CRITICAL ADSORPTION OF FLUIDS AND THE EQUIVALENCE OF EXTRAORDINARY TO NORMAL SURFACE TRANSITIONS

The current understanding of the critical adsorption of fluids on walls or interfaces is reviewed, with the emphasis put on a description of what can be learned from field-theoretic renormalization group (RG) approaches. In particular, results of a recent RG calculation of the universal order-parameter profile are presented. The prototypical continuum model describing critical adsorption is a standard semi-infinite phi4 model with surface terms integral dS(-1/2 g0phi2-h1phi). For h1 = 0, this model describes surface phase transitions called ordinary, special, and extraordinary, depending on whether the surface enhancement g0 is less than, equal, or greater than a certain critical value. For typical chemical systems one expects g0 to be subcritical and h1, to be nonvanishing. Under those conditions the model describes a so-called normal surface transition or, equivalently, critical adsorption of fluids. The critical behavior at this normal transition (and hence at critical adsorption) is the same as at the extraordinary transition. This can be shown by an explicit mapping of appropriate lattice models.