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

Abstract

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...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...pi (benzene case) or SiOH...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.

Silica Surface Features and Their Role in the Adsorption of Biomolecules: Computational Modeling and Experiments

“…Moreover, when adsorption is determined by dispersion interactions with siloxane groups and less by H-bonding, such a correction also becomes essential to determine the proper geometry and adsorption energy of the adsorbed adduct. An example of that is the work of Rimola et al 426 which studied the interaction of benzene, which can be considered as a partial analogue of the Phe molecule, with a hypothetical hydrophobic silica surface constituted by only Si-O-Si bonds; i.e., with no SiOH surface groups. Calculations were run adopting periodic boundary conditions (see Section 4.1) using B3LYP and B3LYP-D* methods, the latter inclusive of the dispersion contribution properly reparametrized 422 to account for the condensed matter nature of the system.…”

“…Along this line, it has been recently demonstrated that hydrophobic interactions indeed play an essential role which is clearly revealed when electrostatic and H-bonding interactions can be considered as constant. 426 In summary, it seems clear that the adsorption mechanism of AA on silica surfaces may be driven by three main kinds of contributions: i) electrostatic forces between AA ions and negatively charged SiO -surface groups, which become particularly favorable when the silica surfaces and AA exhibit a net charge of opposite sign achieved by modulating the pH; ii) H-bonding interactions between AA (either as neutral molecules or ions) and neutral SiOH groups; iii) hydrophobic interactions between the AA side chains and the siloxane Si-O-Si groups.…”

“…Consistently with the number of H-bonds formed, the BSSE-corrected adsorption energies (E C ads ) for catechol on the (001) α-cristobalite and (111) -cristobalite surfaces were computed to be -59 and -49 kJ mol -1 , respectively. Rimola et al 426 have studied the gas-phase adsorption of hydroquinone on periodic surface models based on the sanidine feldspar framework with hydrophobic and hydrophilic surface features at the B3LYP-D level. The most stable structures found considering low surface loading (i.e., with no lateral interactions between adsorbed molecules belonging to adjacent unit cells) are shown in Figure 94.…”

“…426 Small amounts of catechol occur naturally in fruits, vegetables and in some mushrooms. Moreover, the catechol moiety is present in proteins of marine mussels, which have an unusual high content of 3,4-dihydroxy-L-phenylalanine (the catechol functionality).…”

“…Draw with structural and energetic data from Ref. 426 The DFT-D2 scheme has recently been reformulated as DFT-D3 method 421 by including atom-pairwise specific dispersion coefficients and cutoff radii that are both computed from first principles. Moreover, a new non-empirical method to obtain C 6 coefficients from ground state electron density of a molecule or solid has also been proposed.…”