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Salt exclusion in charged porous media: a coarse-graining strategy in the case of montmorillonite clays

Abstract : 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 κL decreases (where κ 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 concentration 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 concentration in the reservoir, the excluded volume effect dominates.
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Marie Jardat, Jean-François Dufrêche, Virginie Marry, Benjamin Rotenberg, Pierre Turq. Salt exclusion in charged porous media: a coarse-graining strategy in the case of montmorillonite clays. Physical Chemistry Chemical Physics, Royal Society of Chemistry, 2009, 11 (12), ⟨10.1039/B818055E⟩. ⟨hal-01917646⟩

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