Re organization of Electrolyte in DL

The IR spectra measured in situ are sensitive to (re)organization of electrolyte (both solute and solvent) species in the DL. On the one hand, this can provide unique information about the DL structure and, on the other hand, it can complicate determination of the spectrum baseline. The spectral changes associated with reorientation of an electrolyte species can be distinguished by using the corresponding SSR (Sections 1.8 and 3.11.4), as shown in the IR studies of perchlorate [157-159, 171], Cr(DMSO)e + [157], sulfate [172], bisulfate [173], nitrate [158] ions at Au, tetraethylammonium ions at Pt [151], perchlorate ions at Ge [174, 175], and ferri/ferrocyanide at Si [176]. The situation is complicated when an electrolyte species with degenerate IR-active modes appears in 

A second-order effect that contributes to the in situ IR spectra consists of change in the interfacial structure of solvent, including the coordination to the surface, solute, and self-organization. Correlation between the structure of solvent and the HL ionic composition and the electrode surface properties is a considerable objective not only in electrochemistry but also in other numerous areas of science and technology dealing with surface modification. A number of systems have been studied to date, including acetonitrile [110, 115, 159, 179], acetone [110, 179], methanol [110, 180], and benzene [110] at a Pt electrode. However, particularly interesting but yet little understood is the most common solvent, water. 

Ataka and co-workers [171] have presented the following interpretation of the absorption bands of water in the ATR-SEIRA spectra of the Au 0.5 M HCIO4 interface measured in the DL region from - -0.1 to -1-1.3 V (SHE) (Fig. 3.41). 

To selectively resolve the boundary water near a nonmetal-water interface, Hasegawa et al. [199] employed the polarized ATR at different angles of incidence in conjunction with principal component analysis (PCA). The surface of a hemicylindrical Si IRE was cleaned by use of ozone cleaner, which yielded Si—OH species at the surface. The vOH band of the interfacial water was similar to that in Fig. 3.41, spectrum 2 which implies that such nanometer-scale information can be obtained on a nonmetallic surface without using the SEIRA effect. Based on the polarization dependence, this spectrum was assigned to H-down water that forms strong symmetric double H bonds with the hydrophilic surface. The surface-perturbed water layer was found to be several monolayers thick, in agreement with the X-ray scattering data [192]. 

The position of the absorption bands of water in the hydration spheres depends on the origin of the ions [200] (Table 3.4, Part A). Certain correlations were also observed between ability of adsorbed ions to structure the water of hydration and IR spectra of interfacial water (Table 3.4, Part B). For a series of aqueous 

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