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. A well-established hydration is seen for both TMA(+) and TBA(+) throughout the concentration range studied. A clear penetration of water molecules, as well as counterions, between the hydrocarbon arms of TBA(+), which remain in an extended configuration, is seen. Global rotation of individual TBA(+) points towards isolated rather than aggregated ions (from dilute up to 1 m concentration). Only for highly concentrated solutions, in which inter-penetration of adjacent TBA(+)s cannot be avoided, does the rotational time increase dramatically. From both structural and dynamic data we conclude that there is absence of hydrophobicity-driven cation-cation aggregation in both TMABr and TBABr solutions studied. The link between these real systems and the theoretical predictions for spherical hydrophobic solutes of varying size does not seem straightforward.

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.

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. The same procedure applied to crystalline phases allows to parametrize the interaction between cations and the chloride anion. Finally, we illustrate the good transferability of the force field to other thermodynamic conditions by investigating concentrated solutions.

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. Effective ion-ion potentials are derived from molecular dynamics simulations and are used within a first-order perturbation theory to derive the best possible description in terms of charged hard-spheres. We show that our model provides a valid description for a series of 1-1 electrolytes.

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. These results imply significant consequences for the description of surfaces. First, realistic models are required for amorphous silica interfaces. Second, experimental amorphous silica hydrophilicity is attributed to charged or uncharged defects, and not to amorphousness. In addition, autoirradiation in nuclear waste glass releases hydrogen atoms from silanol groups and can induce such a transition.

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.

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. In the LiCl/Al simulations, the transition occurs when the interaction model includes the induction of dipoles on the ions of the liquid by their mutual interaction and their interaction with the electrode surface as well as the polarization of the metal by the charges and dipoles on the ions ("image" interactions). When the electrode or ion polarization effects are not included, the transition is no longer observed. The interaction between the induced charges on the metal atoms and the induced dipoles on the ions creates an additional screening, which stabilizes the formation of a crystalline layer at the anode. When the crystallographic plane of the metal is changed to (110) instead of (100), the two first adsorbed layer are crystalline on both the anode and the cathode, but the structure is different: the crystal is formed through an epitaxial mechanism to adapt to the electrode surface structure. In the case of the (110) crystallographic plane, the charging of the adsorbed layer occurs through the formation of nonstoichiometric crystalline layers.

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.

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.

A teoria de transporte em meios porosos tais como argilasdepende do nível de descrição. Na escala macroscópica, equações da hidrodinâmica são utilizadas. Tais descrições a níveldo contínuo são convenientes para tratar o movimento do fluido em sistemas confinados. No entanto, tais equações são válidas se os poros do material são muito maiores do que as moléculas das componentes do sistema. Uma outra abordagem consiste em usar descrições moleculares. Esses dois métodos que correspondem a diferentes níveis de descriçãosão complementares. A ligação entre eles pode ser elucidada usando um procedimento de mudança de escala onde são tomadas médias das leis microscópicas sobre as variáveis rápidas para se obter as leis macroscópicas para tempos longos. Apresentamos esta abordagem no caso de argilas. Primeiramente apresentamos em detalhes os vários níveis de descrição bem como as relações entre eles, enfatizando o domínio de validade das equações hidrodinâmicas. Em seguida, focamos no caso de argilas desidratadas onde a hidrodinâmica não é relevante. Mostramos que é possível derivar um modelo simples para o movimento dos íons césio baseado na diferença entre as escalas de tempo do solvente e das partículas do soluto.

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. These sizes allow us to calculate the conductivity for a mixture of several ionic species without any additional parameter. We have also measured the electrical conductivity of solutions of LiCl, NaCl, and KCl and of KBr and MgCl(2) at 25 degrees C. A very good agreement between theoretical calculations and experimental values is obtained for each studied system.

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. Molecular simulations are used to calibrate a mesoscopic model of the clay micropore in equilibrium with a reservoir. Brownian Dynamics simulations are then used to predict the structure of ions in the pore and the amount of NaCl salt entering the pore as a function of the pore size (the distance L between clay surfaces) and of the electrolyte concentration in the reservoir. These results are also compared to the predictions of a Density Functional Theory, which takes into account the excluded volumes of ions. We show that the calibration of the mesoscopic model is a key point and has a strong influence on the result. We observe that the salt exclusion increases when kappaL decreases (where kappa is the inverse of the Debye length) and that this effect is modulated by the correlations between ions. Two different regimes are revealed. At low concentrations in the reservoir, we observe a regime controlled by electrostatics: the Coulomb attraction between ions increases the amount of salt in the interlayer space. On the opposite, at high concentrations in the reservoir, the excluded volume effect dominates.

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.

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. We also analyze the H-bond network between surface water molecules. The diffusion of water molecules along the surface is slowed down compared to the bulk case. As far as the orientational order and dynamics of the water dipole are concerned, only the component normal to the clay surface is perturbed. We investigate the surface H-bond formation and dissociation dynamics and their coupling to the release of molecules from the first molecular layer. We introduce a simple kinetic model in the spirit of Luzar and Chandler [Nature, 1996, 379, 55] to allow for a comparison with bulk water dynamics. This model semi-quantitatively reproduces the molecular simulation results and suggests that H-bond formation is faster with the surface than in the bulk, while H-bond dissociation is slower.

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. We use polarizable ion interaction potentials which have previously been shown to give excellent agreement with structural, transport, and spectroscopic information of the molten salts, and the activity coefficients calculated in this work agree well with experimental data. The activity coefficients are shown to vary systematically with cation size for a set of trivalent cations.

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. Contrarily to Poisson-Boltzmann predictions, attractive, and repulsive direct forces between proteins are not screened similarly. Moreover, it has been shown that the relative orientations of the charge patterns strongly influence salt-mediated interactions. More precisely, for short distances between the proteins, ions enhance the difference of the effective forces between (i) like-charged and oppositely charged proteins, (ii) attractive and repulsive relative orientations of the proteins, which may affect the selectivity of protein/protein recognition. Finally, such results observed with the simplest models are applied to a more elaborate one to demonstrate their generality.

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. We describe how to build a two-state diffusion-reaction scheme from the microscopic dynamics of ions, controlled by their interaction with a mineral surface. The starting point is an atomic description of the clay interlayer using molecular simulations. These provide a complete description of the ionic dynamics on short time and length scales. Using the results of these simulations, we then build a robust mesoscopic (Fokker-Planck) description. In turn, this mesoscopic description is used to determine the mobility of the ions in the interlayer. These results can then be cast into a diffusion-reaction scheme, introducing in particular the fraction of mobile ions, or equivalently the distribution coefficient Kd. This coefficient is of great importance in characterizing electrokinetic phenomena in porous materials.

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. The coarse-graining procedure leads to a description where only the translational degrees of freedom of the nanoparticles are left, all the degrees of freedom related to the smallest solutes being rigorously averaged out. The authors' contribution aims at quantitatively evaluating the influence of the eliminated forces on the dynamics of the nanoparticles. For this purpose, an effective diffusion coefficient has to be calculated. In practice, this effective diffusion coefficient is taken as an input of a coarse-grained simulation that uses the potential of mean force between nanoparticles. The procedure has been validated by the quantitative comparison between the coarse-grained calculations and BD simulations at the "microscopic" level of description (which explicitly include microions). For a model of aqueous solutions of 10-1 electrolyte with a 1-1 added salt, the agreement is found to be excellent. This new method allows us to compute the diffusion coefficients of nanoparticles with a computation time at least one order of magnitude lower than with explicit BD.

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. The transition between these régimes is linked to the changing local and intermediate-scale structure in the melts. The decoupling phenomenon is associated with the appearance of migration channels in the network which leads to cooperative effects in the Li+ migration.

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. UV-vis/near-IR, time-resolved laser-induced fluorescence spectroscopy experiments, and molecular dynamics (MD) calculations were conducted for dilute to concentrated solutions (ca. 3 mol.kg-1) for a study of the microscopic behavior of DyCl3 binary solutions. Coupling MD calculations and extended X-ray absorption fine structure led to the determination of reliable distances. The MD results were used for a discussion of the parameters used in the BIMSA.

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. In the subsequent paper, a more extensive comparison of the predicted properties of LiF-BeF2 mixtures with experiment is considered.