Ion adsorption on charged mineral surfaces influences the mobility of the solutes in natural waters. Understanding the interfacial ion distribution can provide an insight into adsorption mechanisms. In dilute aqueous conditions, the distribution of ions adsorbed at charged interfaces can be described by classical electrical double layer (EDL) theories. In brine conditions, however, the interfacial structure is less well understood because of complex ion–ion interactions. To determine the dependence of the EDL structure on the salinity of water, we systematically investigated the distribution of Rb<sup>+</sup> adsorbed on the negatively charged muscovite (001) surface in ~0.05–7000 mM RbCl and ~20–1000 mM RbI solutions. In situ observations using high-resolution X-ray reflectivity revealed non-classical variations in adsorbed Rb coverage as a function of dissolved Rb concentration ([Rb]). In solutions containing [Rb] ≤ 20 mM, the adsorbed Rb coverage increased with increasing [Rb] until it reached a plateau, where the saturated coverage matched the amount needed for compensating the muscovite surface charge. Surprisingly, for [Rb] ≥ 20 mM, the Rb coverage began to increase again, indicating the overcompensation of the surface charge. The coverage reached a second plateau at [Rb] = ~1000 mM where it matched the number of the available ditrigonal cavity sites in the muscovite surface. Molecular dynamics simulations showed that adsorption of excess Rb<sup>+</sup> led to additional adsorption of anions, some of which transformed into RbCl and RbI crystals at the surface in contact with solutions undersaturated with respect to the compounds. Overall, these results demonstrate intrinsic complexities of ion adsorption structures in brine conditions, which need to be incorporated in geochemical models in many natural conditions.

Ion cooperativity at the muscovite (001): Brine Interface*

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.