Philippe A. Bopp

Frequency and wave-vector dependent dielectric function of water: Collective modes and relaxation spectra

The longitudinal frequency and wave-vector dependent complex dielectric response function χ(k,ω)=1−1/e(k,ω) is calculated in a broad range of k values by means of molecular dynamics computer simulation for a central force model of water. Its imaginary part, i.e., Im{e(k,ω)}/|e(k,ω)|2, shows two main contributions in the region of small k values: Debye-like orientational relaxation in the lower frequency part of the spectrum and a damped librational resonance at the high frequency wing. The Debye relaxation time does not follow a de Gennes-like pattern: τ(k) goes through a maximum at k≈k*≈1.7 A−1, while the static polar structure factor S(k) peaks at k≈3 A−1. The resonance frequency ω(k) and the decay decrement γ(k) show a dispersion law, indicative of a decaying optical-like mode, the libron. With an approximate normal mode approach, we analyze the origin of this mode on a molecular level which shows that it is due to a damped propagation of molecular orientational vibrations through the network of hydrog...

Structure and dynamics of water confined in single-wall nanotubes.

The structure and dynamics of water confined in model single-wall carbon- and boron-nitride nanotubes (called SWCNT and SWBNNT, respectively) of different diameters have been investigated by molecular dynamics (MD) simulations at room temperature. The simulations were performed on periodically extended nanotubes filled with an amount of water that was determined by soaking a section of the nanotube in a water box in an NpT simulation (1 atm, 298 K). All MD production simulations were performed in the canonical (NVT) ensemble at a temperature of 298 K. Water was described by the extended simple point charge (SPC/E) model. The wall-water interactions were varied, within reasonable limits, to study the effect of a modified hydrophobicity of the pore walls. We report distribution functions for the water in the tubes in spherical and cylindrical coordinates and then look at the single-molecule dynamics, in particular self-diffusion. While this motion is slowed down in narrow tubes, in keeping with previous findings (Liu et al. J. Chem. Phys. 2005, 123, 234701-234707; Liu and Wang. Phys. Rev. 2005, 72, 085420/1-085420/4; Liu et al. Langmuir 2005, 21, 12025-12030) bulk-water like self-diffusion coefficients are found in wider tubes, more or less independently of the wall-water interaction. There may, however, be an anomaly in the self-diffusion for the SWBNNT.

Combining extended x-ray absorption fine structure with numerical simulations for disordered systems

Over the past two decades x-ray absorption spectroscopy has proven to be a valuable tool for the study of the short-range order in a wide variety of materials, including disordered systems such as superionic conductors as well as glasses, amorphous and liquid systems in general. A number of methods have been proposed to analyse EXAFS data. However, in the case of disordered systems, only the ones taking the distribution of atomic environments into account should be retained. Molecular dynamics (MD) simulations are a valuable tool in this respect, as will be shown from results obtained in a supercritical aqueous solution.

Molecular dynamics simulation of an aqueous aluminium(III) chloride solution with three-body interactions†

A molecular dynamics simulation of an aqueous aluminium (III) chloride solution with three-body interactions.

Comparison of the Structure of Liquid Amides as Determined by Diffraction Experiments and Molecular Dynamics Simulations

The intermodular structures of liquid formamide, N-methylformamide and N,N-dimethylformamide at room temperature are studied by means of NVE molecular dynamics computer simulations. Newly developed flexible models are used. X-ray and neutron weighted structure and total radial pair distribution functions are computed from the simulated site-site pair distribution functions. They are compared with experimental results. The agreement is usually satisfactory as far as heavy atom pairs are concerned while the lengths of the hydrogen bonds are found to be systematically too long in the simulations.

A molecular dynamics study of thermal diffusion in a porous medium

It is the main aim of this work to identify the microscopic origin of the perturbations brought about by a porous environment on the thermal diffusion (Soret effect) of a liquid mixture. For this purpose, nonequilibrium molecular dynamics simulations are carried out on model systems representing hydrocarbons in a porous medium. In keeping with previous simulations, a simplified model,i.e., Lennard-Jones spheres, is used for representing a methanedecane mixture, while the porosity is modeled by the inclusion of quasi-harmonic solids of various sizes and shapes. The model parameters are chosen to yield the proper order of magnitude for a silicate and its interactions with the alkanes. The model was first validated by investigating the equilibrium properties of the system. Then the thermal influence of the porous medium was evaluated and the adsorbing behaviour of the alkanes on the pores was characterized. It is found that the Soret coefficient of the equimolar mixture studied here is lowered by about 30% at 75% porosity. We find also that this reduction is strongly dependent on the structure of the porous medium.

Interaction of Aluminum(III) with water. An ab initio study

Hydrated Al(3+) ions [Al(H(2)O)(n)](3+), n = 1-6, were examined with ab Initio self-consistent field (SCF) calculations. The relative contributions of two-, three-, and higher-body terms to the total interaction energy for an [Al(H(2)O)(6)](3+) complex were calculated The sum of all three-body contributions amounts to - 30% of the sum of all pair-additive contributions and is opposite in sign. The three-body energy contributions were also derived for two types of [Al(H(2)O)(2)](3+) complexes. in the first type, both water molecules reside in the first hydration shell of Al(3+) and in the second type there is one in the first shell and one in the second. Altogether 15,500 triplets were investigated and analytical two- and three-body potential energy functions were derived via a fitting procedure

Water–Methanol Mixtures: Simulations of Mixing Properties over the Entire Range of Mole Fractions

Numerous experimental and theoretical investigations have been devoted to the hydrogen bond in pure liquids and mixtures. Among the different theoretical approaches, molecular dynamics (MD) simulations are predominant in obtaining detailed information, on the molecular level, simultaneously on the structure and the dynamics. Water and methanol are the two most prominent hydrogen-bonded liquids, and they and their mixtures have consequently been the subject of many studies; we revisit here the problem of the mixtures. An important first step is to check whether a classical potential model, the components of which are deemed to be satisfactory for the pure liquids, is able to reproduce the known thermodynamic excess properties of the mixtures sufficiently well. We have used the available BJH (water) and PHH (methanol) flexible models because they are by construction mutually compatible and also well suited to study, in a second step, some dynamic property characteristic of hydrogen-bonded liquids. In this article we show that these models, after a slight reparametrization for use in NpT simulations, reproduce the essential features of the excess mixing and molar properties of water-methanol mixtures. Furthermore, in the pure liquids, the agreement of the radial distribution functions with experiment remains as satisfactory as before. Similarly, the translation self-diffusion coefficients D are modified by less than 10%. In the mixtures, they evolve nonmonotonously as a function of mole fraction.

Ab initio studies on the interactions of water with tetramethylurea and tetramethylthiourea

Abstract The interactions of a water molecule with tetramethylurea (TMU) and tetramethylthiourea (TMTU) were studied by ab initio molecular orbital calculations using the supermolecule approach at the HF and MP2 levels. The water molecule was found to be able to form nearly linear hydrogen bonds both with the oxygen and nitrogen atom of TMU. Although the HF SCF interaction energy is significantly smaller in the second case the total interaction energy decreases only by 15% due to the increase in dispersion forces. For a water molecule in the neighbourhood of the carbonyl (thionyl) group the total TMUH 2 O and TMTUH 2 O energies are nearly identical, whereas for the water molecule in the region of the amide group the latter interaction energy is much smaller. Hydrogen bond formation was not observed between the hydrogen atom of the water molecule and any of the nitrogen atoms of the TMTU molecule.

Computer simulations of electrochemical systems

We review briefly the developments and the actual state of the art in the field of molecular level simulations of the electrochemical interface between aqueous ionic solutions and simple metals.