Iron(III) Speciation Observed at Aqueous and Glycerol Surfaces: Vibrational Sum Frequency and X-ray

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.