INVESTIGATIONS OF TOTAL ADSORPTION IN GAS-SOLID SYSTEMS BY TRACER PULSE CHROMATOGRAPHY

The dynamic void volumes of several gas-solid chromatographic systems were measured with varying amounts of nitrogen, carbon dioxide, ethane, or propane adsorbed on silica gel at 77 K. The void volume and total amount of material adsorbed were measured simultaneously as a function of adsorbate pressure by tracer pulse chromatography. The void volume was determined from the measured retention time of a He-3 probe, The dynamic (kinetic) void volume of a column decreased linearly with increasing amounts of adsorbed material due to the increasing volume of adsorbed, stationary phase. The observed decrease in the dynamic void volume per mole of adsorbed material gave a direct measure of the molar volume or density of the adsorbed species. Measured values of the adsorbed-phase molar volume for four adsorbates agreed, within experimental uncertainty, with the classical van der Waals b constants for these gases. The phase boundary between the stationary acid mobile phases in a gas chromatographic column explicitly delineates the normally ill-defined ''Gibbs dividing plane''. The material adsorbed from the gas phase acts as the stationary phase in the column. Thus, the experimental, chromatographic adsorption data give the total amount of material adsorbed rather than the excess amount obtained from classical volumetric and gravimetric techniques. The explicit demarcation between mobile and stationary phases is unique to gas and supercritical fluid chromatography and provides a basis for the only known experimental technique for the direct measurement of total adsorption as well as an explicit differentiation between total and excess adsorption. Because the observed decrease in void volume was equal to the increase in the volume of material adsorbed, a direct method for the measurement of the volume of material adsorbed as a function of adsorbate pressure was developed. Such a ''volumetric'' isotherm is presented for nitrogen on silica gel at 77 K, The determination of volume-based isotherms from the change in void volume provides a novel, but simple and accurate, experimental method for the investigation of phase distribution equilibria in gas-solid systems.