Eric Borguet , Temple University
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 extensive studies on the vibrational dynamics of bulk water, information in the interfacial water dynamics is far less developed. Sum-frequency generation (SFG) is a versatile surface specific technique that can provide spectroscopic and dynamics information on the interfacial water.
The SFG spectroscopy has suggested that the structure and orientation of water molecules at the interface are different from their behavior in 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.
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. 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~ 600 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.
In order to understand the effect of the surface charge and associated electric field on the ultrafast vibrational dynamics of interfacial water, we used IR pump-SFG probe on the aqueous-silica interface at pH=6 where the silica surface is negatively charged. At very low salt concentrations, a vibrational lifetime (T1) of ~200 fs, similar to bulk H2O, is observed for the O-H stretch. This result is also consistent with the experiments of the Shen group that reported that the dynamics of interfacial water was similar to that of bulk water. However, we observe that the vibrational relaxation rate slows by increasing the NaCl concentration to 0.01 M, and remains at T1~700 fs for concentrations as high as 0.5 M. The similarity dynamics for a range of salt concentrations, from 10-2 M to 0.5 M, associated with different extensions of the electric field into the near interface region, 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. Alternatively, the contribution of polarized near interface water to the SFG response may be negligible for NaCl concentrations of 10 mM or more at pH=6, so that only the first layer of incompletely solvated water molecules contributes to the slow vibrational dynamics revealed by our IR pump-SFG probe expeiments.
In order to access faster timescales, we have developed an ultra-broadband infrared non-collinear optical parametric amplifier (NOPA) and applied it to broadband sum-frequency vibrational spectroscopy of OH oscillators at mineral/water interfaces. The NOPA setup is relatively simple due to the use of bulk, rather than periodically poled, nonlinear optical crystals. The use of TIR-geometry for SFG acquisition, combined with the low-noise detection system enables collection of high-SNR spectra, while the broad bandwidth of the NOPA provides SF-vibrational spectra of the entire OH-stretch region without tuning the IR pulses. For single-spectrum acquisition, SNR values on the order of 100-120 for H2O/neat silica and ~35 for H2O/hydrophobic silica can be obtained in relatively short integration times (~60 sec). The bandwidth of the NOPA output should allow access to vibrational dynamics on the sub 30 fsec timescale.
In summary, ACS-PRF support has enabled new understanding of solid-liquid interfaces to be obtained. The knowledge that the vibrational dynamics of water at solid interfaces is much slower than in the bulk can guide our thinking about chemical and physical processes in these environments.