Probing Aqueous Aerosols with Soft X-Ray Spectroscopy

Aerosol-based soft X-ray spectroscopy provides an element-specific probe of solvation and bonding in bulk and interfacial water. By confining liquids into submicron droplets and interrogating them with X-ray photoelectron spectroscopy (XPS), near-edge and extended X-ray absorption fine structure (NEXAFS, EXAFS), and resonant soft X-ray scattering (RSOXS), we obtain Ångström-scale insight into local solvation structure and ion pairing under conditions inaccessible to conventional bulk probes. In concentrated phosphoric acid, O K-edge measurements reveal a transition from hydrated monomeric phosphate to cyclic phosphate–phosphate dimers above ~7 M, providing direct evidence of inter-anionic association. Electronic-structure analysis shows stabilization by polarization and solvent-mediated charge transfer, challenging purely electrostatic descriptions of ionic solutions. Studies of ammonium sulfate aerosols further demonstrate that droplet surfaces exhibit distinct solvation environments and multiple ionic species relative to the bulk. Together, these results establish aerosol-based soft X-ray spectroscopy as a platform for resolving solvation structure and chemical bonding across bulk and interfacial aqueous environments.