Effect of ion-ion correlations on the distribution of halide anions (Cl^-, Br^-, I^-) and Rb⁺ at the muscovite (001)–water interface under high salinity conditions

While electrical double layer models describing classical ion distributions at charged mineral-water interfaces are well-validated, there is still a gap in understanding non-classical interfacial processes at high ionic strength, which is urgently needed to develop models that describe element transport in environmental systems with high salinity.

This study is an extension of previous work on adsorption of alkali metal Rb⁺ and halides Cl^- and I^- on muscovite (001), demonstrating the onset of nonclassical charge overscreening by adsorbed Rb in high salinity solutions (i.e., [Rb] > 0.1 M). Here, we simultaneously probe cation and anion distribution, i.e., Rb and Br, adsorbed at the mica–water interface by element-specific resonant anomalous X-ray reflectivity (RAXR) to answer the question if and how the charge density of the electrolyte anion controls the positional correlations between monovalent ions at charged interfaces. Our new RAXR measurements reveal that the overscreening leads to the adsorption of 0.6 ± 0.1 Br^-/A_UC. During this process, the adsorption height of Rb⁺ (~2 Å) stays constant, while that of Br^- systematically increases from ~3.7 up to 4 Å with increasing [RbBr] from 1 to 2.7M, which is consistent with the height difference between Rb and Br perpendicular to the (111) plane of RbBr crystals. These results provide molecular scale insights into the transition from classical to nonclassical behavior and the initiation of crystal growth/formation at high ionic strength.