The Topmost Water Structure at a Charged Silica/Aqueous Interface Revealed by Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy

Abstract

Despite recent significant advances in interface-selective nonlinear spectroscopy, the topmost water structure at a charged silica surface is still not clearly understood. This is because, for charged interfaces, not only interfacial molecules at the topmost layer but also a large number of molecules in the electric double layer are probed even with second-order nonlinear spectroscopy. In the present study, we studied water structure at the negatively charged silica/aqueous interface at pH 12 using heterodyne-detected vibrational sum frequency generation spectroscopy, and demonstrated that the spectral component of the topmost water can be extracted by examining the ionic strength dependence of the Imχ spectrum. The obtained Imχ spectrum indicates that the dominant water species in the topmost layer is hydrogen-bonded to the negatively charged silanolate at the silica surface with one OH group. There also exists minor water species that weakly interacts with the oxygen atom of a siloxane bridge or the remaining silanol at the silica surface, using one OH group. The ionic strength dependence of the Imχ spectrum indicates that this water structure of the topmost layer is unchanged in a wide ionic strength range from 0.01 to 2 M.

Salting Up of Proteins at the Air/Water Interface

“…18,30,31 Overall, these spectra indicate that salts influence the appearance of the VSFG signal from BSA at the interface. We note the VSFG signal probes interfacial BSA only, and not the bulk BSA due to the electric field induced χ (3) term, [32][33][34][35][36][37][38][39][40][41][42][43] because of the large ionic strength (I=0.71 mol/L) used in this study.…”

New Insights into χ(3) Measurements: Comparing Nonresonant Second Harmonic Generation and Resonant Sum Frequency Generation at the Silica/Aqueous Electrolyte Interface

“…Although there have been simulations with explicit silica surfaces, [64][65][66] we have opted for our simplified model hydrophilic surface described in the experimental section as we have extensively compared its resulting interfacial water structure and generated model SFG spectra from this system 48,56,67 in comparison to experimental homodyne and phase-resolved spectra. 26,33,63,[68][69] In Figure 5A, C and E, we report the log of the ratio of these 3200 cm -1 and 3400 cm -1 hyperpolarizability per molecule values as a function of twist and tilt angle for ppp, ssp and pss polarization combinations, respectively. Although the log ratio values vary considerably depending on the twist and tilt angle, we see that overall the vast majority of twist and tilt angles lead to log ratio values greater than 0 for the ssp and ppp polarization combinations ( Figure 5B, and F, respectively), which corresponds to greater than .…”

Hydrogen bonding interactions of H2O and SiOH on a boroaluminosilicate glass corroded in aqueous solution

“…88,89 The technique has been used extensively to study hydrogen bonding interactions of hydrous species at optically flat surfaces and interfaces. 44,45,90 The spectral region of 3000-3600 cm −1 generally shows two OH stretch vibrational modes (bands) at~3200 cm −1 and~3400 cm −1 . The former is often attributed to strongly hydrogen-bonded OH stretch mode and the latter is generally assigned to weakly hydrogen-bonded one (see Fig.…”

“…These spectral features are also referred as "icelike" (or "solid-like") and "liquid-like" structures, respectively, in the literature. 44,91 The precise assignments of these spectral bands, however, are controversial and still under investigations, 44,90,[92][93][94][95][96][97][98][99][100][101][102] and this study will refer to these spectral features as 3200 cm −1 band and 3400 cm −1 band.…”