Effect of hydrogen-bond on ultrafast spectral diffusion dynamics of water at charged monolayer interfaces.

Ultrafast hydrogen-bond fluctuation dynamics of water at charged monolayer interfaces were studied by the use of steady-state and 2D heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy. Specifically, the effect of hydrogen-bond ability of the interface on the dynamics was investigated by comparing two monolayer interfaces that provide different hydrogen bond abilities: hydrogen bonding octadecylammonium (ODA) monolayer (pH = 2) and non-hydrogen bonding 1,2-dipalmitoyl-3-trimethyl-ammonium propane (DPTAP) monolayer. The steady-state HD-VSFG spectra and their ionic strength dependence revealed that water molecules at both of ODA and DPTAP interfaces are H-down oriented, pointing their H away from the interface, and that the contributions of the electrical double layer in the interfacial spectra of these interfaces are comparable to each other. However, 2D HD-VSFG data clearly indicated that the ultrafast hydrogen-bond fluctuation of water at the ODA interface is significantly suppressed, compared to that at the DPTAP interfaces. The obtained results suggest that the hydrogen-bond fluctuation of the topmost interfacial water at a positively charged interface is significantly affected by the hydrogen-bonding ability of the interface even in the case that the interfacial water molecules act as a hydrogen-bond acceptor to the head group of the monolayer.