Unexpected Behavior of Chloride and Sulfate Ions upon Surface Solvation of Martian Salt Analogue

Abstract

Gas-phase interactions with aerosol particle surfaces are involved in the physicochemical evolution of our atmosphere as well as those of other planets (e.g., Mars). However, our understanding of interfacial properties remains limited, especially in natural systems with complex structures and chemical compositions. In this study, a surface-sensitive technique, ambient pressure X-ray photoelectron spectroscopy, combined with molecular dynamics simulations, were employed to investigate a Martian salt analogue sampled on Earth, including a comparison with a typical sulfate salt (MgSO 4 ) commonly found on both Earth and Mars. For MgSO 4 , elemental depth profiles show that there always exists residual water on the salt surface, even at very low relative humidity (RH). When RH rises, water is well mixed with the salt within the probed depth of a few nanometers. The Cl − -and SO 4 2− -bearing Martian salt analogue surface is extremely sensitive to water vapor, and the surface layer is already fully solvated at very low RH. Unexpected ion-selective surface behavior are observed as RH rises, where the chloride is depleted, while another major anion, sulfate, is relatively enhanced when the surface becomes solvated. Molecular dynamics simulations suggest that, upon solvation with the formation of an ion-concentrated water layer adsorbed on the crystal substrate, monovalent ions experience a higher degree of dehydration than the divalent ions. Thus, to complete their first solvation shell, monovalent ions are driven away from the surface and move toward the water accumulated at the hydrophilic crystal structure.

Adsorbed Water Promotes Chemically Active Environments on the Surface of Sodium Chloride

“…However, because gas phase SO 2 can be taken up together with water vapor as RH rises, it is more likely that Cl – is repelled from the gas–salt surface by competing anions, like SO 4 2– or HSO 4 – . It is interesting that, on other hydrated salts (e.g., MgCl 2 ·8H 2 O crystal), the observed behavior is the reverse, with monovalent ions drawn toward the water–salt interface to complete their solvation shells, while divalent ions are pushed toward the gas phase interface . The S/O ratio begins at 0.5 for RH = 0% as expected with only the SO 2 present but quickly decreases with the presence or water vapor and then remains steady around 0.33, suggesting comparable uptake of the gas phase H 2 O and SO 2 .…”

“…The trace amounts are likely attributable to adsorbed water that cannot be removed from the drop cast samples under these conditions. When SO 2 is introduced, a sharp peak around 535 eV grows over time, which corresponds to oxygen in sulfate. , Like the sodium NEXAFS spectra, after 5 h, the oxygen spectra show features similar to pure Na 2 SO 4 . Again, a RH reduction does not affect the sulfate in the oxygen spectrum.…”

“…Noticeably, around 543 eV, a new structure does appear that, according to the energy, likely originates from water bonded to sulfate. 23 This component is also maintained when the RH is reduced to 0%, indicating that strong bonds are formed.…”

“…Sulfur spectra are shown in Figure b. The spectrum δ shows that, after 50 min, both sulfate and bisulfite (HSO 3 2– ) doublets are observed on the surface (sulfate at 169.6 and 168.5 eV and bisulfite at 168.4 and 167.3 eV), − showing the SO 2 uptake and transformation on the surface. After 140 min, the sulfate doublet grew further (note the lower SF applied in spectrum ε).…”

Probing Complex Chemical Processes at the Molecular Level with Vibrational Spectroscopy and X-ray Tools

“…In Figure 6A,B, we show the N K-edge and S L 2,3 -edge NEXAFS spectra generated from an ammonium sulfate aerosol, a system of much interest in earth's atmosphere 67 and possibly also on other planets. 68 Superimposed on that figure are literature spectra for ammonium generated from a liquid jet 69 and theoretically calculated using time-dependent DFT calculations performed by Carter-Fenk and Head-Gordon. 70 These types of theoretical calculations using coupled clusters 71 and the more recent time-dependent DFT 70 provide profound insight into the nature of solvation and hydrogen bonding embedded in the NEXAFS spectra.…”