Coverage-dependent adsorption of n-hexane and isopropanol on silica: A density-functional study

Abstract

This study delves into the adsorption behavior of both polar and non--polar molecules on silica surfaces, spanning a range from low coverage to full adsorption scenarios. Our results reveal that isopropanol exhibits a self--competitive adsorption mechanism on the surface. However, due to its thermodynamic stability, we anticipate fully covered pores under experimental conditions. In contrast, n--hexane demonstrates a substantial interaction with the hydrophilic substrate, displaying an enhanced adsorption effect whereby the adsorption gets stronger with an increasing number of molecules adhering to the pore´s surface. As a consequence, we predict a complete coverage of the pores under experimental conditions, accompanied by a discernible reduction in the mobility of these attached molecules. This observation carries significance, as alkanes are typically employed as probing molecules for measuring the geometric tortuosity of silica pores, assuming a reflective surface. Caution is advised, particularly for pores with thin diameters, in the order of a few molecular sizes.