Pierre Turq

Application of the molecular dynamics method to a liquid system with long range forces (Molten NaCl)

A significant and characteristic difficulty arising in the use of statistical mechanics to describe the properties of fused salts is due to the extreme range of the electrostatic field; an accurate representation of the Coulomb energy and Coulomb forces may be obtained using Ewalds expansion. Such calculations are tested here on the model case of liquid NaCl at different temperatures. In addition, this method is applied to the description of ionic media by molecular dynamics computations. (A Born-Huggins-Mayer pair potential is used with constants derived from the solid at 298 K.) This treatment introduces cut-off distances and periodic boundary conditions. The influence of these constraints on the conservation of energy and momenta is investigated. Thermodynamic properties of molten NaCl at 1164·5, 1224·5 and 1340·5 K are calculated. The motions of individual ions, anions and cations are described in terms of a velocity self-correlation function and their diffusion coefficients are determined.

Diffusion coefficient and shear viscosity of rigid water models

We report the diffusion coefficient and viscosity of popular rigid water models: two non-polarizable ones (SPC/E with three sites, and TIP4P/2005 with four sites) and a polarizable one (Dang-Chang, four sites). We exploit the dependence of the diffusion coefficient on the system size (Yeh and Hummer 2004 J. Phys. Chem. B 108 15873) to obtain the size-independent value. This also provides an estimate of the viscosity of all water models, which we compare to the Green-Kubo result. In all cases, a good agreement is found. The TIP4P/2005 model is in better agreement with the experimental data for both diffusion and viscosity. The SPC/E and Dang-Chang models overestimate the diffusion coefficient and underestimate the viscosity.

Ions in solutions: Determining their polarizabilities from first-principles

Dipole polarizabilities of a series of ions in aqueous solutions are computed from first-principles. The procedure is based on the study of the linear response of the maximally localized Wannier functions to an applied external field, within density functional theory. For most monoatomic cations (Li(+), Na(+), K(+), Rb(+), Mg(2+), Ca(2+) and Sr(2+)) the computed polarizabilities are the same as in the gas phase. For Cs(+) and a series of anions (F(-), Cl(-), Br(-) and I(-)), environmental effects are observed, which reduce the polarizabilities in aqueous solutions with respect to their gas phase values. The polarizabilities of H((aq)) (+), OH((aq)) (-) have also been determined along an ab initio molecular dynamics simulation. We observe that the polarizability of a molecule instantaneously switches upon proton transfer events. Finally, we also computed the polarizability tensor in the case of a strongly anisotropic molecular ion, UO(2) (2+). The results of these calculations will be useful in building interaction potentials that include polarization effects.

Temperature effect in a montmorillonite clay at low hydration—microscopic simulation

The effect of temperature in the range 0–150°C was studied for homo-ionic montmorillonite clays with Na+ and Cs+ compensating ions in low hydration states. Monte Carlo and molecular dynamics simulations were employed to provide both static and dynamic information concerning the interlayer ions and water molecules, and emphasis was laid on the temperature activation of the diffusion coefficients. Principal structural changes were limited to the interlayer water phase. In the monohydrated systems, neither of the cations was seen to enter into the hexagonal cavities of the clay. Cs+ exhibited clear site-to-site diffusion between sites allowing coordination to six oxygen atoms of the clay sheets, this behaviour persisting to high temperatures. Preferential sites for the Na+ counterion were much less well-defined, even at low temperatures. The behaviour of the water phase in the monohydrated states was similar for the two ions. A rapid approach to bulk dynamics was seen in the transition from monohydrated to bihydrated Na-montmorillonite. A detailed quantitative comparison of the temperature activation of diffusion for a two-dimensional water phase and three-dimensional bulk water is presented for the first time.

Molecular simulation of aqueous solutions at clay surfaces

We report a molecular simulation study of aqueous solutions at montmorillonite clay surfaces. Unlike most previous studies, ours does not focus on the interlayer nanopores, but looks at both kinds of external surfaces of clay particles: basal surfaces along the clay layers, and lateral surfaces through which interlayer and larger interparticle pores are linked. We present results on structural, dynamic and thermodynamic properties and phenomena, including hydration complexes of ions, H bonding networks, modification of the water dynamics with respect to the bulk, and the role of water in the cation exchange between interlayer and interparticle pores.

Polarizabilities of individual molecules and ions in liquids from first principles

Dipole polarizabilities of individual ionic or molecular species are computed in three different liquid systems: liquid water, molten salts and magmatic melts, the last two belonging to the class of ionic liquids. The method is based on a purely first-principles procedure. The liquid water polarizability tensor is found to be nearly isotropic in the molecular framework. Important environmental effects occur in the two ionic systems when the nature and concentration of the cations are changed. The results of these calculations will be useful in the building of interaction potentials which include polarization effects.

Models for electrokinetic phenomena in montmorillonite

Abstract Clays present remarkable electrokinetic features since they exist from very dilute colloidal state to nanoporous materials, depending on the water/clay ratio. The case of low volume fraction Vwater/Vtot which corresponds to compact systems is examined. The ionic distributions have been evaluated by Poisson–Boltzmann like models and compared to discrete solvent simulations. Several electrokinetic properties (electroosmosis and conductance) have been calculated, in the framework of the mean spherical approximation introduced in the Fuoss–Onsager transport theory. It is found that Onsagers limiting laws in terms of external concentration are not valid on the grounds of the Donnan effect.

Intermediate range chemical ordering of cations in simple molten alkali halides

The presence of first sharp diffraction peaks in the partial structure factors is investigated in computer simulations of molten mixtures of alkali halides. An intermediate range ordering appears for the Li + ions only, which is associated with clustering of this species and is not reflected in the arrangement of other ions. This ordering is surprising in view of the simplicity of the interionic interactions in alkali halides. The clustering reflects an incomplete mixing of the various species on a local length scale, which can be demonstrated by studying the complementary sub-space of cations in the corresponding pure alkali halides by means of a void analysis.

Electrostatic Relaxation and Hydrodynamic Interactions for Self-Diffusion of Ions in Electrolyte Solutions

The concentration dependence of self-diffusion of ions in solutions at large concentrations has remained an interesting yet unsolved problem. Here we develop a self-consistent microscopic approach based on the ideas of mode-coupling theory. It allows us to calculate both contributions which influence the friction of a moving ion: the ion atmosphere relaxation and hydrodynamic interactions. The resulting theory provides an excellent agreement with known experimental results over a wide concentration range. Interestingly, the mode-coupling self-consistent calculation of friction reveal a nonlinear coupling between the hydrodynamic interactions and the ion atmosphere relaxation which enhances ion diffusion by reducing friction, particularly at intermediate ion concentrations. This rather striking result has its origin in the similar time scales of the relaxation of the ion atmosphere relaxation and the hydrodynamic term, which are essentially given by the Debye relaxation time. The results are also in agreement with computer simulations, with and without hydrodynamic interactions.

Bridging molecular and continuous descriptions: the case of dynamics in clays

The theory of transport in porous media such as clays depends on the level of description. On the macroscopic scale,hydrodynamics equations are used. These continuous descriptions are convenient to model the fluid motion in a confined system. Nevertheless, they are valid only if the pores of the material are much larger than the molecular size of the components of the system. Another approach consists in using molecular descriptions. These two methods which correspond to different levels of description are complementary. The link between them can be clarified by using a coarse-graining procedure where the microscopic laws are averaged over fast variables to get the long time macroscopic laws. We present such an approach in the case of clays. Firstly, we detail the various levels of description and the relations among them, by emphasizing the validity domain of the hydrodynamic equations. Secondly, we focus on the case of dehydrated clays where hydrodynamics is not relevant. We show that it is possible to derive a simple model for the motion of the cesium ion based on the difference on time scale between the solvent and the solute particles.