Halide Identity Shapes Ion Pairing and Solvent Dynamics in Aqueous Lithium Halide Electrolytes

Abstract

Predicting whether electrolytes associate or dissociate in water is challenging because direct ion attraction, hydration, solvent caging, and collective reorganization operate on comparable free-energy scales, and subtle changes in their balance can reverse the preferred ion-pairing state. Here, we develop machine-learned potentials trained on density-corrected r2SCAN to investigate how periodic trends across the halide series govern the thermodynamics, structure, and dynamics of aqueous lithium halide electrolytes. The resulting potentials of mean force reveal a systematic shift from the strongly stabilized contact ion pair in LiF toward solvent-shared and dissociated configurations for LiCl, LiBr, and LiI, with apparent dissociation constants spanning nearly 4 orders of magnitude. This thermodynamic progression emerges from the balance among direct ion attraction, hydration shell stability, solvent caging, and water-mediated screening. From F– to I–, the halide hydration shell becomes increasingly diffuse and labile, with faster water exchange and orientational relaxation. These changes provide a microscopic dynamical basis for the progression from structure-making to structure-breaking behavior, accompanied by a shift from strongly suppressed toward modestly enhanced water mobility. The same hydration contrast governs the coupling between ion separation and solvent reorganization during association and dissociation. Together, these results establish a unified molecular connection among association–dissociation equilibria, hydration shell dynamics, and solvent transport in aqueous electrolytes.