Effect of Electric Fields on the Ultrafast Vibrational Relaxation of Water at a Charged Solid–Liquid Interface as Probed by Vibrational Sum Frequency Generation

The effect of the surface electric field on the ultrafast vibrational dynamics of interfacial water is studied using IR pump-sum frequency generation (SFG) probe spectroscopy. At very low salt concentrations, a vibrational lifetime (<i>T</i>₁) of ∼200 fs, similar to bulk H₂O, is observed for the O–H stretch at the H₂O/silica interface at pH 6, where the silica surface is negatively charged. However, <i>T</i>₁ increases to ∼700 fs by increasing the NaCl concentration to 0.01 M. The observation of similar dynamics for a range of salt concentrations, associated with different extensions of the electric field, suggest that the surface electric field is screened faster than predicted by classical electrical double-layer theories and that the Debye length may not be the appropriate measure of the depth sampling of the SFG response. An interfacial excess of cations is hypothesized to explain the faster decay of the static electric field than predicted by the Gouy–Chapman theory

Spectroscopy and Dynamics of the Multiple Free OH Species at an Aqueous/Hydrophobic Interface

Sum frequency generation (SFG) spectra and free induction decay (FID) measurements of the H₂O/octadecylsilane (ODS)/silica interface in the free OH spectral region (∼3700 cm^–1) show spatially inhomogeneous behavior. The SFG spectra and FIDs suggest an inhomogeneous response of the free OH, consisting of at least two distinct species at the interface with short and long coherence times. In most areas of the sample, an OH band at ∼3680 cm^–1 with a short dephasing (<150 fs), assigned to the free OH of water interacting with the hydrophobic methyl group of ODS, was observed in agreement with previously reported SFG spectra of the H₂O/ODS/silica interface. In a small fraction (∼20%) of the sample areas, a more intense peak at ∼3700 cm^–1 was observed in the SFG spectrum characterized by significantly longer dephasing (∼760 fs) in the SFG-FID. Based on the peak position, as well as control experiments on octadecydimethylmethoxysilane (ODMS) monolayers and deuterium substitution experiments at the water/ODS/silica interfaces, two possible assignments for the new feature are provided. The long dephasing can be due to the free OH of the Si–OH of incompletely cross-linked/tethered ODS molecules. Alternatively, a contribution of water molecules trapped in nano pores of silica surface and/or confined between the ODS molecules can explain the long coherence. Either way, the long coherence can be attributed to the OH species decoupled from bulk water