Influence of Polarizability on the Prediction of the Electrical Double Layer Structure in a Clay Mesopore: A Molecular Dynamics Study

Abstract

The prediction of the water and ion density profiles in the electrical double layer (EDL) is a key insight for macroscopic models that intend to model transport and swelling properties in clay materials. Unfortunately, the structure of the EDL, and especially the ion distribution, cannot be probed directly by measurements, because EDL features are inherently disturbed by direct measurement techniques. In recent years, molecular dynamics has provided a growing set of information on the properties of the EDL, including the properties of the diffuse layer, located beyond the Stern or compact layer, and in which a diffuse cloud of ions screens the remaining uncompensated surface charge. Molecular dynamics results are, however, dependent on the force field used to run the simulations. In this study, we investigated the influence of the choice of a force field on the structural and dynamic properties of the EDL present at montmorillonite surface/water interfaces in a 50 Å wide slit-shaped mesopore. Simulations were run in the presence and in the absence of added NaCl, and the results were compared to ion distributions in the diffuse layer predicted with a Poisson–Boltzmann model. The simulations evidenced the strong influence of the consideration of polarizability of ions and water molecules on the predicted structural and dynamic properties of the EDL. While nonpolarizable force fields gave results in good agreement with the prediction of the Poisson–Boltzmann theory, our tested polarizable force field, PIM, showed very significant deviations, which could be mainly attributed to the enhanced formation of ion pairs.