Experimentally Quantifying Anion Polarizability at the Air/Water Interface

The adsorption of anions from aqueous solution on the air/water interface controls important heterogeneous chemistry in the atmosphere and is thought to have similar physics to anion adsorption at hydrophobic interfaces more generally. While much theoretical and experimental work over the last 20 years has made clear that the adsorption of large, polarizable anions is thermodynamically favorable, determining the role of polarizability in adsorption has proven surprisingly challenging. Simple physical models make clear that nonpolarizable anions will not adsorb, but trends in adsorption are difficult to rationalize based on polarizability and in some theoretical approaches adsorption is observed without change in anion polarization. Because there are no experimental studies of interfacial anion polarizability, one possible explanation of these results is that theoretical descriptions suffer from systematic error. In this study we use interface specific nonlinear optical spectroscopy to extract the spectral response of the interfacial ClO$_{4}^{-}$ anion. We find that (i) the interfacial environment induces a break in symmetry of the anion due to its solvation anisotropy (ii) the Raman depolarization ratio, a measure of the ratio of the change in two components of ClO$_{4}^{-}$'s polarizability tensor with change in nuclear positions, is $>2\times$ larger than its value in the adjoining bulk phase and interfacial concentration dependent and (iii) using a simple theoretical description we find that our measured changes in depolarization ratio are consistent with known changes in surface potential and tension with small changes in bulk ClO$_{4}^{-}$ concentration. The notion that interfacial anion polarizability differs from that in bulk, and that this polarizability is coverage dependent, is not accounted for in any current theoretical treatment of ions at the air/water interface.