Sea-spray aerosol particles have major yet poorly understood influence on the state of the atmosphere. Although non-linear vibrational spectroscopy is a reliable technique for probing the nature of aerosol interfaces, resolving the spectral features into specific structural and dynamical properties of the interface poses substantial difficulties. Here, computer simulations are used to disentangle strictly surface-sensitive contributions from bulk-dependent effects at a model sea-spray aerosol, which allows for a detailed, molecular-level characterization of the interfacial properties.

Chemical separations, particularly liquid extractions, are pervasive in academic and industrial laboratories, yet a mechanistic understanding of the events governing their function are obscured by interfacial phenomena that are notoriously difficult to measure. In this work, we investigate the fundamental steps of ligand self-assembly as driven by changes in the interfacial H-bonding network using vibrational sum frequency generation. Our results show how the bulk pH modulates the interfacial structure of extractants at the buried oil/aqueous interface via the formation of unique H-bonding networks that order and bridge ligands to produce self-assembled aggregates. These extended H-bonded structures are key to the subsequent extraction of Co 2+ from the aqueous phase in promoting micelle formation and subsequent ejection of said micelle into the oil phase. The combination of static and time resolved measurements reveals the mechanisms underlying complexities of liquid extractions at high [Co 2+ ]:[DEHPA] ratios by showing an evolution of interfacially assembled structures that are readily tuned on a chemical basis by altering the compositions of the aqueous phase. The results of this work point to new mechanistic principles to design separations through the manipulation of surface charge, electrostatic screening, and the associated H-bonding networks that arise at the interface to facilitate organization and subsequent extraction File list (2)download file view on ChemRxiv DEHPA_pH_MS_submitted.pdf (1.11 MiB) download file view on ChemRxiv Supporting Information_DEHPA_submitted.pdf (384.42 KiB)

Aqueous solutions of FeCl3 have been widely studied to shed light on a number of processes from dissolution, mineralization, biology, electrocatalysis, corrosion, to microbial biomineralization. Yet there are little to no molecular level studies of the air–liquid FeCl3 interface. Here, both aqueous and glycerol FeCl3 solution surfaces are investigated with polarized vibrational sum frequency generation (SFG) spectroscopy. We also present the first ever extreme ultraviolet reflection–absorption (XUV-RA) spectroscopy measurements of solvated ions and complexes at a solution interface, and observe with both X-ray photoelectron spectroscopy (XPS) and XUV-RA the existence of Fe­(III) at the surface and in the near surface regions of glycerol FeCl3 solutions, where glycerol is used as a high vacuum compatible proxy for water. XPS showed Cl– and Fe­(III) species with significant Fe­(III) interfacial enrichment. In aqueous solutions, an electrical double layer (EDL) of Cl– and Fe­(III) species at 0.5 m FeCl3 concentration is observed as evidenced from an enhancement of molecular ordering of water dipoles, consistent with the observed behavior at the glycerol surface. At higher concentrations in water, the EDL appears to be substantially repressed, indicative of further Fe­(III) complex enrichment and dominance of a centrosymmetric Fe­(III) species that is surface active. In addition, a significant vibrational red-shift of the dangling OH from the water molecules that straddle the air–water interface reveals that the second solvation shell of the surface active Fe­(III) complex permeates the topmost layer of the aqueous interface.

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