Molecular modelling of the adsorption of aromatic and aromatic sulfonate molecules from aqueous solutions onto graphite

Carbonaceous layers on mineral surfaces are often graphite-like and are dep ressed in Rotation separation by soluble inorganic salts of naphthalene. Th e adsorption of sulfonated and unsulfonated aromatic molecules onto a flat graphite surface has been studied using semi-empirical quantum mechanical m olecular modelling techniques. Comparison with ab initio calculations has j ustified this computational choice. No bonding between the surfactant and t he surface is observed. Molecules reorient upon geometry optimisation, favo uring an arrangement with the aromatic rings parallel to the graphite surfa ce. The inclusion of solvent effects (via a dielectric continuum model) inc reases the inclination of the aromatic sulfonates relative to the graphite surface, due to interactions between the hydrophilic sulfonate group and th e aqueous solution. Of the aromatic sulfonates examined, the one exception to the generalisation that molecules adsorb Rat was naphthalene sulfonate, which preferred to orient vertically with the sulfonate group upwards when solvent effects are included. This is in agreement with previous experiment al results. (C) 1999 Elsevier Science Ltd. All rights reserved.