NaCl salts in finite aqueous environments at the fine particle marine aerosol scale

We investigated pure sodium/chloride aqueous droplets at the microscopic level, which comprise from about 10k to 1M water molecules and whose salt concentrations are 0.2$m$ (saline water) and 0.6$m$ (sea water), by means of molecular dynamics simulations based on an ab initio-based polarizable force field. The size of our largest droplets is at the fine (i.e. sub micron) particle marine aerosol scale. We investigated from our simulations ion spatial distributions, ion aggregates (size, composition, lifetime and distribution), and the droplet surface potential, for instance. Most of the droplet properties depend on the droplet radius $R$ according to the standard formula $A=A_\infty R / (R + 2 \delta)$, where $A_\infty$ is the bulk magnitude of the quantity $A$ and $\delta$ is a distance at a few~A scale. Regarding ions, they form a weak electrostatic double layer extending from the droplet boundary to 20~A within the droplet interior. Free Na$^+$ and ion aggregates are more repelled from the boundary than free Cl$^-$. Because of our weak salt concentrations, the perturbation induced by the ion presence on the water ordering (relative to pure water droplets) is weak and its effect is maximum in between the Na$^+$ and Cl$^-$ layers lying close to the droplet boundary. Lastly, the magnitude of the parameters $\delta$, as estimated form our simulations and earlier ones, shows that estimating properties from 10k water droplets will provide results that differ from those of 1M systems by about 10\%. Salty 10k water droplets are thus well suited to theoretically investigate water/vapor interface phenomena in sub micron marine aerosols.