MULTICOMPONENT VAPOR ADSORPTION ON SILICA-GEL

Mass spectrometric tracer pulse chromatography was used to measure the adsorption isotherm of benzene on silica gel at 0 degrees C. In addition, the retention volumes and standard free energies of adsorption of eight probe solutes were measured as a function of the amount of benzene adsorbed. Preadsorption of benzene significantly decreased the retention volume and improved peak symmetry of the probe solutes relative to those values observed for bare silica gel. The isotherm and retention data were used to test the applicability of current models for adsorption of binary mixtures. The regression results indicate that the models can be forced to fit the thermodynamic data; however, the resulting regression parameters have little or no physical significance. Neither the scaled particle theory nor Martire's unified theory of chromatography explicitly fits the experimental data because the particular system of benzene adsorbed on silica gel at 0 degrees C violates almost every assumption used in the derivation of the theoretical models. Most significantly, the surface of silica gel is not energetically homogeneous, and benzene is not an ideal adsorbate by any measure; i.e., it does not consist of a linear collection of identical segments. The data set is presented, however, as a realistic-system representative of those encountered in most types of chromatography.