Speaker
Description
Charged aqueous interfaces are the subject of extensive investigation due to their prevalence in both natural and industrial processes, with importance ranging across the biological, environmental, and chemical sciences. At such phase boundaries, the excess surface charge generates an electric field that penetrates through the electrolyte, perturbing the ion distributions and electric potential, as well as generating a rotational torque on the water dipoles and thus altering the distribution of molecular orientations. These alterations can persist up to several 10s of nanometers (i.e., 100s of molecular layers) and have important consequences for the role of the electrolyte in interfacial dynamics and reactivity. An important model that is extensively used to describe such charged interfaces is the Gouy-Chapman (GC) model, along with its Stern adaptation (Gouy-Chapman-Stern, GCS model), which describes either a single dielectric regime (GC), or two distinct layers (GCS): the compact layer in close proximity to the charged interface, and the diffuse layer beneath. While this model has successfully been employed to describe the field-driven anisotropy of the electric potential and interfacial ion excess, it nevertheless treats the constituent water molecules as a continuous medium. Clearly, therefore, many questions about the participation of the most abundant component of the electrolyte remain unanswered.
In this work, we directly probe these open questions using our recently developed depth-resolved vibrational spectroscopy technique that combines the phase-resolved sum- and difference-frequency generation (SFG and DFG) responses from an interface. By studying the depth-dependency of the water response, we elucidate the connection between the anisotropy in electric potential and any induced molecular reorientation. This firstly allows us to experimentally demonstrate that the induced water response features at least two distinct layers rather than the single-layer description of the GC model. Beyond this, using the two-layer GCS description, we then extract the spectra of the compact and diffuse layers, showing that they report distinctly different molecular structures and environments. Finally, through concentration-dependent studies, we extract the decay length of the induced orientational anisotropy of the water molecules within the diffuse layer and compare this to the theoretically predicted Debye screening length, finding significant discrepancies and thus highlighting the insufficiencies of the GCS model in describing the response of molecular water.
