Effect of carbon deposition on the surface of aerosil on the structure of adsorption sites and water and benzene interlayers 1HNMR spectroscopy study

Abstract

¹ H NMR spectroscopy of water and benzene adsorbed on the surface, in combination with bulk freezing, have been used to study the effect of carbonization of the silica surface on the types of adsorption sites and the thickness of the adsorption layer at the water/carbonized silica interface. Two types of carbosil surface sites participate in adsorption of benzene; these are characterized by chemical shifts of 4.6–5 ppm and between -0.7 and -0.9 ppm. The relative amounts of these sites do not depend on the degree of carbonization. A higher degree of carbonization results in a change in the sites for water adsorption, with oxygen-containing sites of the carbon component become dominant. Changes in the chemical potential of adsorbed water at adsorbent/water/ice interfaces are based on the temperature dependence of the thickness of the unfrozen water layer. It has been shown that the Gibbs energy at the adsorbent/water interface can be measured on the basis of the above dependence. Perturbation of the water structure by the silica surface is restricted to nine monolayers of adsorbed water located near the surface. A low degree of silica surface carbonization results in an increase in the thickness of the interface aqueous interlayer perturbed by the surface to 40–50 molecular diameters. This carbosil is characterized also by a maximum Gibbs energy of the adsorbent/water interface of 820 mJ m^-2. Increase in the carbon component contribution results in a decrease in the thickness of the interface aqueous interlayer to nine molecular diameters and a decrease in the Gibbs energy to 180 mJ m^-2.