A multianalytical approach to characterize acidic adsorption sites on a quartz powder

In this paper three methods are described to estimate the number of adsorption sites and detect adsorption sites of differing nature on mineral surfaces. The methods are solution depletion adsorption isotherms, adsorption microcalorimetry, and desorption by solvent extraction followed by surface analysis using Electron Spectroscopy for Chemical Analysis (ESCA). The number of adsorption sites is obtained from the adsorbed amount of a test molecule on a mineral surface from the ESCA data using equations based on a substrate/overlayer model, and from the adsorption isotherms the number of sites available to accommodate a monolayer of test molecules is used. Information about whether the adsorption sites are of different or similar nature is provided by the desorption method using extraction in solvents of different polarities. More quantitative information concerning interactions between test molecules and adsorption sites, and possible interaction strength distribution due to sites of differing nature, is obtained from the adsorption enthalpies measured by microcalorimetry. The systems studied included strong basic test molecules (either a fatty diamine, octadecyl amine or pyridine) adsorbed on quartz powder from a nonpolar medium, n-octane. For the desorption study, subsequent extraction was carried out in pure n-octane, followed by ethanol. Adsorption/desorption of basic test molecules on quartz is important in applications of asphalt systems where fatty amines are often added to strengthen adhesion between bitumen and stone aggregates. The quartz powder studied here has acidic adsorption sites of differing nature on the surface. The strong interaction sites are consistent with iron oxide and/or oxohydroxide (strongest interactions with the fatty diamine where both nitrogen atoms can interact) and geminal hydroxyl groups. The weaker interaction sites are consistent with hydroxyl (silanol) groups forming hydrogen bonds with the basic test molecules. (C) 2000 Elsevier Science B.V. All rights reserved.