Modulating multivalent ion interaction in angstrom-scale confinement through solvent environment

Abstract

When the size of the confinement approaches that of individual molecules and ions at the angstrom-scale (Å-scale), intriguing ion–ion and water–ion interactions emerge. Water, being a unique solvent, has asymmetric dielectric constants and discrete configurations within Å-scale confinement. However, how these distinctive characteristics of water affect ion behaviors as well as how they translate to other solvents remains largely unexplored. Here, with the robust platform of functionalized two-dimensional molybdenum disulfide nanochannels, we systematically probed how the solvent environment regulates ion dynamics, uptake, and selectivity in Å-scale confinement, using rare-earth elements (REEs) as a model system due to their periodic properties and practical relevance. By tuning solvent composition, we find that both ion uptake and selectivity are governed by the interplay between binding-site deprotonation controlled by solvent-dependent acidity and dielectric effects mediated by the solvent environment. Maximum uptake was observed at intermediate solvent ratios where acidity and dielectric properties are balanced. Increasing deprotonation in dimethylformamide-rich systems shifts selectivity toward heavier REEs. Dynamic tests further reveal that the preference for heavy element Yb arises from stronger binding and improved channel accessibility, facilitated by carboxylate formation and lighter-element-assisted channel opening. Collectively, these findings highlight solvent composition as a powerful lever for tuning ion behavior in 2D channels, and provide mechanistic insight into solvent-mediated approaches to selective REE recovery and separation.