimental results, in both the one- and two-phase regions, are well described by a universal surface scaling function G(z/xi,h(1)t(-Delta(1))), where h(1) approximately Deltasigma, t=[T(c)-T]/T(c) is the reduced temperature relative to the critical temperature (T(c)), and Delta(1) is a universal surface critical exponent. These results are in conformity with theoretical predictions.