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

Abstract

Historically, different pH dependent behaviour at the mineral oxide/aqueous electrolyte interface has been observed by non-resonant second harmonic generation (SHG) and resonant sum frequency generation (SFG), despite a general understanding that both techniques are dominated by the response of water. Here we compare the two at the silica/aqueous interface at high salt concentration and as a function of pH to shed light on the origins of both measurements. From this comparison and SHG measurements at the silica/air interface, we conclude that SHG originates from the net order of water and the silica substrate, with the latter dominating the observed intensities below pH 6.5. In contrast, SFG is dominated by the higher SF activity, yet lower number density, of waters that contribute in the low wavenumber range, according to molecular dynamic simulations. Furthermore, spectral resolution in SFG of oppositely oriented water populations prevents cancellation of signal making it more difficult to relate SF intensity to the net order of water.

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

“…For nonresonant SHG, the water are also expected to dominate the signal although other oscillators such as the silica may also contribute. 45 Moreover, the magnitude ratio of (2) and χ (3) would have to be the same on and off resonance for the signal phase angle to be identical in both experiments. Nevertheless, the change in HD-SHG signal phase is intrinsically related to the change in the diffuse layer thickness, which should play a significant role in the resonant SFG spectra as well.…”

A hydrogen-bonding structure in self-formed nanodroplets of water adsorbed on amorphous silica revealed via surface-selective vibrational spectroscopy

“…While deprotonation behavior of silanol groups is complicated with having multiple pK a , 30 there is no doubt that silica surface in contact with aqueous solution has negative charge at pH > 2 due to deprotonation of the surface silanol groups (SiOH → SiO -+ H + ). [30][31][32] Because the adsorbed water partially behaves like liquid, it is possible for the hydration layer to accept the proton released from the silanol groups. However, while it is expected that the surface negative charge makes water molecules be "silica-oriented" (hydrogen atoms toward the silica) owing to electrostatic interaction, our spectra indicate that water molecules are mainly air-oriented even at 90%RH.…”

Cloud history can change water–ice–surface interactions of oxide mineral aerosols: a case study on silica

“…This provides additional evidence that the surfacesolution system significantly changes with the iteration of the TP. Regardless of the vague behavior of the confined liquid signal, optimizing this setup may ultimately allow us to extend our studies to investigate interfacial water melted between ice and solid surface or the so-called quasi-liquid layer (Nagata et al, 2019;Li and Somorjai, 2007;Rosenberg, 2005;Döppenschmidt et al, 1998), which also has industrial applications (e.g., ski sports or frozen-food packaging).…”