Salt enrichment and dynamics in the interface of supercooled aqueous droplets

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 have not been yet investigated. We report direct computational evidence that the lower 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 form, the number density of Chloride ion in nanodroplets 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 the free energy diffusive barrier crossing found in bulk solution. The water 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. Reduction of adduct formation between macromolecules and salt may open up the possibility to directly ionize a biological sample from a physiological solution in experiments.