Role of Ions on the Surface-Bound Water Structure at the Silica/Water Interface: Identifying the Spectral Signature of Stability

Abstract

We explore the influence of salt addition on the structure of water interacting closely with a charged silica surface. Isolating these surface effects is challenging, even with surface-specific techniques like sum frequency generation (SFG), because of the presence of aligned water nanometers to microns away from the charged silica. Here we combine zeta potential and SFG intensity measurements with the maximum entropy method and reported heterodyne second harmonic and sum frequency generation results to deconvolute from the total signal intensity the SFG spectral contributions of the waters adjacent to the surface. Deconvolution reveals that at very low ionic strength the surface water structure is similar to that of a neutral silica surface near the point of zero charge with waters oriented in opposite directions. This result suggests the known metastability of silica near the PZC and the stability of silica in low ionic strength solutions may originate from the same source, these oppositely oriented surface-bound waters. Orientation changes are induced upon adding salt, which lead to a decrease in the total amount of aligned water at the surface. File list (2) download file view on ChemRxiv Rehl Manuscript.pdf (1.99 MiB) download file view on ChemRxiv Rehl Supporting Information.pdf (2.26 MiB)

Gate Alignment of Liquid Water Molecules in Electric Double Layer

“…The 3,200 cm −1 mode did not exhibit as much amplitude change as the other two modes, and remained positive in the whole potential range. We attributed this mode to "upward" OH groups donating H-bonds to the SiO 2 surface (highlighted in green in Figures 2C,D), similarly to the findings in Refs (Urashima et al, 2018;Rehl and Gibbs, 2021). Combining the fitting results and discussions above, we summarized the microscopic pictures of the interface.…”

“…Fitting parameters are also tabulated in Table 1. The fitting involved a low frequency mode ∼2,800-3,000 cm −1 besides the 3,200 cm −1 and 3,400 cm −1 modes, as in previous studies (Ostroverkhov et al, 2004;2005;Nihonyanagi et al, 2011;Pezzotti et al, 2019;Wang et al, 2019;Rehl and Gibbs, 2021). When the surface charge varied from negative to positive, the 3,400 cm −1 mode changed from a large positive amplitude to a negative one.…”

Charge-Induced χ(3) Susceptibility in Interfacial Nonlinear Optical Spectroscopy Beyond the Bulk Aqueous Contributions: The Case for Silica/Water Interface

“…There has been a recent revival of the studies on the often overlooked charge-induced χ (3) contributions from the polarized bulk water molecules near the charged interface to the SHG and SFG signal. − The main focus of these studies are trying to quantitatively understand the phase-resolved SFG-VS spectra of the charged aqueous interface and to address the phase-referenced SHG measurement data of the quartz/water interface . These efforts not only led to the corrected description of the charge induced χ (3) contributions of the polarized water molecules in general, − but also resulted in a series systematic treatment of the spectral lineshapes of the phase-resolved SFG-VS spectra of the charged aqueous interfaces involving the χ (3) contributions from polarized bulk water molecules near the charged interface. − With the explicit definition of the two phase angles, one of the charge-induced χ (3) terms and another between the χ (3) and the χ (2) , which are both experimentally measurable, Geiger and co-workers recently discovered that, for the SHG from the charged silica/water interface, there exists an additionally new imaginary nonresonant χ (3) term other than the traditionally considered χ (3) contributions from polarized bulk water molecules near the charged interface . We shall show that the χ (3) contributions from the bulk phase are actually complex and likely more complicated than what has been considered.…”

Charge Induced Chi(3) Susceptibility in Interfacial Nonlinear Optical Spectroscopy Beyond the Bulk Aqueous Contributions: The Case for Silica/Water Interface

“…SHG and SFG-VS has been used extensively in studying the charged silica/water interface. 1,4,14,[18][19][20][21][22][23][24][25][26][27] The interpretations based on non-resonant SHG and SFG-VS data and by considering of the  (3) contributions exhibited substantial disagreements and have been puzzling the community. 4,15 After reviewing the significantly different SHG signal data from the charged silica/water and the charged Langmuir monolayer covered air/water interface, [1][2][3] the necessity to consider the bulk silica  (3) contributions at the charges silica/water interface was recognized, and the details are yet to be hammered out.…”

“…7 These efforts not only led to the corrected description of the charge induced  (3) contributions of the polarized water molecules in general, [5][6][7] but also resulted to a series systematic treatment of the spectral lineshapes of the phase-resolved SFG-VS spectra of the charged aqueous interfaces involving the  (3) contributions from polarized bulk water molecules near the charged interface. [8][9][10][11][12][13][14] With the explicit definition of the two phase angles, one of the charge induced  (3) term and another between the  (3) and the  (2) ， which are both experimentally measurable, 12 Geiger and co-workers recently discovered that for the SHG from the charged silica/water interface there exists an additionally new imaginary non-resonant  (3) term other than the traditionally considered  (3) contributions from polarized bulk water molecules near the charged interface. 15 We shall show that the  (3) contributions from the bulk phase is actually complex and likely more complicated than what has been considered.…”