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

Abstract

Carbonaceous layers on mineral surfaces are often graphite-like and are depressed in flotation separation by soluble inorganic salts of naphthalene. The adsorption of sulfonated and unsulfonated aromatic molecules onto a flat graphite surface has been studied using semi-empirical quantum mechanical molecular modelling techniques. Comparison with ab initio calculations has justified this computational choice. No bonding between the surfactant and the surface is observed. Molecules reorient upon geometry optimisation, favouring an arrangement with the aromatic rings parallel to the graphite surface. The inclusion of solvent effects (via a dielectric continuum model) increases the inclination of the aromatic sulfonates relative to the graphite surface, due to interactions between the hydrophilic sulfonate group and the aqueous solution. Of the aromatic sulfonates examined, the one exception to the generalisation that molecules adsorb flat was naphthalene sulfonate, which preferred to orient vertically with the sulfonate group upwards when solvent effects are included. This is in agreement with previous experimental results.