NaCl Salts in Finite Aqueous Environments at the Fine Particle Marine Aerosol Scale

We investigated isolated sodium/chloride
aqueous droplets at the
microscopic level, which comprise about 5k to 1M water molecules and
whose salt concentrations are 0.2 M (brackish water) and 0.6 M (seawater),
by means of molecular dynamics simulations based on an 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 Na+ and ion aggregates are more repelled from the boundary than
free Cl–. Most of the droplet properties depend
on the droplet radius R according to the standard
formula A = A∞(1 – 2δ/R), where A∞ is the bulk magnitude of the quantity A and δ is a length at most at the nanometer 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. This length is about 1 order of magnitude larger than that
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 Kölher equation
to model submicron salty aerosols.