What the Diffuse Layer (DL) Reveals in Non-Linear SFG Spectroscopy

Following our recent work [Phys. Chem. Chem. Phys. 20:5190-99 (2018)] that provided the means to unambigously define and extract the three water regions at any charged interface (solid-liquid and air-liquid alike), denoted the BIL (Binding Interfacial Layer), DL (Diffuse Layer) and Bulk, and how to calculate their associated non-linear Sum Frequency Generation Spectroscopy (SFG) chi(2)(omega) spectroscopic contributions from Density Functional Theory (DFT)-based ab initio molecular dynamics simulations (DFT-MD/AIMD), we show here that the chi(2)(DL) (omega) signal arising from the DL water region carries a wealth of essential information on the microscopic and macroscopic properties of interfaces. We show that the chi(2)(DL) (omega) signal carries information on the surface potential and surface charge, the isoelectric point, EDL (Electric Double Layer) formation, and the relationship between a nominal electrolyte solution pH and surface hydroxylation state. This work is based on DFT-MD/AIMD simulations on a (0001) alpha-quartz-water interface and on the air-water interface, with various surface quartz hydroxylation states and various electrolyte concentrations. The conclusions drawn make use of the interplay between experiments and simulations. Most of the properties listed above can now be extracted from experimental chi(2)(DL) (omega) alone with the protocols given in this work, or by making use of the interplay between experiments and simulations, as described in this work.