Abstract
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) χ2(ω) spectroscopic contributions from Density Functional Theory (DFT)-based ab initio molecular dynamics simulations (DFT-MD/AIMD), we show here that the χDL2(ω) 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 χDL2(ω) 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) α–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 χDL2(ω) alone with the protocols given in this work, or by making use of the interplay between experiments and simulations, as described in this work.
Highlights
Non-linear vibrational Sum Frequency Generation Spectroscopy (SFG) probes non-centrosymmetric media, such that one tunable Infrared (IR) beam and one fixed visible beam are mixed at the interface between two media, and the recorded SFG intensity is due to the second-order susceptibility χ(2) (ω )of the interface
We move the focus to the Diffuse Layer (DL)-SFG contribution, showing how the χ DL (ω ) signal provides a direct and quantitative measure of the ∆φDL electrostatic potential across the DL, and the net surface charge, but we show that this signal is directly sensitive to the ion distribution at interfaces, providing a means to discriminate if an Electric Double Layer (EDL) is formed at a given charged interface
12% dep), or are composed of the neat quartz–liquid water (QW) interface accomodating varied concentrations of excess K+ cation located in the BIL (Binding Interfacial Layer), i.e., 1.6 M, 4.2 M and 7.1 M
Summary
Non-linear vibrational Sum Frequency Generation Spectroscopy (SFG) probes non-centrosymmetric media, such that one tunable Infrared (IR) beam and one fixed visible beam are mixed at the interface between two media, and the recorded SFG intensity is due to the second-order susceptibility χ(2) (ω )of the interface. Non-linear vibrational Sum Frequency Generation Spectroscopy (SFG) probes non-centrosymmetric media, such that one tunable Infrared (IR) beam and one fixed visible beam are mixed at the interface between two media, and the recorded SFG intensity is due to the second-order susceptibility χ(2) (ω ). Since χ(2) (ω ) is zero in media with inversion symmetry, like the bulk of liquids and many solids, the SFG signal will only arise from the molecules at the interface between the two media where the centrosymmetry is broken. The surface-specificity of SFG makes this technique a perfect tool to probe the microscopic arrangement of molecules in the interfacial layers, and for this reason, it has been widely used to unravel the water structure at aqueous interfaces. The unique molecular arrangement at aqueous interfaces is at the basis of all the cited phenomena and its characterization is pivotal. There is no need to stress the relevance of aqueous interfaces in many fields, from the key role played by water–air interfaces in atmospheric chemistry [1,2,3,4], through the effect of the interfacial water structure on protein folding and peptide bond formation [5,6,7], to the importance of silica–water interfaces in pollutant transport in underground water in mineral dissolution and for drug delivery in our bodies [8,9].
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