Salt enrichment and dynamics in the interface of supercooled aqueous droplets

01 February 2022, Version 2
This content is a preprint and has not undergone peer review at the time of posting.


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.


aqueous clusters
droplet chemisty
diffusion coefficient in confinement
sodium chloride
native mass spectrometry
ions in solution
vapour-liquid interface
contact-ion pair species
solvent-separated ion pair

Supplementary materials

Supplementary material
The Supplementary material has the following content: (S1) Details of the computational methods and models. (S2) The trajectories of the distance of Na+ and Cl- ions from the droplet center of mass and the interionic distance of a sole NaCl pair in 776 H2O water-molecule droplet, modeled with TIP4P/2005 at various temperatures. (S3) Dipole moment orientation of NaCl. (S4) Ion probability density profiles using TIP4P/2005. (S5 & S6) Ion probability density profiles using SWM4-NDP. (S7) Probability density profiles for droplets with 4 NaCl. (S8) Probability density profiles and discussion for highly charged droplets with counterions. (S9) Dynamics.


Comments are not moderated before they are posted, but they can be removed by the site moderators if they are found to be in contravention of our Commenting Policy [opens in a new tab] - please read this policy before you post. Comments should be used for scholarly discussion of the content in question. You can find more information about how to use the commenting feature here [opens in a new tab] .
This site is protected by reCAPTCHA and the Google Privacy Policy [opens in a new tab] and Terms of Service [opens in a new tab] apply.