Critical adsorption at the liquid/vapor surface of multicritical mixtures

Abstract

Preferential adsorption at the liquid/vapor surface of a critical binary liquid mixture gives rise to a position dependent order parameter Θ(z). Near an ordinary critical point the excess adsorption, is the reduced temperature and β and ν are the usual critical exponents. Bulk multicritical points will modify critical adsorption due to exponent renormalization. The critical binary liquid mixture D 2 O + 3-methylpyridine exhibits a closed-loop coexistence curve with the two phase region existing between upper (T UC ) and lower (T LC ) critical temperatures. On addition of ordinary water, T UC and T LC approach each other and coalesce for a specific mole fraction of H 2 O where the closed-loop coexistence curve has shrunk to a point at T DCP = T UC = T LC . At this double critical point (DCP) the exponents β and ν are expected to be renormalized to 2β and 2ν. We compare ellipsometric measurements of the adsorption near this double critical point with the same phenomena near an ordinary single critical point. Surprisingly we find that near the DCP Γ~ t −1.0 experimentally, rather than ~t −0.6 expected theoretically. The measured Γ dependence is believed to be due toβ not renormalizing in the one phase region; however an adequate theoretical interpretation of this effect is at present lacking.