Salt Enrichment and Dynamics in the Interface of Supercooled Aqueous Droplets

Abstract

The interconversion reaction of NaCl between the contact-ion pair (CIP) and the solvent-separated ion pair (SSIP) as well as the free-ion state in cold droplets has not yet been investigated. We report direct computational evidence that the lower is the temperature, the closer to the surface the ion interconversion reaction takes place. In supercooled droplets the enrichment of the subsurface in salt becomes more evident. The stability of the SSIP relative to the CIP increases as the ion-pairing is transferred toward the droplet’s outer layers. In the free-ion state, where the ions diffuse independently in the solution, the number density of Cl– shows a broad maximum in the interior in addition to the well-known maximum in the surface. In the study of the reaction dynamics, we find a weak coupling between the interionic NaCl distance reaction coordinate and the solvent degrees of freedom, which contrasts with the diffusive crossing of the free energy barrier found in bulk solution modeling. The H2O self-diffusion coefficient is found to be at least an order of magnitude larger than that in the bulk solution. We propose to exploit the enhanced surface ion concentration at low temperature to eliminate salts from droplets in native mass spectrometry ionization methods.

Conical Shape Fluctuations Determine the Rate of Ion Evaporation and the Emitted Cluster Size Distribution from Multicharged Droplets

“…Figure c shows that a sole I – and a sole Cl – in a droplet, modeled by SWM4-NDP, have a significant probability to be found near the surface, and similarly, the multiple ions also reside in the same location. For a sole Cl – , we have found that in addition to the well-known maximum of the number density on the droplet surface, there is also a significant broad maximum in the droplet interior . In the presence of multiple Cl – ions, the maximum near the surface is maintained, but as expected, the interior maximum disappears.…”

“…For a sole Cl − , we have found that in addition to the well-known maximum of the number density on the droplet surface, there is also a significant broad maximum in the droplet interior. 52 In the presence of multiple Cl − ions, the maximum near the surface is maintained, but as expected, the interior maximum disappears. Thus, for the polarizable chaotropic ions (Cl − and I − ), the location of the multiple ions is the same as that of the single ion in the droplet's subsurface or surface.…”