Aqueous solutions of tetraalkylammonium halides: ion hydration, dynamics and ion-ion interactions in light of steric effects.

Molecular simulations have allowed us to probe the atomic details of aqueous solutions of tetramethylammonium (TMA) and tetrabutylammonium (TBA) bromide, across a wide range of concentrations (0.5 to 3-4 molal). We highlight the space-filling (TMA(+)) versus penetrable (TBA(+)) nature of these polyatomic cations and its consequence for ion hydration, ion dynamics and ion-ion interactions.

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.

A transferable ab initio based force field for aqueous ions.

We present a new polarizable force field for aqueous ions (Li(+), Na(+), K(+), Rb(+), Cs(+), Mg(2 +), Ca(2 +), Sr(2 +), and Cl(-)) derived from condensed phase ab initio calculations. We use maximally localized Wannier functions together with a generalized force and dipole-matching procedure to determine the whole set of parameters. Experimental data are then used only for validation purposes and a good agreement is obtained for structural, dynamic, and thermodynamic properties.

Primitive models of ions in solution from molecular descriptions: a perturbation approach.

The development of simple, primitive model descriptions for electrolyte solutions is usually carried out by fitting the system parameters to reproduce some experimental data. We propose an alternative method, that allows one to derive implicit solvent models of electrolyte solutions from all-atom descriptions. We obtain analytic expressions for the thermodynamic and structural properties of the ions, which are in good agreement with the underlying explicit solvent representation, provided that ion association is taken into account.

Hydrophobic transition in porous amorphous silica.

Realistic models of amorphous silica surfaces with different silanol densities are built using Monte Carlo annealing. Water-silica interfaces are characterized by their energy interaction maps, adsorption isotherms, self-diffusion coefficients, and Poiseuille flows. A hydrophilic to hydrophobic transition appears as the surface becomes purely siliceous.

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.

Potential-induced ordering transition of the adsorbed layer at the ionic liquid/electrified metal interface.

The potential-driven ordering transition of a LiCl layer adsorbed on the (100) surface of a metallic aluminum electrode is studied by molecular dynamics simulations. The transition causes a sharp peak in the potential dependence of the differential capacitance of the interface. This result is in qualitative agreement with recently reported experimental work on the interface between a room temperature ionic liquid and a well-defined Au(100) surface.

Models of electrolyte solutions from molecular descriptions: the example of NaCl solutions.

We present a method to derive implicit solvent models of electrolyte solutions from all-atom descriptions; providing analytical expressions of the thermodynamic and structural properties of the ions consistent with the underlying explicit solvent representation. Effective potentials between ions in solution are calculated to perform perturbation theory calculations, in order to derive the best possible description in terms of charged hard spheres. Applying this method to NaCl solutions yields excellent agreement with the all-atom model, provided ion association is taken into account.

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.

Electrical conductivity of mixed electrolytes: Modeling within the mean spherical approximation.

The purpose of this study is to predict the electrical conductivity of an electrolyte solution containing several simple ionic species. Explicit equations of the MSA-transport theory for the electrical conductivity in this complex solution are given. The theoretical conductivity of simple salts is first compared to experimental results of the literature to deduce the sizes of the ions.

Salt exclusion in charged porous media: a coarse-graining strategy in the case of montmorillonite clays.

We study the exclusion of salt from charged porous media (Donnan effect), by using a coarse-grained approach. The porous medium is a lamellar system, namely a Montmorillonite clay, in contact with a reservoir, which contains an electrolyte solution. We develop a specific coarse-graining strategy to investigate the structural properties of this system.

Structure and dynamics of water at a clay surface from molecular dynamics simulation.

We report a molecular dynamics study of the structure and dynamics of water at a clay surface. The negative charge of the surface and the presence of surface oxygen atoms perturbs water over two to three molecular layers, while the nature of the counterions (Na(+)or Cs(+)) has only a small effect. In the first molecular layer, approximately half of the water molecules are H-bonded to the surface.

Calculation of activities of ions in molten salts with potential application to the pyroprocessing of nuclear waste.

The ability to separate fission products by electrodeposition from molten salts depends, in part, on differences between the interactions of the different fission product cations with the ions present in the molten salt "solvent". These differences may be expressed as ratios of activity coefficients, which depend on the identity of the solvent and other factors. Here, we demonstrate the ability to calculate these activity coefficient ratios using molecular dynamics simulations with sufficient precision to guide the choice of suitable solvent systems in practical applications.

Toward the description of electrostatic interactions between globular proteins: potential of mean force in the primitive model.

Monte Carlo simulations are used to calculate the exact potential of mean force between charged globular proteins in aqueous solution. The aim of the present paper is to study the influence of the ions of the added salt on the effective interaction between these nanoparticles. The charges of the model proteins, either identical or opposite, are either central or distributed on a discrete pattern.

Conductivity-viscosity-structure: unpicking the relationship in an ionic liquid.

The relationships between the ionic mobility, the viscosity, and the atomic-scale structure are investigated in computer simulations of mixtures of LiF and the network glass-forming material BeF(2). The simulations span a wide range of compositions, across which the fluidity of the system changes greatly due to the break-up of the Be-F network by the addition of the LiF. The relationship between the conductivity and viscosity passes from that expected for independently diffusing ions in the dilute mixtures to strongly decoupled Li+ migration through a viscous network at higher concentrations.

New coarse-graining procedure for the dynamics of charged spherical nanoparticles in solution.

A multiscale strategy based on the Brownian dynamics (BD) simulation method is presented here. It leads to an approximate but realistic reproduction of the dynamics of charged nanoparticles in suspension. This method is particularly suited to systems containing highly dissymmetric electrolytes with added salts, such as micellar suspensions or protein solutions.

A multiscale approach to ion diffusion in clays: building a two-state diffusion-reaction scheme from microscopic dynamics.

The mobility of particles is generally lowered by the presence of a confining medium, both because of geometrical effects, and because of the interactions with the confining surfaces, especially when the latter are charged. The water/mineral interface plays a central role in the dynamics of ions. The ionic mobility in clays is often understood as an interplay between the diffusion of mobile ions and their possible trapping at the mineral surfaces.

Experimental and molecular dynamics studies of dysprosium(III) salt solutions for a better representation of the microscopic features used within the binding mean spherical approximation theory.

This work is aimed at a predictive description of the thermodynamic properties of actinide(III) salt solutions at high concentration and 25 degrees C. A new solution of the binding mean spherical approximation (BIMSA) theory, based on the Wertheim formalism, for taking into account 1:1 and also 1:2 complex formation, is used to reproduce, from a simple procedure, experimental osmotic coefficient variation with concentration for three binary salt solutions of the same lanthanide(III) cation: dysprosium(III) perchlorate, nitrate, and chloride. The relevance of the fitted parameters is discussed, and their values are compared with available literature values.

Lanthanide salts solutions: representation of osmotic coefficients within the binding mean spherical approximation.

Osmotic coefficients of aqueous solutions of lanthanide salts are described using the binding mean spherical approximation (BIMSA) model based on the Wertheim formalism for association. The lanthanide(III) cation and the co-ion are allowed to form a 1-1 ion pair. Hydration is taken into account by introducing concentration-dependent cation size and solution permittivity.

Experimental determination of water activity for binary aqueous cerium(III) ionic solutions: application to an assessment of the predictive capability of the binding mean spherical approximation model.

This work is aimed at a description of the thermodynamic properties of actinide salt solutions at high concentration. The predictive capability of the binding mean spherical approximation (BIMSA) theory to describe the thermodynamic properties of electrolytes is assessed in the case of aqueous solutions of lanthanide(III) nitrate and chloride salts. Osmotic coefficients of cerium(III) nitrate and chloride were calculated from other lanthanide(III) salts properties.

A first-principles description of liquid BeF2 and its mixtures with LiF: 2. Network formation in LiF-BeF2.

A polarizable ionic interaction potential, constructed from first-principles calculations, is used to examine the structure, vibrational spectra, and transport properties of molten mixtures of LiF and BeF2 across a range of compositions. The simulations reproduce the experimentally measured vibrational frequencies of the fluoroberyllate (BeF4(2-)) ions, which form in the melt, as well as conductivity and viscosity values across the composition range. Examination of the structures of the melts reveals the emergence of a slowly relaxing network of BeF4 units as the concentration of BeF2 is increased.

A first-principles description of liquid BeF2 and its mixtures with LiF: 1. Potential development and pure BeF2.

The construction of an interaction potential for BeF2 and its mixtures with LiF on a purely first-principles basis is described. The quality of the representation of the forces on the ions obtained from ab initio electronic structure calculations by various potentials, which include many-body interaction effects to different extents, are considered. The predictions of the properties of pure BeF2 obtained in simulations with a polarizable potential are then compared with experimental values.

Uranyl(VI) nitrate salts: modeling thermodynamic properties using the binding mean spherical approximation theory and determination of "fictive" binary data.

This work is aimed at a description of the thermodynamic properties of highly concentrated aqueous solutions of uranyl nitrate at 25 degrees C. A new resolution of the binding mean spherical approximation (BIMSA) theory, taking into account 1-1 and also 1-2 complex formation, is developed and used to reproduce, from a simple procedure, experimental uranyl nitrate osmotic coefficient variation with concentration. For better consistency of the theory, binary uranyl perchlorate and chloride osmotic coefficients are also calculated.