Adsorption of critical and supercritical fluids

We develop a crossover theory for critical adsorption of pure fluids in a semi-infinite system. In our previous publication [Phys. Lett. A 251, 212 (1999)] we applied the theory to the analysis of experimental data for adsorption of liquid SF6 on the critical isochore only. In this article we extend the theory on the noncritical isochores and present a comparison of the theoretical predictions for the surface excess (Gibbs) adsorption with experimental data for CO2/silica and SF6/graphite systems. Good representation of experimental data is achieved in the range of temperatures from the saturated temperature up to 1.15 Tc and densities 0.5ρc⩽ρ⩽1.5ρc. The optimization of the model to the excess isotherms in both systems indicates that they have surface critical behavior in the universality class of normal transition. However, in this case model does not reproduce the excess adsorption data for SF6/graphite system at temperatures τ=T/Tc−1⩽0.02 at ρ=ρc. Analysis of the excess adsorption data along the critical isochore in SF6/graphite system indicates that the surface field h1 vanishes linearly with τ as T→Tc, which corresponds to the ordinary transition.