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

TL;DR

This study uses molecular dynamics simulations to analyze the microscopic structure and properties of sodium chloride aqueous droplets at the submicron scale, revealing ion distributions, surface potentials, and vapor densities relevant to marine aerosols.

Contribution

It provides detailed insights into ion arrangements, surface effects, and vapor interactions in saltwater droplets at the molecular level, extending understanding to the submicron aerosol scale.

Findings

Ions form a weak double layer extending 2 nm inside the droplet.

Na+ ions are more repelled from the droplet boundary than Cl- ions.

Water vapor densities follow a Kelvin relation with a negative Tolman length.

Abstract

We investigated isolated sodium/chloride aqueous droplets at the microscopic level, which comprise from about 5k to 1M water molecules and whose salt concentrations are 0.2$m$ (brackish water) and 0.6$m$ (sea water), by means of molecular dynamics simulations based on an \emph{ab initio}-based polarizable force field. The size of our largest droplets is at the submicron particle marine aerosol scale. From our simulations, we investigated ion spatial distributions, ion aggregates (size, composition, lifetime and distribution), droplet surface potentials and the densities of the water vapor surrounding the droplets. Regarding ions, they form a weak electrostatic double layer extending from the droplet boundary to 2~nm within the droplet interior. Free $\mathrm{Na^+}$ and ion aggregates are more repelled from the boundary than free $\mathrm{Cl^-}$. Most of the droplet properties depend on the droplet radius $R$ according to the standard formula $A=A_\infty(1 - 2 \delta/R) $, where $A_\infty$ is the bulk magnitude of the quantity $A$ and $\delta$ is a length at most at the~nm scale. Regarding the water vapor densities they obey a Kelvin relation corresponding to a surface tension whose Tolman length is negative and at the 1~nm scale. That length is about one order of magnitude larger than for pure water droplets, however it is weak enough to support the reliability of a standard Kelvin term (based on planar interface surface tensions and water densities) and of the related K{\"o}lher equation to model sub-micron salty aerosols.