Jean-François Dufrêche

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.

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.

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.

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.

Equilibrium and electrokinetic phenomena in charged porous media from microscopic and mesoscopic models: electro-osmosis in montmorillonite

Ion concentration profiles and electro-osmosis have been theoretically investigated by complementary methods. The counterion profiles of these charged porous media are obtained from microscopic (molecular dynamics) and mesoscopic (Brownian dynamics and Poisson-Boltzmann equation) models. Thus, the limit of validity of the more macroscopic approaches has been quantified. The electro-osmotic flow is evaluated from a similar multi-scale description. A proper evaluation of the Kubo relations adapted to this dynamical process allowed the value of this transport quantity to be obtained from the microscopic model. The comparison with the classical Smoluchowski theory of this electrokinetic phenomenon shows that the latter cannot be applied unless slip hydrodynamic boundary conditions are taken into account. The corresponding slipping length is roughly equal to 6 Å.

Transport equations for concentrated electrolyte solutions: Reference frame, mutual diffusion

Transport coefficients of dissociated electrolytes in aqueous solution are studied in the framework of the primitive model of ionic solutions in which the solvent is a dielectric continuum. Simple explicit expressions are obtained by using a combination of Smoluchowski equation for the dynamics and the mean spherical approximation (MSA) for the equilibrium pair correlation function. The case of symmetrical and dissymmetrical electrolytes is examined. In the case of the mutual diffusion, a reference-frame correction, that can be obtained from density measurements has to be taken into account for molar concentrations. The combined Smoluchowski/MSA theory of the primitive model is found to be self-consistent since it is able to describe simultaneously the different transport and equilibrium properties up to high concentrations (1–2 mol l−1).

Recycling metals by controlled transfer of ionic species between complex fluids: en route to “ienaics”

Recycling chemistry of metals and oxides relies on three steps: dissolution, separation and material reformation. We review in this work the colloidal approach of the transfer of ions between two complex fluids, i.e. the mechanism at the basis of the liquid-liquid extraction technology. This approach allows for rationalizing in a unified model transformation such as accidently splitting from two to three phases, or uncontrolled viscosity variations, as linked to the transformation in the phase diagram due to ion transfer. Moreover, differences in free energies associated to ion transfer between phases that are the origin of the selectivity need to be considered at the meso-scale beyond parameterization of an arbitrary number of competing “complexes”. Entropy and electrostatics are taken into account in relation to solvent formulation. By analogy with electronics dealing about electrons transported in conductors and semi-conductors, this “ienaic” approach deals with ions transported between nanostructures present in colloidal fluids under the influence of chemical potential gradients between nanostructures coexisting in colloidal fluids. We show in this review how this colloidal approach generalizes the multiple chemical equilibrium models used in supra-molecular chemistry. Statistical thermodynamics applied to self-assembled fluids requires only a few measurable parameters to predict liquid-liquid extraction isotherms and selectivity in multi-phase chemical systems containing at least one concentrated emulsified water in oil (w/o) or oil in water (o/w) microemulsion.

Synergism by coassembly at the origin of ion selectivity in liquid-liquid extraction.

In liquid-liquid extraction, synergism emerges when for a defined formulation of the solvent phase, there is an increase of distribution coefficients for some cations in a mixture. To characterize the synergistic mechanisms, we determine the free energy of mixed coassembly in aggregates. Aggregation in any point of a phase diagram can be followed not only structurally by SANS, SAXS, and SLS, but also thermodynamically by determining the concentration of monomers coexisting with reverse aggregates. Using the industrially used couple HDEHP/TOPO forming mixed reverse aggregates, and the representative couple U/Fe, we show that there is no peculiarity in the aggregates microstructure at the maximum of synergism. Nevertheless, the free energy of aggregation necessary to form mixed aggregates containing extracted ions in their polar core is comparable to the transfer free energy difference between target and nontarget ions, as deduced from the synergistic selectivity peak.

Ion transport in porous media: derivation of the macroscopic equations using upscaling and properties of the effective coefficients

In this work, we undertake a numerical study of the effective coefficients arising in the upscaling of a system of partial differential equations describing transport of a dilute N-component electrolyte in a Newtonian solvent through a rigid porous medium. The motion is governed by a small static electric field and a small hydrodynamic force, around a nonlinear Poisson–Boltzmann equilibrium with given surface charges of arbitrary size. This approach allows us to calculate the linear response regime in a way initially proposed by O’Brien. The O’Brien linearization requires a fast and accurate solution of the underlying Poisson–Boltzmann equation. We present an analysis of it, with the discussion of the boundary layer appearing as the Debye–Hückel parameter becomes large. Next, we briefly discuss the corresponding two-scale asymptotic expansion and reduce the obtained two-scale equations to a coarse scale model. Our previous rigorous study proves that the homogenized coefficients satisfy Onsager properties, namely they are symmetric positive definite tensors. We illustrate with numerical simulations several characteristic situations and discuss the behavior of the effective coefficients when the Debye–Hückel parameter is large. Simulated qualitative behavior differs significantly from the situation when the surface potential is given (instead of the surface charges). In particular, we observe the Donnan effect (exclusion of co-ions for small pores).

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.