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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.

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+, Mg2+, Ca2+ and Sr2+) 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, UO22+. The results of these calculations will be useful in building interaction potentials that include polarization effects. [ABSTRACT FROM AUTHOR]

Small-angle neutron scattering experiments were performed at 25 °C on solutions of tetraphenylphosphonium chloride and sodium tetraphenylborate in deuterated water and in deuterated dimethylformamide. The results show a strong attraction between either the organic cations or the organic anions when the salts are dissolved in water. On the contrary, in the organic solvent the attraction between the equally charged organic ions is found to be very weak. This is the first structural evidence of hydrophobic association of small organic ions in water. [ABSTRACT FROM AUTHOR]

We investigate the possibility of describing by a continuous solvent model the dynamics of solutes confined down to the molecular scale. We derive a generalised Langevin equation (GLE) for the generic motion of a solute in an external potential using the Mori–Zwanzig formalism. We then compute the corresponding memory function from molecular simulations, in the case of cesium ions confined in the interlayer porosity of montmorillonite clays, with a very low water content (only six solvent molecules per ion). Previous attempts to describe the dynamics of cesium in this system by a simple Langevin equation were unsuccessful. The purpose of this work is not to carry out GLE simulations using the memory function from molecular simulations, but rather to analyse the separation of time scales between the confined ions and solvent. We show that such a separation is not achieved and discuss the relative contribution of the ion–surface, ion–solvent and ion–ion interactions to the dynamics. On the picosecond time scale, the ion oscillates in a surface-and-solvent cage, which relaxes on much longer time scales extending to several nanoseconds. The resulting overall dynamics resembles that of glasses or diffusion inside a solid by site-to-site hopping. [ABSTRACT FROM PUBLISHER]

The question is discussed whether small-angle neutron scattering measurements and their interpretation with continuum-solvent models can yield reliable information about hydrophobic association of small organic ions in water. [ABSTRACT FROM AUTHOR]

Highlights: [•] Ion binding to humic substances is assessed by Monte Carlo and zetametry. [•] Mobility is derived via an all-ion MSA transport theory. [•] Conductimetry shows small variation of mobility even for divalent counterions. [ABSTRACT FROM AUTHOR]

Abstract: We present the first microscopic description of the exchange of water and ions between clay interlayers and microporosity. A force field based on ab-initio calculations is developed and used in classical molecular dynamics simulations. The latter allow to compute the potential of mean force for the interlayer/micropore exchange for water, Na+ and Cs+ cations and Cl− anions. For the simulated water content (water bilayer, with interlayer spacing 15.4Å) and salt concentration in the micropore (0.52moldm−3) the exchange is found to be almost not activated for water and cations, whereas the entrance of an anion into the interlayer is strongly unfavorable (). Calculations of the diffusion tensor in the interlayer and in the micropore complete the study of the exchange dynamics. [Copyright &y& Elsevier]

Abstract: The safety analysis of geological disposal of nuclear waste relies on a comprehensive understanding of the structural and dynamic properties of the geological medium hosting the repository. In this context, a vast research program has been undertaken on the migration of radionuclides (mainly ions) in clays, under the supervision of ANDRA. In parallel to the experimental determination of empirical parameters describing the transport at the macroscopic scale, some theoretical efforts have been made to base such a description on a microscopic view of the transport mechanisms. We review here some of the theoretical approaches developed up to now in our research group, with a particular focus on their complementarity. [Copyright &y& Elsevier]