Adsorption isotherms on silica for methanol and 1-hexanol modifiers from supercritical carbon dioxide

Abstract

Adsorption isotherms were determined for methanol and 1-hexanol on silica from supercritical carbon dioxide at four temperatures and three mobile phase densities. Maximum stationary phase concentrations were extrapolated from linear least squares fits of the data to the Langmuir equation for monolayer adsorption. From these results, maximum surface area coverages were calculated using the mean molecular area of each modifier. The maximum, molar stationary phase concentration of methanol was found to exceed that of 1-hexanol under all experimental conditions; however, in each case the surface area coverage by hexanol was calculated to be larger. The capacity factors of several substituted and unsubstituted aromatic hydrocarbons were determined in 0–1% (w/v) methanol modifier carbon dioxide. From the linearity of capacity factor versus modifier concentration plots, the ability of the solutes to compete with methanol for active column sites was determined. Unsubstituted aromatic hydrocarbons do not appear to compete with the modifier for direct adsorption onto modifier-sorbing active sites.