Structural hysteresis in dilute aqueous electrolytes from path-dependent ion pairing

Dilute aqueous electrolyte solutions are typically regarded as rapidly equilibrating systems in which ion-pair structures are uniquely determined by thermodynamic conditions. Here, we show that this assumption does not fully capture ion-pair behavior. Using Zn²⁺-F^- system as a model, we demonstrate that ion pairing can retain pronounced pathway dependence under identical thermodynamic conditions, resulting in distinct long-lived states and structural hysteresis. ¹⁹F NMR spectroscopy reveals that contact ion pairs (CIPs), although constituting less than 2% of total fluoride species, display reproducible population differences (∼0.4%) that persist for up to 21 months at room temperature. Complementary UV-vis measurements and analogous observations in Cd²⁺, Co²⁺, and Cu²⁺ systems support the generality of this phenomenon. Thermal perturbation leads only to partial convergence between states, indicating finite barriers to ion-pair reorganization. Ab initio molecular dynamics simulations further show that the underlying free-energy landscape features asymmetric barriers and a preference for solvent-separated configurations (2SSIP-like), providing a molecular-level origin for the persistence of distinct states. These results demonstrate that ion-pair distributions in dilute aqueous electrolytes are not uniquely defined by thermodynamic variables alone but can depend on preparation history over experimentally accessible timescales. This study identifies structural hysteresis as an emergent feature of ion pairing in aqueous solutions and highlights the role of preparation history in determining solution structure.