Adsorption of critical and supercritical fluids

We develop a crossover theory for critical adsorption of pure fluids in a s emi-infinite system. In our previous publication [Phys. Lett. A 251, 212 (1 999)] we applied the theory to the analysis of experimental data for adsorp tion 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 th eoretical predictions for the surface excess (Gibbs) adsorption with experi mental data for CO2/silica and SF6/graphite systems. Good representation of experimental data is achieved in the range of temperatures from the satura ted temperature up to 1.15 T-c and densities 0.5 rho(c) less than or equal to rho less than or equal to 1.5 rho(c). The optimization of the model to t he excess isotherms in both systems indicates that they have surface critic al behavior in the universality class of normal transition. However, in thi s case model does not reproduce the excess adsorption data for SF6/graphite system at temperatures tau = T/T-c -1 less than or equal to 0.02 at rho = rho(c). Analysis of the excess adsorption data along the critical isochore in SF6/graphite system indicates that the surface field h(1) vanishes linea rly with tau as T --> T-c, which corresponds to the ordinary transition. (C ) 2000 American Institute of Physics. [S0021-9606(00)50406-1].