Neutralization pathways in charge-imbalanced aqueous nano-clusters

Neutralization between hydronium and hydroxide ions is a fundamental aqueous reaction, yet its mechanism in charged water droplets remains largely unexplored. Using ab initio molecular dynamics simulations, we investigate neutralization in nanometer-sized water clusters containing multiple excess ions and a net charge of ±1. Net charge strongly polarizes the droplet surface, which provides a different environment for proton transport from that of a neutral cluster. We show that neutralization proceeds through short, concerted proton transfer events along hydrogen-bonded water chains comprising up to five water molecules, but in contrast to bulk water, all successful pathways are confined to the droplet surface. Hydronium ions preferentially reside at the surface, while hydroxide ions are stabilized in the subsurface regions, creating an intrinsic asymmetry that directs proton transport from the surface inward. Neutralization is preceded by collective chain compression and hydrogen bond reorientation and involves simultaneous competition among multiple excess protons in charged clusters. These results provide a microscopic basis for the charging mechanisms of analytes in electrospray ionization coupled to mass spectrometry, and accelerated proton-coupled chemistry in electrosprayed droplets.