Reorganization of Water at Aqueous Aluminum Chloride (AlCl ₃ ) Interfaces: Vibrational Sum Frequency Generation and Molecular Dynamics Simulations

AlCl₃ hydration states and complexation are not well understood both in solutions 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 an AlCl₃ aqueous bulk solution and at the air-aqueous interface using interface-selective vibrational sum frequency generation (SFG), Raman and infrared spectroscopies, 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 that the interfacial depth of trivalent ions can be significantly larger than that of 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 to 2.5 <i>m</i> 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 <i>m</i> and 0.5 to 2.5 <i>m</i>, respectively, indicative of interfacial depth increases and a change in average orientation above 1.5 <i>m</i>. Extensive evaluation of SFG spectra, Fresnel-corrected using several approaches, shows the same trends. The nonmonotonic 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 the solution phase. Spectral evidence of a strong Al³⁺ hydration shell and the acidic behavior of the Al³⁺ ions is obvious in the Raman and infrared spectra of the bulk solution. We show that the MD dipole potential is directly related to the MD second-order susceptibility of the interface, χ_SFG-MD⁽²⁾, both of which correlate up to ∼35 Å 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.