Analysis of the Surface Heterogeneity of Unmodified and Modified Silica by Capillary Inverse Gas-Solid Chromatography at Finite Dilution

The adsorption energy distributions (AEDs) of several molecular probes on two silica samples were determined from their chromatographic retention data. Adsorption data were obtained using the elution-by-characteristic points (ECP) method of capillary inverse gas chromatography at finite dilution. The diffuse rear profiles of the bands obtained with samples of different sizes overlay consistently. The numerical expectation-maximization (EM) method has been applied to the calculation of the AEDs from the unfitted experimental adsorption isotherms, using the Langmuir model for the local adsorption isotherm. Two silica samples, both unmodified and reacted with trimethylchlorosilane, were used. Results were obtained with methanol, diethyl ether, 1-chlorobutane, dichloromethane, and toluene. Their AEDs exhibit two high-energy peaks, around 45-60 and 50-70 kJ/mol, respectively. The differences observed for the specific capacities of adsorption and for the energy distributions of the unmodified and modified silicas are correlated with the chemical structure of the probe and the adsorbent surface. 17 refs., 10 figs., 4 tabs.