Molecular Spectroscopy Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
J Chem Phys. 2019 Feb 7;150(5):054705. doi: 10.1063/1.5081077.
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.
通过稳态和二维异频探测振动和频光谱（HD-VSFG）技术研究了荷电单层界面上水的超快氢键波动动力学。具体来说，通过比较两种提供不同氢键能力的单层界面，研究了界面氢键能力对动力学的影响：氢键十八烷基铵（ODA）单层（pH=2）和非氢键 1,2-二棕榈酰基-3-三甲铵丙烷（DPTAP）单层。稳态 HD-VSFG 光谱及其离子强度依赖性表明，在 ODA 和 DPTAP 界面上的水分子均为 H-朝下取向，即 H 指向远离界面的方向，并且这些界面的界面光谱中的双电层贡献彼此相当。然而，二维 HD-VSFG 数据清楚地表明，与 DPTAP 界面相比，在 ODA 界面上水的超快氢键波动显著受到抑制。所得结果表明，即使在界面水分子充当单层头基的氢键受体的情况下，带正电荷界面上最顶层界面水的氢键波动也会受到界面氢键能力的显著影响。
