Molecular dynamics study of the interface between water and 2-nitrophenyl octyl ether

We present results of molecular dynamics simulations of the interface between water and 2-nitrophenyl octyl ether (NPOE). This system is analyzed in detail using a procedure to calculate intrinsic profiles of several important properties (density, radial distribution functions, hydrogen bonds, molecular orientation, self-diffusion). The interface was found to be molecularly sharp but corrugated by thermal fluctuations. Using a method based on capillary wave theory, we have estimated the interfacial tension and obtained good agreement with values calculated from the virial route. The results were compared to simulations of the water/nitrobenzene interface. The presence of an alkyl chain in NPOE introduces an added degree of hydrophobicity, which causes an increase in the interfacial tension. Furthermore, interfacial NPOE molecules are less organized than nitrobenzene and show a distinct dynamic response. These results shed light on the observed differences between these two organic liquids in electrochemical studies

What does an ionic liquid surface really look like? Unprecedented details from molecular simulations

We present the first intrinsic analysis of the surface of the [bmim][PF6] room-temperature ionic liquid. Our detailed analysis reveals unprecedented details about the structure of the interface by providing the relative prevalence of different molecular orientations. These results suggest that experimental data should be reinterpreted considering a distribution of molecular arrangements

Molecular dynamics study of 2-nitrophenyl octyl ether and nitrobenzene

The pure organic liquids nitrobenzene (NB) and 2-nitrophenyl octyl ether (NPOE) have been studied by means of molecular dynamics simulations. Both solvents are extremely important in various interfacial processes, mainly connected with ion transfer taking place across the interface with water. Thermodynamic (mass density, enthalpy of vaporization, isothermal compressibility, dipole moment) and dynamic (viscosities and self-diffusion coefficients) properties of both liquids have been calculated and are in very good agreement with the experimental data. In the case of NB, several potentials have been tested and the obtained results compared and discussed. In most cases, the OPLS all-atom potential gives results that are in better agreement with available experimental values. Atomic radial distribution functions, dihedral and angle distributions, as well as dipole-orientation correlation functions are used to probe the structure and interactions of the bulk molecules of both organic solvents. These were seen to be very similar in terms of structure and thermodynamics, but quite distinct in terms of dynamic behavior, with NPOE showing a much slower dynamic response than NB. A simulation study of the simple Cl- and K+ ions dissolved in both solvents has been also undertaken, revealing details about the diffusion and solvation mechanisms of these ions. It was found that in both liquids the positive potassium ion is solvated by the negative end of the molecular dipole, whereas the negative chloride ion is solvated by the positive end of the dipole

Gas-phase molecular structure and energetics of anionic silicates

The gas-phase stabilities of linear, branched and cyclic silicates made of up to five silicon atoms were studied with density functional theory (DFT). The starting geometries for the DFT calculations at the B3LYP/6-311+G(2d,2p) level of theory were obtained from classical molecular dynamics simulations. We have observed that geometric parameters and charges are mainly affected by the degree of deprotonation. Charges on Si atoms are also influenced by their degree of substitution. The enthalpy of deprotonation of the neutral species was found to decrease with the size of the molecule, while the average deprotonation enthalpy of highly charged compounds increased with molecular size. Furthermore, the formation of rings in highly charged silicates is enthalpically preferred to chain growth. These observations result from two competing effects: the easier distribution of negative charge in silicates with low charge density and the strong intramolecular repulsions present in silicates with high charge density. As a consequence, highly charged silicates in the gas phase tend to be as small and as highly condensed as possible, which is in line with experimental observations from solution NMR

A systematic molecular simulation study of ionic liquid surfaces using intrinsic analysis methods

In this paper, we apply novel intrinsic analysis methods, coupled with bivariate orientation analysis, to obtain a detailed picture of the molecular-level structure of ionic liquid surfaces. We observe pronounced layering at the interface, alternating non-polar with ionic regions. The outermost regions of the surface are populated by alkyl chains, which are followed by a dense and tightly packed layer formed of oppositely charged ionic moieties. We then systematically change the cation chain length, the anion size, the temperature and the molecular model, to examine the effect of each of these parameters on the interfacial structure. Increasing the cation chain length promotes orientations in which the chain is pointing into the vapor, thus increasing the coverage of the surface with alkyl groups. Larger anions promote a disruption of the dense ionic layer, increasing the orientational freedom of cations and increasing the amount of free space. The temperature had a relatively small effect on the surface structure, while the effect of the choice of molecular model was clearly significant, particularly on the orientational preferences at the interface. Our study demonstrates the usefulness of molecular simulation methods in the design of ionic liquids to suit particular applications

Molecular simulation of silica/surfactant self-assembly in the synthesis of periodic mesoporous silicas

Understanding the synthesis of periodic mesoporous silica (PMS) is crucial for a more efficient use of these materials and is a necessary first step toward a rational design strategy for the templated synthesis of porous solids. In this paper, the early stages of the synthesis process of PMS materials are simulated directly by molecular dynamics, using realistic atomistic models. It is the first time that such computationally demanding calculations have been attempted. By comparing the self-assembly of cationic surfactants in the presence and absence of silicates, we are able to show that silica promotes the formation of larger aggregates than in a simple surfactant/water solution. The formation of these larger micelles is explained by a strong interaction of the silicate molecules with the surfactant head groups. This strong interaction increases the local concentration of silica at the surface of the micelles, which induces the formation of more condensed silicate species. The surfactant/silica structures observed here are potentially important intermediates in PMS synthesis

Methanol dissociation on bimetallic surfaces: validity of the general Bronsted-Evans-Polanyi relationship for O-H bond cleavage

Density functional theory (DFT) calculations were employed to study the dissociation of the O-H bond in methanol on several planar and stepped bimetallic transition metal surfaces, composed of elements showing high or moderate activity towards this reaction, namely, Ni, Rh, Ru, Ir, Pd, Au, Zn and Cu. The activation energies for the O-H bond cleavage were compared with those estimated using a Bronsted-Evans-Polanyi (BEP) relationship for the RO-H bond breakage on pure metal transition surfaces, relating the activation energy for the reaction with the adsorption energies of the reaction products, RO center dot and H-center dot adsorbed on the surface of the catalyst. Furthermore, the average differences between the values of the activation energies calculated with the exhaustive determination of the full reaction path and location of the transition state on each surface model and the activation energies obtained from the BEP relationship with the simple calculation of the adsorption energies of the RO center dot and H-center dot species are similar to 0.14 eV. This suggests that the BEP relationship developed upon the consideration of data for dissociation of the O-H bond in alcohols and water on pure metal surfaces is also valid for a qualitative prediction of the methanol activation energy on bimetallic surfaces

A DFT study of the NO dissociation on gold surfaces doped with transition metals

The NO dissociation on a series of doped gold surfaces (type TMn@Au(111) or TMn@Au(110), with TMn = Ni, Ir, Rh, or Ag and referring n to the number of dopant atoms per unit cell) was investigated through periodic density functional theory calculations. Generally, doping of Au(111) and Au(110) matrices was found to strengthen the interaction with NO species, with the exception of Ag, and was found to increase the energy barrier for dissociation, with the exception of Ni on Au(111). The calculations suggest that the NO dissociation is only possible in the case of the Ir@Au(110) bimetallic surface but only at high temperatures. The increase of the contents of Ir on Au(110) was found to improve significantly the catalytic activity of gold towards the NO dissociation (E-act = similar to 1 eV). Nevertheless, this energy barrier is almost the double of that calculated for NO dissociation on pure Ir(110). However, mixing the two most interesting dopant atoms resulted in a catalyst model of the type Ir@Ni(110) that was found to decrease the energy barrier to values close to those calculated for pure Ir surfaces, i.e., similar to 0.4 eV, and at the same time the dissociation reaction became mildly exothermic. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4790602

Solvation free energy profile of the SCN- ion across the water-1,2-dichloroethane liquid/liquid interface. A computer simulation study

The solvation free energy profile of a single SCN- ion is calculated across the water-1,2-dichloroethane liquid/liquid interface at 298 K by the constraint force method. The obtained results show that the free energy cost of transferring the ion from the aqueous to the organic phase is about 70 kJ/mol, The free energy profile shows a small but clear well at the aqueous side of the interface, in the subsurface region of the water phase, indicating the ability of the SCN- ion to be adsorbed in the close vicinity of the interface. Upon entrance of the SCN- ion to the organic phase a coextraction of the water molecules of its first hydration shell occurs. Accordingly, when it is located at the boundary of the two phases the SCN- ion prefers orientations in which its bulky S atom is located at the aqueous side, and the small N atom, together with its first hydration shell, at the organic side of the interface