Bisulfate Dehydration at Air/Solution Interfaces Probed by Vibrational Sum Frequency Generation Spectroscopy

The structure and organization of ions at vapor/solution interfaces have great implications for the reactivity and growth of atmospheric aerosols. Considering the ionic components of aqueous aerosols, sulfate species are one of the most prevalent due to high levels of SO2(g) emission to the atmosphere from biofuel burning and volcanic eruptions. Atmospheric SO2(g) can undergo direct gas phase oxidation or experience dissolution and subsequent oxidation to sulfate species within aqueous aerosols, where, depending on the pH level, sulfate may exist as SO42-, HSO4-, or H2SO4. Here we probe the molecular environment experienced by the bisulfate anion (HSO4-) at vapor/solution interfaces for H2SO4, Na2SO4, and MgSO4 solutions via vibrational sum frequency generation (VSFG) spectroscopy. VSFG is an inherently interface specific nonlinear optical spectroscopy and is a powerful tool for the study of interfacial structure and organization. Our VSFG results are compared to bisulfate behavior in bulk aqueous solution observed using Raman and infrared spectroscopies. The presence of Na+ and Mg2+ is observed to perturb HSO4- anion hydration compared to H+ which manifests as a blue shift in the observed SO3 symmetric stretching mode frequency of HSO4-. This perturbation is greatly exaggerated for interfacial HSO4- anions residing within vapor/solution interfaces relative to bulk solution. Mg2+ ions are found to disrupt the net bisulfate population hydration within the vapor/solution interfaces tested, while Na+ ions only influence a subpopulation of the interfacial bisulfate distribution. This difference is attributed to the much greater propensity for aqueous solvation that Mg2+ exhibits compared to Na+. Our results are interpreted with a perspective toward understanding interfacial acid dissociation for the bisulfate anion and the role that this may play for tropospheric acidic aerosols.