Anisotropic Water Structure at Charged Interfaces Studied by Depth-Resolved Vibrational SFG/DFG Spectroscopy

The molecular water structure at charged aqueous interfaces is shaped by interfacial electric fields, which can induce significant anisotropy in the molecular orientations extending over nanometer-scale distances. Despite its great relevance, very little is known about the details of this depth-dependent anisotropic water structure, mainly due to the lack of appropriate experimental techniques. Here, we present a depth-resolved study of the water anisotropy at the interface with insoluble charged surfactants using a newly developed technique, which allows for directly correlating nonlinear vibrational spectra with depth information on the nanometer scale. We demonstrate that the obtained data allows for a reconstruction of the nonlinear vibrational responses as a function of depth. The results for the case of low-salinity solutions show the presence of two pronounced regions within the interfacial anisotropy with largely deviating degrees of preferential molecular orientations. A spectral analysis of the depth-dependent vibrational responses furthermore reveals that the natural local hydrogen-bond structure of bulk water remains largely unperturbed throughout the interfacial region, including water in direct proximity to the surface charges. These findings significantly refine our understanding of the anisotropic water structure at the interface with hydrophilic charged surfactants and showcase the large potential of our depth-resolved spectroscopic technique.