Depth-resolved SFG/DFG spectroscopy of charged aqueous interfaces Alvaro Diaz-Duque, Alexander Fellows, Martin Wolf, Martin Thämer Fritz Haber Institut der Max Planck Gesellschaft, Deutschland Charged aqueous interfaces are omnipresent in our world, being a crucial ingredient in both natural systems including our physiology, the atmosphere, or geological structures, as well as technical devices such as electrochemical cells. Because of this importance such interfaces have been subject to extensive theoretical and experimental studies with the goal to obtain a microscopic understanding of their nature. The presence of interfacial charges and the generated electric fields leads to the formation of substantial anisotropies over a relatively large length scale in the interfacial region. These anisotropies result in depth dependent variations in chemical and physical properties in the electrolyte such as gradients in chemical compositions, depth specific molecular structures and modified dynamics of the water network. Over the past century important advances have been made in experiment and theory yielding a detailed molecular picture on interfacial ion distributions or the evolution of the electric potential with depth (e.g. the Gouy-Chapman (GC) or Gouy-Chapman-Stern (GCS) models), however, much less is known about the depth-depended properties of the main constituent in such electrolyte systems, the water. In the majority of the commonly used models the water is treated as a homogeneous solvent although from the current knowledge that we have on the complexity of the water structure and dynamics it becomes clear that this assumption is too simplistic. In this contribution, we investigate the depth dependent water structure at charged interfaces experimentally using the recently developed depth resolved nonlinear vibrational spectroscopy (SFG/DFG method)(1). This method allows for the analysis of preferential molecular orientations and intermolecular forces and correlates this information with spatial coordinates orthogonal to the interface. By probing both positive and negative charges, as well as their mixtures, and varying the sub-phase salt concentration, the depth-dependent structure of water in response to the residual (screened) field is extracted. The expected results from the GC and GCS models are then compared to the experimental observations to highlight any shortcomings. References 1. J. Phys. Chem. C 126, 26, 10818–10832 (2022).
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