Ion Spatial Distributions at the Air- and Vacuum-Aqueous K2CO3 Interfaces

The spatial distribution of electrolyte ions at water interfaces remains a topic of considerable interest to the atmospheric, geochemical, and physical sciences communities. Here, the depth-resolved spatial distributions of K+ and CO32- from 0.5 and 1.1 M aqueous solutions of potassium carbonate (K2CO3) are measured by synchrotron-based X-ray photoelectron spectroscopy (XPS) in combination with a liquid microjet. The ion distributions determined from the intensities of the K 2p and C 1s orbitals are consistent with the K+ cation residing on average slightly closer to the interface than the CO32- anion. The interface of this solution is the broadest yet reported for an electrolyte solution by depth-resolved XPS, consistent with earlier molecular dynamics simulations of aqueous Na2CO3 that showed a large (>1 nm) ion depletion layer at the interface. Results are compared, where possible, between liquid jets running in vacuum (1 x 10(-4) mbar) and jets in an equilibrated background vapor pressure that is determined by the temperature of the solution (6 mbar in this case). The ion spatial distributions and the molecular-level pictures of the air- and vacuum-aqueous electrolyte interfaces as derived by XPS are identical.