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

Abstract

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