Clay high surface area to volume ratio enhances their ability to influence the water organization in their vicinity. Their surface charge, which varies as a function of physical-chemical conditions of the surrounding water, results in a complex organization of solute ions as a function of distance from the surface. This organization is known as the electrical double layer where counter-ions are electrostatically attracted to the surface whereas co-ions are repelled from it. Abundant and convincing spectroscopic evidence has emerged of a complex layering of water molecules and ionic species adsorbed at specific distance from the charged surfaces, a feature that can be captured by surface complexation models derived from Stern theory. The Gouy-Chapman model, or its modified version based on the resolution of the full Poisson-Boltzmann equation (MGC), is almost always used to calculate the electrostatic potential value in the diffuse region between the Stern layers and the bulk solution. Clay surfaces present several advantages with regards to molecular simulations owing to the 2D pseudo-periodicity of the clay particles and this system is especially fitted to enable accurate comparison between the MGC prediction and the actual ions organization at the solid-water interface. Amongst clay minerals, montmorillonite has been the matter of a broad range of molecular dynamic studies, but the vast majority of them involved only monovalent cations in interlayer spaces without the presence of anions. Few studies have been focused on divalent cations and/or charged surfaces in contact with large pores where the diffuse layer can fully extend without overlapping.  In the present study we address the problem of MGC model predictions accuracy in a clay/water system in the presence of divalent cations and monovalent anions using molecular dynamic calculations. MGC models departure implications on clay diffusion models will be discussed.

Quantification of the breakdown of modified Gouy-Chapman model for 2:1 salt background electrolyte: A molecular dynamic study

248th National Meeting (2014)

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