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

AlCl3 hydration states and complexation are not well understood both in solution and at the air-aqueous interface despite its potential significance in natural waters. To address this knowledge gap, we investigated AlCl3 aqueous solutions using interface-selective sum frequency generation (SFG) vibrational spectroscopy and molecular dynamics (MD) simulation. Our findings indicate that AlCl3 has significant and non-monotonic impacts at the interface, and a broadening interfacial width from 5 Å to more than 35 Å. Aluminum retains full hydration without evidence of contact ion pairing or complexation with Cl. Across all concentrations studied, we observe significant perturbation of the hydrogen bonding environment with the strongest SFG signal enhancement at 1.5 m indicative of an expanded SFG-active interfacial region, that is, a significantly increased interfacial width. However, at the highest concentrations studied, 2.0 and 2.5 m AlCl3, a reversal in the intensity trend is observed that is not explained by transition moment increases. Concentration dependent molecular dynamic simulation reveals ion stratification with several Al3+ maxima as the concentration increases and differences in water orientation in the surface and subsurface layers. The dipole potential and at the interface up to ~ 35 Å correlates with the spectral observations of increasing and then decreasing intensities. We then acknowledge this correlation that implies that the water molecules are ordered such that the interfacial depth increases (larger interfacial region of nonzero-centrosymmetry) up to 1.5 m and then a reversal is then indicative of a shrinking interfacial width caused by a condensed interfacial layering of the Al3+.