Like-Charge Ion Pairing and Enhanced Interfacial Diffusion in Charged Aqueous Droplets

Chemical reactions in charged aqueous microdroplets often proceed orders of magnitude faster than in bulk solution, yet the molecular origins of this acceleration remain unclear. Using molecular dynamics simulations of charged water nanodroplets containing Na + , Cl-, and OHions, we examine two molecular-level factors that may contribute to enhanced interfacial reactivity: ion pairing and radially dependent diffusion. Multiple hydroxide ions exhibit an enhanced tendency to form contact-ion pairs relative to Na + and Clions despite their like charge. These contact ion pairs are stabilized by hydrogen-bonded water bridges that form transient O 4 H 6 2structures. These correlations are most pronounced in the subsurface region, where ion density and orientational ordering of interfacial water are enhanced. Diffusion in this layer is moderately accelerated, with coefficients up to 1.5 times larger than in the droplet interior. Although this enhancement alone cannot explain the large reaction-rate increases reported in microdroplet experiments, the combination of increased encounter frequencies and stabilized ion pairs suggests a molecular mechanism by which interfacial confinement and fluctuating electric fields may promote reactive ion configurations in charged droplets. Finally, the larger depth of the OHions from the interface relative to alkali and halide ions may help rationalize differences in ionization efficiency between negative and positive modes in electro-spray ionization.