Water Structure at the Buried Silica/Aqueous Interface Studied by Heterodyne-Detected Vibrational Sum-Frequency Generation

Abstract

Complex χ(2) spectra of buried silica/isotopically diluted water (HOD-D2O) interfaces were measured using multiplex heterodyne-detected vibrational sum frequency generation spectroscopy to elucidate the hydrogen bond structure and up/down orientation of water at the silica/water interface at different pHs. The data show that vibrational coupling (inter- and/or intramolecular coupling) plays a significant role in determining the χ(2) spectral feature of silica/H2O interfaces and indicate that the doublet feature in the H2O spectra does not represent two distinct water structures (i.e., the ice- and liquid-like structures) at the silica/water interface. The observed pH dependence of the imaginary χ(2) spectra is explained by (1) H-up oriented water donating a hydrogen bond to the oxygen atom of silanolate, which is accompanied by H-up water oriented by the electric field created by the negative charge of silanolate, (2) H-up oriented water which donates a hydrogen bond to the neutral silanol oxygen, and (3) H-down oriented water which accepts hydrogen bonds from the neutral silanol and donates hydrogen bonds to bulk water molecules. The broad continuum of the OH stretch band of HOD-D2O and a long tail in the low frequency region represent a wide distribution of strong hydrogen bonds at the silica/water interface, particularly at the low pH.

Interfacial solvation can explain attraction between like-charged objects in aqueous solution

“…surface charge density but that water molecules indeed flip around with increasing pH34,35,[37][38][39] -which corresponds to increasing negative charge -in line with the behaviour suggested by MD simulations(Figs. 1 and 2).…”

The elusive silica/water interface: isolated silanols under water as revealed by vibrational sum frequency spectroscopy

“…35 Consistent with the proposed assignment, the reversed sign indicates that the OH groups of isolated silanols have an opposite net polar orientation from those in water molecules aligned by the negative surface electric field (i.e. water hydrogens are oriented on average towards the surface, 22 while those in silanols point away from the surface). Had the surface been positively charged, all three modes would have had the same phase (i.e.…”

“…Although the precise assignments of the two bands remain a source of debate, 21,22,25,26 they are consistently linked to water vibrations in the interfacial region. Indirect information of the fate of silanols under water has been inferred by SHG, where depending on the pH history of the silica substrate, two or three successive jumps in the second harmonic response have been observed with increasing pH.…”

Phase-referenced nonlinear spectroscopy of the α-quartz/water interface

“…Yet, phase information has not been recovered in traditional SHG detection schemes, as they only collect the square modulus of the signal. Although phase information from SHG and vibrational sum frequency generation (SFG) signals can be readily obtained through coherent interference of the signal of interest with an external910111213141516 or internal1718 phase standard, applications of such reference techniques to determine the phase of SHG signals generated at buried interfaces, such as charged oxide/water interfaces, is challenging due to the presence of dispersive media on both sides of the interface. In addition, the interface between water and α-quartz, the most abundant silicate mineral in nature192021, has been theoretically predicted to produce a more ordered interfacial water layer than amorphous silica222324, although this has not yet been probed using even traditionally detected SHG, Shen and co-workers applied nonlinear optics under resonant conditions to the α-quartz/water interface but did not report measurements taken at different rotational angles182325.…”

“…as the non-centrosymmetric bulk generally produces second harmonic signals that overpower surface SHG signals by orders of magnitude to the point where the surface signal is indistinguishable from the bulk response. Doing so under non-resonant conditions, however, would circumvent interaction terms between possibly potential-dependent χ (2) and χ (3) contributions to which resonantly enhanced SHG26 or SFG16 studies of charged interfaces may be sensitive.…”