Physisorption of aromatic organic contaminants at the surface of hydrophobic/hydrophilic silica geosorbents: a B3LYP-D modeling study

The adsorption of benzene and benzene-1,4-diol on two all-silica surface models derived from the framework of sanidine mineral with either hydrophobic or hydrophilic properties has been studied by means of periodic calculations based on local Gaussian basis function and the B3LYP-D functional, which includes dispersion contribution as an empirical correction to the pure B3LYP energy. The aromatic molecules have been docked on different adsorption sites of the two surfaces using the electrostatic potential of the separate parts as a guide to ensure the best matching between electrophilic/nucleophilic regions. The inclusion of dispersion in the definition of the functional method dramatically affects both the intermolecular geometries and the adsorption energies, these latter being, in all cases, underestimated without the inclusion of the dispersive contribution. The adsorption of the aromatic molecules on the hydrophobic silica surface is dictated by dispersion and weak CH center dot center dot center dot O(Si)O interactions. The entity of the interaction for benzene on the hydrophilic surface is close to the value of the sublimation energy of the benzene molecular crystal, thus showing that adsorbate self-aggregation and adsorption to the silica surface are competing processes. For hydrophilic surfaces dispersion is still large despite the fact that adsorption energies are almost doubled with respect to the hydrophobic surface due to H-bonding interactions through either SiOH center dot center dot center dot pi (benzene case) or SiOH center dot center dot center dot OH (benzene-1,4-diol case). The computed infrared spectra of the adsorbed molecules reveal small perturbations in the CH, CCH and CCC ring modes, which are sensitive to the adsorbate/adsorbent features, so that these bands can be used as fingerprints for the interpretation of experimental spectra. The present work may contribute to a better understanding of the sorption of typical organic contaminants in common earth's inorganic soils, which is of relevance for environmental concerns.