Abstract
The solvation of alkali and halide ions in the aqueous environment has been a subject of intense experimental and theoretical research with multidisciplinary interests; yet, a comprehensive molecular-level understanding has still not been obtained. In recent years, electron spectroscopy has been increasingly applied to study the electronic and structural properties of aqueous ions with implications, especially in atmospheric chemistry. In this work, we report core and valence level (Cl 2p, Cl 3p, and K 3p) photoelectron spectra of the common alkali halide, KCl, doped in gas-phase water clusters in the size range of a few hundred water molecules. The results indicate that the electronic structure of these nanosolutions shows a distinct character from that observed at the liquid–vapor interface in liquid microjets and ambient pressure setups. Insights are provided into the unique solvation properties of ions in a nanoaqueous environment, emerging properties of bulk electrolyte solutions with growing cluster size, and sensitivity of the electronic structure to varying solvation configurations.
Highlights
Aqueous alkali and halide ions are involved in a myriad of natural and technological processes, yet their structural and electronic properties are not fully understood
It was reported that the binding energies (BEs) of nanosolvated alkali and halide ions are insensitive to the counterions in water clusters with a few hundred water molecules,[11] similar to what has been observed in bulk solutions.[16]
To evaluate the similarity of the electronic structure of our pure water clusters to bulk water, we consider the vertical binding energy (VBE) of the water 1b1 valence level, which decreases with increasing cluster size and bridges the transition from single molecules to bulk.[30]
Summary
Aqueous alkali and halide ions are involved in a myriad of natural and technological processes, yet their structural and electronic properties are not fully understood. Clusters play a central role in probing the detailed interplay of ions with water molecules, in both experiments and theory.[1−6] Clusters can be considered as simplified models of bulk solutions, but they are in many ways unique as the solvation structures and dynamics are subject to, e.g., the high surface-to-volume ratio and distinct thermodynamic properties. It was reported that the binding energies (BEs) of nanosolvated alkali and halide ions are insensitive to the counterions in water clusters with a few hundred water molecules,[11] similar to what has been observed in bulk solutions.[16] Notable BE discrepancies remained, especially for the anion core levels that were attributed to calibration uncertainty in bulk level studies. The effect of changing salt concentration was addressed with signatures of ion pairing observed in the core level spectra.[12]
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