MOLECULAR ADSORPTION - REPULSIVE VERSUS ATTRACTIVE SUPERCRITICAL-FLUID MIXTURES

The behavior and structure of supercritical fluid (SCF) mixtures in co ntact with an adsorptive solid surface have been determined by the non uniform Ornstein-Zernike integral equations for the molecular distribu tions. In this work, we examine the behavior of attractive and repulsi ve mixtures (A = SCF and B = solute) with respect to adsorption on pla nar substrates. Properties and fluid structures in the vicinity of the wall (W) are explored. The behavior of a dilute solute B adsorbed to wall W from solvent A (i.e. system AS B/W) is described for the cases where ASB form either an attractive SCF mixture or a repulsive SCF mix ture for a temperature where the solvent A is slightly supercritical a nd for density conditions where A is subcritical, critical, and superc ritical. We examine the degree of preferential adsorption of B vis-g-v is the competitive interactions with A and W. This shows the relative adsorptions, while covering a range of states. We find evidence of sol ute rejection by the SCF toward the wall, when the mixture A + B chang es character from 'attractive' to 'repulsive'. The numerical work is v ery demanding in computer memory and time. Thus, full numerical conver gence has not been achieved in the calculations reported here. We have attained partial convergence. However, the qualitative trends are con sidered unaltered. The mechanisms of solute adsorption in SCF have bee n demonstrated at a fundamental molecular level. (C) 1998 Elsevier Sci ence B.V.