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Hydrogen bonds are responsible for many of the properties of water.  Fast molecular rearrangements of this dynamic network, resulting from the breaking and reforming of hydrogen bonds on the ultrafast time scale, dictate many fundamental phenomena that take place in aqueous media.  The rate at which water molecules accept the energy dissipated during the course of chemical reactions in aqueous media affects the reaction dynamics.  Despite the extensive studies on the vibrational dynamics of bulk water, information in the interfacial water dynamics is not developed yet.  Sum-frequency generation (SFG) is a versatile surface specific technique that can provide spectroscopic and dynamics information on the interfacial water.

In IR pump-SFG probe, a technique of choice for the study of vibrational dynamics at the surface, an IR pulse excites a vibrational mode and combination of second IR and visible generates SFG which probes the ground state bleaching.  The intensity of SFG as a function of time between IR pump and IR probe gives the information on the vibrational relaxation mechanism.

            The SFG spectroscopy has been suggested that the structure and orientation of water molecules at the interface has been shown to be different from the adjacent bulk.  It is important to investigate how the ultrafast dynamics is different for interfacial water compared to the bulk.  The water/silica interface is an ideal model system to explore the effect of ordering of water on vibrational dynamics.  In this system the surface charge, interfacial electric field and interfacial water structure can conveniently be changed via bulk pH.  The surface charge density at the silica/water interface is known to be close to zero at pH range of 2-5 and starts increasing from pH~6.  At pH>10 most of the surface silanols are deprotonated leading to a strong interfacial electric field.  

            Using IR pump-SFG probe spectroscopy we have found that at high pH, where the electric field resulting from deprotonation of silanol groups polarizes several layers of water molecules, fast vibrational dynamics similar to the dynamics of bulk water is observed.  At the neutral surface, where the structural ordering of interfacial water as well as the thickness of interfacial water sampled is smaller than at the charged surface, the vibrational lifetime of O-H stretch becomes more than two times longer (T₁~ 570 fs).  The longer vibrational lifetime is a result of reduced hydrogen bonding induced intermolecular coupling between interfacial water species that have lost part of their solvation shell compared to bulk.

            The coupling of vibrational modes in H₂O implies that the O-H stretching vibration is not localized on a single O-H bond, resulting in rapid spectral diffusion and fast intermolecular energy transfer.  In order to decouple O-H groups within an individual water molecule, on the one hand, and between surrounding water molecules, on the other, steady state and ultrafast spectroscopy of bulk water as well as the steady state SFG spectroscopy, but not the dynamics, of interfacial water have focused on the O-H (O-D) stretching vibration of dilute HDO in D₂O (H₂O).  The rate of inter and intramolecular energy transfer from excited OH modes of dilute HDO in D₂O decreases compared to pure H₂O since the frequency of the O-D stretching mode is far below that of the O-H mode.  Furthermore, decoupling the O-H and O-D oscillators within a single HDO species reduces intramolecular energy transfer and so information on the intrinsic lifetime of the decoupled O-H vibration can be obtained.  Thus by investigating HOD rather than H₂O, we simplify the dynamics. 

            We probed the vibrational dynamics of the O-H stretching mode in the hydrogen bonded region at the HDO:D₂O/silica interface at low and high bulk pH.   In contrast to the H₂O/silica system where two peaks were observed in the SFG spectra, we observed a single peak whose frequency redshifts upon increasing the bulk pH in the SFG spectra of the HDO/silica interface.  From the IR pump-SFG probe measurements, the vibrational lifetime of the OH stretch at the HDO/silica interface is observed to be frequency dependent with faster relaxation at the red compared to the blue side of the hydrogen bonded spectral region.  At the charged surface, the measured dynamics is faster than in bulk water.  On the contrary, at neutral pH, the interfacial vibrational dynamics is slower than in the bulk.