Investigations of Total Adsorption in Gas-Solid Systems by Tracer Pulse Chromatography

Abstract

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 123He 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

Supercritical Fluid Chromatography and Extraction

“…This work defines different types of void volumes and adsorbed-phase volume, discusses excess and total adsorption, and reviews experimental methods (29). Liu et al used tracer pulse chromatography to measure the decrease in void volume caused by four different mobile phases adsorbed to a stationary phase of a chromatographic column at 77 K (30). Volumetric isotherms were generated, and the values of the van der Waals b constants for four mobile phases were estimated and found in agreement with accepted values.…”

Gas Chromatographic Measurement of Void Volume and Mobile Phase Volume: Illustration of the Concepts of Excess and Total Adsorption

“…Direct measurement of total adsorption is not feasible with volumetric or gravimetric techniques because the void volume measured using those methods represents only the volume of a system with nothing adsorbed (i.e., V 0 ). Currently, chromatographic methods present the only way to obtain both to-tal and excess adsorption, since the volume of the mobile phase (eluent) in a chromatographic column can be measured, under certain conditions, both with and without material adsorbed on the surface of the stationary phase (7,8). In that case, the physical boundary between the stationary and mobile phases defines the position of the Gibbs dividing surface, z δ .…”

“…Experimental measurement of V M as a function of n i total forms the basis for determining the molar volume of adsorbed species (7,8).…”

“…This is possible because the volume occupied by the mobile phase may vary with the amount of adsorbate condensed on a solid stationary phase if the eluent is excluded from the adsorbed layer. This information can be utilized (8) to characterize adsorption data as either total or excess and consequently specify the location of the Gibbs dividing surface. In this paper, as suggested by Zhu (15) the void volume minus the volume of the adsorbed phase, (eq 6) is defined as the mobile-phase volume of the column.…”

“…It is believed that chromatography is presently the only experimental technique that can be used to distinguish between the two types of adsorption, and thus offers a clear illustration of the abstract concept of the interfacial plane. Another interesting aspect of gassolid chromatographic studies is the idea that the adsorbed layer volume obtained from experimental measurements can be directly correlated to a molecular size parameter of the gas adsorbed, that is, the van der Waals b constant (7,8). This demonstrates an additional advantage of gas-solid chromatographic techniques as a promising tool for measuring physical properties of materials adsorbed in thin layers.…”