Unraveling The Critical Hydration Effect on Size-Dependent and Ion-Specific Ion Pairing Inside Sub-1nm Graphene Nanoslits

The intriguing phenomena of ion pairing and salt aggregation inside two-dimensional nanoslits have recently been reported in experiments and numerical simulations, leading to the anomalous gate-voltage modulated ion transport and memoresistive properties. An in-depth grasp of the intricate mechanisms governing this ion pairing effect is imperative for engineering advanced materials with specific functionalities and heightened performance. Nevertheless, despite extensive investigations into the dielectric properties of confined water and its confining medium, a unified understanding of how ion-pairing effects are governed by the interplay of hydration effect and nanoconfinement has remained elusive.<br/>This study delves into the ion pairing effect within sub-1nm graphene nanoslits and offers a novel insight into the mechanisms driven by hydration effects. Conducting atomic-level molecular dynamics (MD) simulations of alkali ion electrolytes within multilayered graphene membranes, we unveil a nuanced dependence on slit size and ion-specific behaviors influencing the ion pairing phenomenon, which are confirmed in our experiments. Furthermore, we conclude that the complicated dehydration and renormalization processes primarily determine the propensity for nanoconfined ion-pairing effects, by displaying a comprehensive analysis of the structural and energetic characteristics of water molecules within the hydration shells. We validate this mechanism by elucidating both slit-size and ion-specific effects: as the slit size decreases from 1nm to 7 Å, the structure of the hydration shell becomes increasingly ordered, resulting in a corresponding reduction in entropic free energy; on the other hand, the ion-specific effect is driven by distinct enthalpic changes associated with the formation of larger ion clusters, rather than the mere presence of individual ion pairs.