Sum-Frequency Vibrational Spectroscopic Study on Charged Water Interfaces
Yu-Chieh Wen1*
1Institute of Physics, Academia Sinica, Taipei, Taiwan
* presenting author:Yu-Chieh Wen,
Charged water interfaces are ubiquitous and responsible for many important processes in nature and modern technology. Water molecules, solvated ions, and the (charged) substrate interact strongly with one another through hydrogen bonding, electrostatic force, and van der Waals force within a distance of a few monolayers away from the charged surface, forming an interface-specific bonding network. It is mainly this interfacial region, labeled as “the bonded interface layer (BIL)”, that governs the properties and functionality of the interface, but despite extensive studies over the years, still little is known about its microscopic structure. We have developed a new sum-frequency spectroscopy scheme that allows deduction of vibrational spectra of the BIL and the electrical double layer (EDL) separately and, hence, structural and charging information of water interfaces. Application of this scheme to a prototype lipid/aqueous interface as a demonstration revealed an interfacial hydrogen-bonding water layer structure that responds sensitively to the charge state of the lipid headgroup and its interaction with specific ions. We also applied this technique to studies on ion affinity at long-chain alcohol monolayer/water interfaces, chosen as a prototype for non-ionic organic/water interfaces. The spectra of the EDL allowed us to find densities of specifically adsorbed ions and surface pH/pOH quantitatively. The result showed significantly stronger affinity of OH- than H3O+ at the alcohol/water interface, suggesting that the interface is preferentially basic at bulk pH 7. This novel spectroscopic technique provides unique opportunities to search for better understanding of water interfaces at a deeper molecular level.

Keywords: Laser Spectroscopy, Nonlinear Optics, Interfacial Water