Gas chromatographic determination of the absolute values of retention volumes and of adsorption heats for hydrocarbons on silica gels of various structures

Abstract

1. 1. From chromatograms obtained at various temperatures, the adsorption heat values of C 1-C 10 hydrocarbons were determined on silica gels of varying porosity. For fine pore silica gels increased adsorption heats were observed with a reduction of mean pore diameter and with an increase in the number of carbon atoms in the n-paraffin molecule. With enlargement of pores, a maximum linear dependence is established of differential adsorption heats on the number of carbon atoms in the molecule. 2. 2. The difference in the adsorption heats of unsaturated and saturated hydrocarbons with the same numbers of carbon atoms, which characterizes the effect of specific interactions of π-electron bonds with the hydroxyl groups of the silica gel surface, is practically independent of geometrical structure. 3. 3. Absolute values (related to unit surface) of the retention volumes V Rs were obtained for normal hydrocarbons. On large pore silica gels V Rs is practically independent of s and on fine pore silica gels it increases with s, due to the increase of adsorption energy with constriction of pores, particularly for larger molecules.