Effect of Droplet Size and Counterions on the Spatial Distribution of Ions

Charged droplets have become a new environment for accelerating chemical reactions by orders of magnitude relative to their bulk analogues. Nevertheless the reaction mechanisms still remain unknown. Here we investigate the ion spatial distributions and surface charge in aqueous droplets with diameters in the range of 5 nm to 16 nm with and without counterions using molecular dynamics. The charge carriers are Na, and Cl ions ions. We demonstrate the convergence of ion spatial distributions. Scaling of the ion distributions reveals underlying universal behavior. The convergence allows one to extrapolate the simulation results from nanoscopic dimensions to larger ones, which are still inaccessible to atomistic modeling.

The surface excess charge and electric field are also computed. We find that the surface excess charge layer in the presence of Na and Cl ions is approximately 1.5 nm-1.7 nm thick and that approximately 55%-33 % (from smaller to larger droplets) of the total number of ions reside in this layer. For the first time droplet sizes that are accessible to experimental scrutiny are modeled atomistically.