The reaction of water molecules with the MgO(001) surface was studied with density-functional theory using periodic boundary conditions for a better understanding about the formation of H + and HO À ions on the MgO(001) terrace through the structural, reaction barrier, and thermodynamic studies. This process is relevant as the initial hydroxylation step of this surface, and it is part of a catalyzed hydrolysis mechanism.The geometries and the dissociation energies of one, two, and three water molecules adsorbed on a clean MgO(001) were obtained and the type of adsorption assessed. Transition states for the dissociation processes were computed. The results show that the adsorption of a single water molecule does not lead to dissociation. For the dimer and trimer of water molecules, one molecule dissociates while the others coadsorbed stabilize the H + and HO À ionic species on the surface. In the two cases, the dissociation products on the surface converged through the formation of hydrogen bonds among the formed hydroxyl and water molecules. As a consequence of these interactions, the protonated surface oxygen anions coexist with the adsorbed hydroxyl ions. The variation of the Gibbs free energy for the adsorption and dissociation processes was calculated in the 100À600 K temperature range including electronic, vibrational, rotational, and translational contributions. The entire process (adsorption + dissociation) is spontaneous up to about 401.2 and 471.1 K for adsorption of two and three water molecules, respectively. The computed energetic barriers are 23.2 and 24.9 kJ/mol for the dissociation of one H 2 O in the clusters of two and three water molecules, respectively.

Organophosporous VX agent O-ethyl S-(2-diisopropylethylamino)ethyl methylphosphonothioate is one of the main nerve agents. For this reason, the search for ways to deactivate it is very important. In this work, hydrolysis and adsorption reactions of a VX-like compound (O,S-dimethyl methylphosphonothioate, DMPT) on the MgO(001) surface were studied by density-functional theory (DFT) using periodic boundary conditions. A degradation reaction mechanism was proposed and theoretically investigated on two types of MgO(001) surfaces: the terrace and the Al-doped. Conformations, free-energy differences, transition states, reaction barriers, and minimum-energy paths were computed. We found that the P–S bond, related to the agent toxicity, breaks via hydrolysis occurring spontaneously throughout the analyzed temperature range, 100–600 K. In the dissociative chemisorption of the DMPT molecule, the formation of the MgO:[PO(CH3)(OCH3)]+[SCH3]− intermediate is catalytically favored from a temperature of about 335 K for the Al-doped surface, a value considerable smaller than the 500 K value for the same process on the terrace. At 335 K, the dissociation fragments on the Al-doped surface are less stable in comparison to the hydrolysis products. The possible reconstitution of the P–S bond on both surfaces does not occur according to kinetic analysis; however, the electronic energy barrier for the direct dissociation reaction on the Al-doped sites is about 49.0 kJ/mol lower than the value for the terrace. After recombination with the OH– and H+ ions, the HOPO(CH3)(OCH3) and HSCH3 products do not accumulate on either surface because these molecules desorb below the DMPT dissociation temperatures. The Al-doped sites of MgO(001) are thus more active in the catalytic hydrolysis process of the VX-like organophosphorus compound than is the nondoped surface.

The understanding of intermolecular interactions on asphaltene nanoaggregates has important implications over the petroleum industry. Here, we investigated the aggregation of asphaltene and resin using a hierarchical approach that combines Density Functional Theory (DFT) with dispersion corrections and molecular mechanics (MM) calculations. From molecular models already established in the literature for specific types of asphaltene (A) and resin (R) molecules, we employed MM simulations to calculate the potential energy surface to obtain the best conformations for the possible dimers A-A, A-R, and R-R, as well as the relevant combinations of trimers A-R-A, A-A-R, and A-A-A, which have been further relaxed by the DFT calculations. Indeed, the formation of the dimer A-A is energetically more favored with respect to A-R and R-R mainly due to the enhanced effect on the intermolecular interaction of the aromatic region of A. In this context, the trimers have shown to be almost 3 times more stable than the dimers. Our result suggests that the nanoaggregates have a charge density well distributed but centralized between the aromatic ring π-orbitals, whereas A molecules are added, creating a tightly packed structure. In this case, the contribution from the aliphatic chains is just steric to stabilize the aggregate and shield the aromatic center for new interactions. Although the π−π stacking can guide the nanoaggregate formation, the presence of the R molecule leads to a possible disaggregation. When R molecules are inserted, the growth of the nanoaggregate seems to be yet continued due to a dipole moment and radius of gyration increasing. However, we observed charge rearrangement from the aromatic center of π−π interaction to aliphatic chains with heteroatoms that displaces into the structure. The HOMO-1 and HOMO degeneracy is broken in this time, being more significant in trimers derived from particular dimer conformation. The formation energy in some asphaltene and resin conformations is decreased compared to the asphaltene nanoaggregates with larger aromatic islands. Therefore, R rich in heteroatoms may be seen like an inner destabilizer naturally present in oil. These findings can guide new methods to control the stability of asphaltene aggregates by external chemical agents through the degradation mechanism directly upon the aliphatic chains.

Aluminosilicate clays like Montmorillonite (MMT) and Muscovite Mica (MT) have siloxane cavities on the basal plane. The hydroxyl groups localized in these cavities and van der Waals (vdW) forces contribute significantly to adsorption processes. However, the basal sites are found to be difficult to characterize experimentally. Here, (001) surfaces of MMT and MT clays were investigated using first-principles calculations to understand how these silicate surface sites are influenced by hydroxyl groups and the effective role of inner layer vdW interactions. Based on density-functional theory (DFT) within the generalized gradient approximation (GGA), different types of exchange-correlation functionals were tested to check the effect of vdW dispersion correction. Noncontact atomic force microscopy (nc-AFM), X-ray absorption spectroscopy (XAS) in the near-edge region and solid-state nuclear magnetic resonance (SS-NMR) spectroscopy were simulated. In both clays, the oxygen surface sites are directly affected by the intralayer interaction through hydroxyl groups. Our results indicated that the chemical environment of the hydroxyl groups is distinct in the MMT and MT structures. The vdW correction was essential for a better description of the surface oxygen sites and correctly describes the similarity between both clays. Particularly, the bulk apical oxygen sites in the MT structure are less influenced by vdW interaction. Compared to MMT, the silicon surface sites of MT are more sensitive to the intralayer changes in Si-Oapical-Al and with less effect of the hydroxyl groups. These results provide a clear understanding of influence of the siloxane cavity on the oxygen and silicon surface sites in aluminosilicates.