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The process of equilibration of the tetradecane-water interface in the presence of sodium hexadecane-benzene sulfonate is studied using intensive atomistic molecular dynamics simulations. Starting as an initial point with all of the surfactants at the interface, it is obtained that the equilibration time of the interface (several microseconds) is orders of magnitude higher than previously reported simulated times. There is strong evidence that this slow equilibration process is due to the aggregation of surfactants molecules on the interface. To determine this fact, temporal evolution of interfacial tension and interfacial formation energy are studied and their temporal variations are correlated with cluster formation. To study cluster evolution, the mean cluster size and the probability that a molecule of surfactant chosen at random is free are obtained as a function of time. Cluster size distribution is estimated, and it is observed that some of the molecules remain free, whereas the rest agglomerate. Additionally, the temporal evolution of the interfacial thickness and the structure of the surfactant molecules on the interface are studied. It is observed how this structure depends on whether the molecules agglomerate or not.

This paper describes the characterization of structural, energetic, and electric properties of the molecular complexes between Orinoco belt resins through the application of computational molecular mechanics (MM), semi-empirical parametrization (PM6), and density functional theory (DFT) (PW91 and HCTH) in conjunction with the double numeric and polarized (DNP) basis set. The resin sources for the studied molecules are Orinoco belt vacuum residues (Carabobo, Hamaca, Merey, Merey-Mesa, and Zuata). Molecular structures of these compounds were proposed from analysis of experimental characterization. The study of molecular interactions has shown that these resins are able to form stable van der Waals complexes, where their computed stability has a large dependence upon the applied theory level. A qualitative description of the formation of these complexes could be obtained with these methodologies. In particular, MM overestimates resin interaction energies when compared to PM6 and DFT (PW91 and HCTH) results. ...

Density functional theory (DFT) calculations combined with surface thermodynamic arguments and the Gibbs-Curie-Wulff equilibrium morphology formalism have been employed to explore the effect of the reaction conditions, temperature (T), and gas-phase partial pressures (p h2 and p h2s ) on the stability of low Miller index ruthenium sulfide (RuS 2 ) surfaces. The calculated thermodynamic surface stabilities and the resulting equilibrium morphology models suggest that unsupported RuS 2 nanoparticles in HDS conditions are like to a polyhedron with six, eight, and twelve (100), (111), and (102) faces, respectively. The area of these faces covers about 40%, 37%, and 23% of the total particle, respectively. The atomic basins of the outermost individual atoms of the exposed surfaces were determined using the quantum theory of atoms in molecules methodology. Direct visualization of these basins has shown that a hole just at the middle of the outermost sulfur basins provides access to uncovered metal sites. Analysis of the electrostatic potential mapped onto a selected electron density isocontour (0.001 au) on the exposed surface reveals a very high potential reactivity of these holes toward electrodonating reagents. Consequently, the high attraction between these uncovered sites and S atoms coming from reagent polluting molecules makes these kinds of particles quite active for HDS catalysis.

The energy of formation and the Lewis acid strength of sulfur vacancies or coordinative unsaturated sites on the MoS2 edges were studied using density functional theory for periodic systems and an electrostatic potential-based methodology. The results suggest that the more energetically favorable sites are located on the sulfur edges; however, their Lewis acid strength is considerably smaller than the site acidity at the molybdenum edges. The acid strength for the reported most hydrodesulfurization active site of RuS2 was also determined. In general, the Lewis acid for the site on RuS2 is 100% smaller than the sites on the Mo edges and around 20% larger than the most favorable site on the S edges of MoS2. Binding of the pyridine molecule in the eta1 adsorption configuration on the considered sites has corroborated the trend of Lewis acidity suggested by the electrostatic potential methodology.

The nature of the interaction of water molecules with the surface of a set of experimentally well-studied alkane−amidethiols adsorbed on a gold(111) surface and its effect on the water contact angle have been studied by carrying out a systematic determination of the topology of electrostatic potential using ab initio density functional theory methods for periodic systems. The obtained results have shown that the water contact angle decreases systematically as the number of minima by surface cell unit and the electrostatic potential magnitude at these minima, and consequently the adsorption strength, increases. Thus, this electrostatic potential magnitude can be used as a measure of the efficiency and ability of a chemical group for wetting of a surface by water.

Binding between water and models of poly(ethylene oxide), (CH2−CH2−O)n, n = 2−40, has been studied using the topographic features of the electrostatic potential, V(r), and standard density functional theory methods. It was found that, in general, the contour around the minima of the oxygen atoms overlap forming a negative-valued spiral coiled around a positive-valued helix. The positive zone defines a helical groove in the O−C−C−O units where minima lone pairs critical points are located. Topological analysis of the water molecule has also suggested that the attractive electrostatic effect between the positive water O−H zone and the negative PEO lone pairs plays an important role in the hydrogen bonding of the PEO−water system. Thus, the V(r) topology predicts a coil of water molecules around the PEO chain forming hydrogen bonding with two sites of ether oxygens. This coil is formed in such a way that more water molecules accumulate on the cavities surrounding the poly(ethylene oxide)'s oxygen atoms where...

In this work, the topology of the electrostatic potential using density functional theory for periodic systems was used to study the nature of the interaction of water with laponite surfaces; an uncharged sheet model was also used. The topological analysis predicts that for uncharged surfaces the adsorption mode is such that the water molecules are adsorbed almost parallel to the surface. For laponite surfaces, where there is a net charge, the adsorption mode involves electrostatic repulsion between the negative lone pairs on the water molecules and the ones on the surface oxygen atoms. As a consequence, the water molecules bind to the surface in a perpendicular and tilted approach, minimizing the repulsive interactions. The advantage of using the topology of the electrostatic potential as an efficient method to describe the electrostatic interactions between adsorbates and surfaces is also discussed.

To study the effect of the surface Ru sulfur coordination number and of the surface S−H and Ru−H species into the thiophene adsorption on RuS2, a topologic study of the Laplacian of the electronic density of selected (100) and (111) surfaces was carried out. It was found that a nonbonded local charge concentration on the S atom of the thiophene interacts with a local minimum on the outermost Ru atoms of the surfaces. This interaction is strongly affected by a nonbonded local charge concentration located on the outermost S atoms of the surface. Both interactions combine in such way that the strength of the thiophene adsorption on the unhydrogenated surfaces shows small changes. The main role of the S−H bond is to move away the surface sulfur local charge concentrations from the Ru atoms while the Ru−H species favors the hydride attacks to a local minimum located at the Cα of the thiophene molecule.

A topological analysis of the Laplacian of the electronic charge density of bulk 3d transition metals has been performed. This analysis shows that the atomic graph for the atoms of the fcc metals (Ni and Cu) is an octahedron with six vertexes (corresponding to local charge concentrations) linked by twelve edges and bounded by eight faces (corresponding to local charge minima). For the bcc case (V, Cr, and Fe), the corresponding graph is a cube with eight vertexes, twelve edges, and six faces. For the early hcp metals (Sc and Ti), the graph is a trigonal bipyramid with five vertexes, nine edges, and six faces, whereas for Co, it is a trigonal prism with six vertexes, nine edges, and five faces. Despite the different kinds of the graphs obtained, we have been able to correlate the value of the Laplacian at the local minima with the experimental heats of adsorption (which show a systematic decrease) for the O2 and CO molecules on the 3d transition metals. This result suggests that the bonds formed by these m...

The topology of the electron density for face-centered cubic transition metals (β-Co, Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au) was studied. All of the electron density critical points in the unit cell w...

A numerical method to analyze the topology of the electronic density regardless of how it was obtained (analytically or numerically) was implemented for the Extreme 94 program. The method allows th...

A numerical method that analyzes the topology of the Laplacian of the electronic density regardless of how it was obtained (analytically, numerically, or even experimentally) was implemented for the BUBBLE program. The method allows the study of a complex system where the electronic correlation is important, by density functional theory regardless the kind of basis set used, and allows the analysis of the Laplacian of the resulting charge density with the topological theory of Bader. The method was applied to CO and NO molecules and the Cu13, Cu45, and Fe21 clusters modeling the (100) Cu and (001) Fe surfaces, respectively.

MCSCF-CAS and SCF ab initio calculations were carried out for the abstraction of H from CH[sub 4] by MgO and LiO molecules. Effects of the electron correlation on internuclear distances, reaction energies, and Laplacian of the charge density [del][sup 2][rho](r) of the O valence shell were studied. An excellent correlation between the values of -[del][sup 2][rho](r) at an external saddle point of this shell and the activation barrier for the H abstraction from CH[sub 4] already reported was obtained. In this way, the use of -[del][sup 2][rho](r) has been extended to include the prediction of the chemical reactivity of a reaction involving a homolytic break of a pair of bonding electrons. The changes in the charge redistribution in going from the reactants to the transition state and to the products was also determined in terms of variations of -[del][sup 2][rho](r) at the critical points of the valence shells. 22 refs., 7 figs., 5 tabs.

The 14 N NQR spectra of quinoline, 2-chloroquinoline, 1,5-naphthyridine and its hydrated form, 1,8-naphthyridine, 1,3-benzodiazine, 2,3-benzodiazine, 1,2-benzodiazine, and 1,4-benzodiazine were obtained at 77 K. Also the N electric field gradient EFG for quinolines, naphthyridines, and benzodiazines was calculated using an ab initio method with a 6-31G * set