Abstract
Anion concentrations at the air/water interface of saline droplets are important in atmospheric and environmental chemistry, because gaseous halogens emitted from the droplet surface mediate various key tropospheric chemical processes. In this study, the adsorption of water on alkali halide nanocrystals (KBr, KCl, KF, NaCl) on SiO2 and their deliquescence was in-situ investigated by noncontact atomic force microscopy (AFM) in an amplitude-modulation mode based on electrostatic forces. We also used ambient-pressure X-ray photoelectron spectroscopy in a synchrotron facility to complement our AFM results. For KBr, KCl and NaCl, deliquesced droplets show negative surface potentials relative to the surrounding region, indicating the preferential segregation of Br- and Cl- anions to the air/solution interface, even in the presence of a liquid/solid interface located a few nanometers away. This enhancement of anion concentration at the thin-droplet surface is more drastic for larger anions, meaning that heterogeneous reactions of gas-phase molecules with saline droplets to emit gaseous halogens can be more significant with larger anions.
