Like-charge anion pairing as a mechanism for ion co-localization in charged aqueous droplets

Aqueous droplets serve as microreactors for chemical reactions in atmospheric and laboratory-generated aerosols. These droplets frequently carry excess charge, ranging from a small fraction of the Rayleigh limit to nearly the maximum charge sustainable before spontaneous fission. The effect of the droplet charge on chemical reactivity remains insufficiently understood. Here we identify like-charge contact ion-pairing of OH^- ions as a mechanism for transient co-localization of pre-reactive species. Using molecular dynamics simulations of charged water nanodroplets containing multiple OH^- ions, we quantify the equilibrium constant, lifetime, and formation rate of contact ion-pairs, with Cl^- and Na⁺ ions included for comparison. We find that OH^- and Cl^- exhibit a significantly higher propensity to form contact ion-pairs than Na⁺ ions. Hydroxide contact ion-pairs are stabilized by hydrogen-bonded water bridges forming transient [OH^-(H₂O)_nOH^-] structures (n = 1-3) with lifetimes of ≈18 ps. These configurations are enriched in the droplet subsurface region, where ion density is elevated and water self-diffusion is up to twofold faster than in the bulk-like interior. Although diffusion enhancement alone does not account for reported rate accelerations in droplets, it increases ion-ion encounter frequencies in regions where pairing is most probable. We suggest that rather than directly producing radicals, these contact ion-pairs act as transient, pre-reactive configurations that locally concentrate charge and restructure the hydrogen-bond network facilitating proton-transfer events within the hydrogen-bonded bridges and lowering barriers for electron detachment due to the presence of nearby charges.