Re-Organization of Water at Aqueous Aluminum Chloride (AlCl3) Interfaces: Vibrational Sum Frequency Generation and Molecular Dynamic Simulations

AlCl3 hydration states and complexation are not well understood both in solution and at the air-aqueous interface despite their potential significance in natural waters and their industrial and energy related applications. Here, we investigated Al3+ and Cl- ion behaviors in AlCl3 aqueous bulk solution and at the air-aqueous interface using interface-selective vibrational sum frequency generation (SFG), Raman, and infrared spectroscopies, classical molecular dynamics (MD) simulation as well as molecular-informed reduced modeling. Our reduced modeling reveals relatively long-range effects for Al3+ as compared to monovalent ions such as Na+ indicating the interfacial depth of trivalent ions can be significantly larger than monovalent ions at the air-water interface. MD simulations reveal interfacial stratification and multiple layering of the ions. Compression of the Al3+ and Cl- distributions with increasing concentrations from 0.5 m to 2.5 m is also observed in the subsurface regions. Significant SSP and PPP polarized SFG OH spectral intensity increases are observed from 0.5 to 1.5 m, and 0.5 to 2.5 m respectively, indicative of interfacial depth increases and a change in average orientation above 1.5 m. Extensive evaluation of SFG spectra, Fresnel corrected using several approaches, show the same trends. The non-monotonic trend points to a changing structure in surface and subsurface water orientation and hydrogen bonding environment generally consistent with the MD simulation of stratification and water orientation changes. Furthermore, solvent shared ion pairing is implicated with MD simulation radial distribution analysis and consistent with infrared spectral identification of the hexaaqua aluminum ion in solution phase. Spectral evidence of a strong Al3+ hydration shell and acidic behavior of the Al3+ ions is obvious in the Raman and infrared spectra of the bulk solution. The MD dipole potential and at the interface up to ~ 35 Å correlates with the spectral observations of increasing and then saturating intensities suggesting that both ion stratification and interfacial depth determine the water orientations at an air-water interface of 1-3 electrolyte solutions.