Unravelling the Anomalous Dielectric Permittivity of Nanoconfined Electrolyte Solutions

International audienceThe dielectric properties of sodium chloride solutions confined in a hydrophilic nanocavity were investigated by means of molecular dynamics simulations. Unlike what is observed in the bulk phase, three dielectric regimes were evidenced, namely an anomalous increase in the dielectric permittivity at low concentrations (with respect to confined pure water), a dielectric plateau at intermediate concentrations and finally a bulk-like behavior for salt concentrations higher than a critical value. It was shown that this peculiar behavior results from the competition between dielectric saturation due to the electric field generated by ions (which tends to lower the dielectric permittivity) and the ion-induced perturbation of pre-oriented water molecules inside the nanocavity which gain some rotational degrees of freedom (entropic contribution) leading to an increase in dipolar fluctuations responsible for the increase in the dielectric permittivity

Water Confinement in Nanoporous Silica Materials

International audienceThe influence of the surface polarity of cylindrical silica nanopores and the presence of Na + ions as compensating charges on the structure and dynamics of confined water has been investigated by molecular dynamics simulations. A comparison between three different matrixes has been included: a protonated nanopore (PP, with SiOH groups), a deprotonated material (DP, with negatively charged surface groups), and a compensated-charge framework (CC, with sodium cations compensating the negative surface charge). The structure of water inside the different pores shows significant differences in terms of layer organization and hydrogen bonding network. Inside the CC pore the innermost layer is lost to be replaced by a quasi bulk phase. The electrostatic field generated by the DP pore is felt from the surface to the centre of pore leading to a strong orientation of water molecules even in the central part of the pore. Water dynamics inside both the PP and DP pores shows significant differences with respect to the CC pore in which the sub-diffusive regime of water is lost for a superdiffusive regime

A new lateral method for characterizing the electrical conductivity of ion-exchange membranes

International audienceA new method for determining the electrical conductivity of ion-exchange membranes was implemented with four commercial membranes (AMX, CMX, MK-40 and MA-41). It is based on lateral resistance measurements without direct contact between electrodes and membranes. The cell configuration made it possible to determine the membrane conductivity over a wide range of electrolyte concentrations (measurements were carried out in the range 10-5-5x10-1 M). The structural parameters of the different membranes were inferred from AC conductivities and the microheterogeneous model. They were found in good agreement with literature results obtained by normal measurements (i.e. with current lines oriented normally to the membrane surface), thus confirming the reliability of the proposed method. The main advantage of our method is the possibility to characterize ion-exchange membranes even at low salt concentration unlike usual non-contact methods based on normal measurements

The averaged potential gradient approach to model the rejection of electrolyte solutions using nanofiltration: Model development and assessment for highly concentrated feed solutions

International audienceSome of the recent publications in nanofiltration modelling converge on the importance of dielectric effects and numerous models have been developed in order to take them into account. However several works reported lately in the literature suggest a screening of image charges effect at high electrolyte concentration and the predominance of the Born effect, due to the change of dielectric constant inside the confined nanopore regarding that of the feed solution. In pursuit of an exhaustive and simple model for nanofiltration, a new approach is developed that account for both dielectric phenomena. Based on the Steric, Electric and Dielectric Exclusion (SEDE), the introduction of an average potential gradient approximation is shown to greatly improve the computational performance of the model without being detrimental to its predictive accuracy. The results obtained with this simplified model (SEDE-APG) are compared to the original SEDE model and an excellent agreement is obtained even in the case of electrolyte mixtures. Ultimately this model is confronted to experimental data of separation obtained for moderately to highly concentrated feed flows and exhibits promising result

Computation of the hindrance factor for the diffusion for nanoconfined ions: molecular dynamics simulations versus continuum-based models

Special Issue: Thermodynamics 2011 ConferenceInternational audienceWe report the self-diffusion coefficients and hindrance factor for the diffusion of ions into cylindrical hydrophilic silica nanopores (hydrated silica) determined from molecular dynamics (MD) simulations. We make a comparison with the hindered diffusion coefficients used in continuum-based models of nanofiltration (NF). Hindrance factors for diffusion estimated from the macroscopic hydrodynamic theory were found to be in fair quantitative agreement with MD simulations for a protonated pore, but they strongly overestimate diffusion inside a deprotonated pore

How does the electronic continuum model perform in the prediction of the surface tension of salt solutions?

International audienceThe electronic continuum (EC) model uses a scaling of the charges of the ions in order to model implicitly the polarization into nonpolarizable models. This scaling procedure is applied here to two standard nonpolarizable force fields to investigate the salt concentration dependence of the surface tension and density of NaCl aqueous solutions. The composition of the interface and the orientation of the water molecules at the water surface are reported for different combinations of force fields

Advanced electrokinetic characterization of composite porous membranes

International audienceThe effect of the streaming current flowing through the porous structure of composite membranes during tangential electrokinetic measurements was investigated. It was shown that neglecting this additional path for streaming current may have dramatic implications in the interpretation of the experimental data and on the determination of the membrane zeta potential. Experimental measurements of both streaming current and electrical conductance were performed with two different composite polymer membranes. By following the procedure proposed by Yaroshchuk and Luxbacher, Langmuir 26 (2010) 10882-10889, in the present work it was possible to determine separately the zeta potential of the membrane surfaces and that of their underlying porous structures. This experimental procedure was shown to provide useful information on the functionalization of an ultrafiltration polyethersulfone membrane by positively charged 4-benzyltriphenylphosphonium groups. Notably we found that the chemical modification leads to a charge reversal (from negative to positive) of the porous substructure of the membrane while the overall charge of the external surface remains negative, although with diminished magnitude

Zhu et al. Reply

International audienceA Reply to the Comment by S. Gekle and A. Arnold. Original Article: Stephan Gekle and Axel Arnold, Comment on "Anomalous Dielectric Behavior of Nanoconfined Electrolytic Solutions", Phys. Rev. Lett. 111, 089801 (2013)

Caractérisation expérimentale d’une unité de distillation membranaire AGMD pour un couplage avec une thermofrigopompe

National audienceLa croissance démographique mondiale entraine une augmentation des besoins en froid commercial et domestique, en rafraîchissement ou climatisation ainsi qu’en eau potable. Dans l'objectif d’un développement social et industriel durable, il apparaît indispensable pour certaines populations, en particulier dans les sites isolés, que de nouveaux systèmes économes en énergie pour la production d'eau potable soient développés. L’objectif général est de coupler une thermofrigopompe (TFP) à une unité de distillation membranaire pour la production de froid et le dessalement de l’eau de mer (figure 1). La technique utilisée pour le dessalement est l’AGMD, air gap membrane distillation. C’est un procédé thermique et membranaire (figure 2) basé sur la création d’un gradient de pression de vapeur généré par un gradient de température de part et d’autre d’une membrane microporeuse hydrophobe. Une étude de caractérisation expérimentale de l’AGMD est effectuée sur une installation pilote. Plusieurs paramètres influents tels que les températures, les débits, l’épaisseur de l’air gap et le type d’écoulement sont été étudiés. Lors des essais nous avons utilisé de l’eau de mer synthétique (35g/l). Les résultats montrent que le flux de perméat augmente lorsque la température et le débit d’alimentation augmentent et lorsque l’épaisseur de l’air gap diminue. Le taux de rejet de sel est de 99,9%. Le type d’écoulement (co-courant ou contre-courant) à une faible influence sur le flux de perméat. Cette étude expérimentale confirme que le couplage d’une thermofrigopompe avec une unité de distillation membranaire est réalisable grâce aux faibles températures de fonctionnement (25 à 65°C) qui correspondent à la gamme de température en production de chaleur de la TFP. Une modélisation du comportement du pilote a été effectuée et validée. Ce modèle permet de caractériser les phénomènes mécaniques et thermiques dans l’installation. Il sera utilisé pour dimensionner un prototype de TFP couplé à l’AGMD puis pour simuler une installation de climatisation et de production d’eau douce pour des bâtiments à taille réelle