@article {642, title = {How Ion Condensation Occurs at a Charged Surface: A Molecular Dynamics Investigation of the Stern Layer for Water{\textendash}Silica Interfaces}, journal = {The Journal of Physical Chemistry C}, volume = {120}, year = {2016}, month = {Jan-21-2016}, pages = {963-973}, abstract = {

We investigate the Stern layer of charged silicawater interfaces by calculating the ionsurface interaction from molecular dynamics simulations. The McMillanMayer potentials of mean force between a charged oxygen site and a lithium or cesium cation have been calculated. Contact ion pairs (CIPs) are important for the adsorption and desorption of ions, especially for lithium. An activation energy appears, which can result in a large estimated relaxation time. In the case of lithium, time scales needed to bind or unbind ions to and from the surface are found to be very long (up to the order of seconds for some surfaces), which implies that molecular dynamics cannot always be fully equilibrated. This work provides a new image of the Stern layer: it is not a continuous layer but a set of Bjerrum pairs. As a matter of fact, quantitative (macroscopic) treatments of such systems with localized surface charges require a three-dimensional model, contrary to the more commonly used one- or two-dimensional theoretical treatments.

}, issn = {1932-7447}, doi = {10.1021/acs.jpcc.5b08836}, url = {http://pubs.acs.org/doi/10.1021/acs.jpcc.5b08836}, author = {Hocine, Sarah and Hartkamp, Remco and Bertrand Siboulet and Duvail, Magali and Benoit A. Coasne and Turq, P. and Jean-Francois Dufreche} } @article {184, title = {Ion-specific adsorption and electroosmosis in charged amorphous porous silica}, journal = {Phys. Chem. Chem. Phys.}, volume = {17}, year = {2015}, month = {Oct-14-2015}, pages = {24683 - 24695}, abstract = {

Monovalent and divalent aqueous electrolytes confined in negatively charged porous silica are studied by means of molecular simulations including free energy calculations. Owing to the strong cation adsorption at the surface, surface charge overcompensation (overscreening) occurs which leads to an effective positive surface next to the Stern layer, followed by a negatively charged diffuse layer. A simple Poisson-Boltzmann model in which the single-ion potential of mean force is introduced is shown to capture the most prominent features of ion density profiles near an amorphous silica surface. Nevertheless, due to its mean-field nature, which fails to account for correlations, this simple model does not predict overscreening corresponding to charge inversion at the surface. Such an overscreening drastically affects the transport of confined electrolytes as it leads to flow reversal when subjected to an electric field. A simple continuum theory is shown to capture how the electro-osmotic flow is affected by overscreening and by the apparent enhanced viscosity of the confined electrolytes. Comparison with available experimental data is discussed, as well as the implications of these phenomena for \ζ-potential measurements.

}, issn = {1463-9076}, doi = {10.1039/C5CP03818A}, author = {Hartkamp, Remco and Bertrand Siboulet and Jean-Francois Dufreche and Benoit A. Coasne} } @article {63, title = {Structure and Dynamics of an Electrolyte Confined in Charged Nanopores}, journal = {The Journal of Physical Chemistry C}, volume = {118}, year = {2014}, month = {Feb-2014}, pages = {5061{\textendash}5072}, abstract = {
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Molecular Dynamics simulations are used to investigate the structure and dynamics of an aqueous electrolyte (NaCl) confined within a nanomembrane, which consists of a nanopore with a diameter 3 nm having a negatively charged surface. Both nanomembranes with a diffuse charge and with local charges are considered (in both cases, two surface charge densities are considered, \−0.9 e/nm2 and \−1.8 e/nm2). For all nanomembranes, significant layering of water and ions in the vicinity of the nanomembrane surface is observed. While the distribution of water and chloride ions is nearly insensitive to the nanomembrane charge and type, the arrangement of sodium cations within the nanomembrane depends on the system being considered. The water and ion density profiles in the nanomembranes are compared with the predictions of a modified Poisson\–Boltzmann equation in which charge image, solvation effects, and dispersion interactions with the surface are taken into account [Huang et al. Langmuir, 2008, 24, 1442]. The self-diffusion coefficient for a given species is smaller than its bulk counterpart and is at most 75\% of the bulk value. While the self-diffusion coefficients for water and sodium cations decrease with decreasing the overall negative charge of the nanomembrane, the self-diffusion coefficient for the chloride anions is nearly independent of the nanomembrane type and charge. We also estimate the dynamics of the confined aqueous electrolyte by calculating time correlation functions which allow estimating solvation, ion pairing, and residence times.

}, isbn = {1932-7447}, doi = {10.1021/jp4098638}, author = {Pierre-Andre Cazade and Hartkamp, Remco and Benoit A. Coasne} } @article {90, title = {Structure and transport of aqueous electrolytes: From simple halides to radionuclide ions}, journal = {Journal of Chemical Physics}, volume = {141}, year = {2014}, month = {Sept-28-2014}, pages = {124508}, type = {Article}, abstract = {

Molecular simulations are used to compare the structure and dynamics of conventional and radioactive aqueous electrolytes: chloride solutions with sodium, potassium, cesium, calcium, and strontium. The study of Cs+ and Sr2+ is important because these radioactive ions can be extremely harmful and are often confused by living organisms for K+ and Ca2+, respectively. Na+, Ca2+, and Sr2+ are strongly bonded to their hydration shell because of their large charge density. We find that the water molecules in the first hydration shell around Na+ form hydrogen bonds between each other, whereas molecules in the first hydration shell around Ca2+ and Sr2+ predominantly form hydrogen bonds with water molecules in the second shell. In contrast to these three ions, K+ and Cs+ have low charge densities so that they are weakly bonded to their hydration shell. Overall, the structural differences between Ca2+ and Sr2+ are small, but the difference between their coordination numbers relative to their surface areas could potentially be used to separate these ions. Moreover, the different decays of the velocity-autocorrelation functions corresponding to these ions indicates that the difference in mass could be used to separate these cations. In this work, we also propose a new definition of the pairing time that is easy to calculate and of physical significance regardless of the problem at hand. (C) 2014 AIP Publishing LLC.

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}, issn = {0021-9606}, doi = {10.1063/1.4896380}, author = {Hartkamp, Remco and Benoit A. Coasne} }