Recombination of Autodissociated Water Ions in a Nanoscale Pure Water Droplet

The recombination of water ions has diverse scientific
and practical
implications, ranging from acid–base chemistry and biological
systems to planetary environments and applications in fuel cell and
carbon conversion technologies. While spatial confinement affects
the physicochemical properties of water dynamics, its impact on the
recombination process has rarely been studied. In this work, we investigate
the dynamics of water, the water ion distribution, and the ion recombination
process in water droplets as a function of droplet size through molecular
dynamics simulations and adaptive quantum mechanical/molecular mechanical
calculations. We compare the dynamics of recombination in water droplet
sizes ranging from 100 to 18 000 waters, both in their interiors
and on their surfaces. We found that the self-diffusion of water dramatically
decreases in droplets with a diameter below 2.2 nm. Using a classical
RexPoN force-field, we found that the ions in 1000 H2O’s
spend almost 50% of the time on the surface and 0.5 nm beneath it
with a slight preference for OH– ion to reside longer
on the surface. We estimate that, on average, recombination in these
drops occurs at 400 ps in 1000 H2O’s and 1 ns in
3000 H2O’s. We also found that recombination is
not limited by the local structure of the surface or the size of the
droplet but can be influenced by the geometry of the water wire connecting
the ions as they approach each other, which can often prevent recombination.
Our results provide insights to the reaction microenvironments presented
by nanoscopic water droplets.