Unveiling structural and charging properties of water interfaces by surface nonlinear optical spectroscopy
Yu-Chieh Wen1*
1Institute of Physics, Academia Sinica, Taipei, Taiwan
* Presenter:Yu-Chieh Wen, email:ycwen@phys.sinica.edu.tw
Charged water interface are ubiquitous and responsible for many natural phenomena and of great importance in the development of advanced energy conversion and storage devices. Our physical picture of such an interface is an electric double layer composed of Stern layer and diffuse layer. The Stern layer controls microscopic reaction pathways, ion desolvation, and charge transfer at the interface, and, hence, directly dictates reactivity and functions of aqueous interfacial systems. Despite its importance, current knowledge on the microscopic structure of Stern layer and the ionic/molecular interactions therein is very limited. The difficulty lies in the paucity of experimental techniques for deducing structural and charging information about such a layer.
In this talk, I shall present our recent development and application of a hybridized phase-sensitive second harmonic generation and sum frequency vibrational spectroscopy (PS-SHG/SFVS). We proposed a new SHG scheme that allows direct characterization of the water surface charge selectively for any planar aqueous interface accessible by light. The achievement comes from the realization that the imaginary part of the effective surface nonlinear susceptibility for nonresonant SHG is solely contributed from the dc-field-reoriented water molecules in the electrical double layer (EDL), through which absolute values of the surface charge density and surface potential of the EDL can be quantified with the intrinsic third-order nonlinear susceptibility of bulk water without need of any prior interfacial information. Application of this technique to a mixed surfactant monolayer on water reveals the manifest effect of the chain-chain interactions among the monolayer on adsorption of soluble ionic surfactants. With the surface charge density known from the PS-SHG, we can further calculate the SFG vibrational spectrum of the EDL, and in turn, find the vibrational spectrum of the Stern layer from a PS-SFG measurement. Application of the new scheme to a prototype lipid/aqueous interface as a demonstration revealed the Stern layer structure that responds sensitively to the charge state of the lipid headgroup and its interaction with specific ions. Another application to prototype hydrophilic/hydrophobic water interfaces demonstrates its capability of determining densities of specifically adsorbed ions and surface pH/pOH, offering new insight into Hofmeister effect in surface physical chemistry.
Such hybridized SHG-SFG scheme offers new opportunities for separate investigations on charging and structural properties of various water interfaces on the microscopic level. The study will, therefore, lead to advances in our knowledge of interdisciplinary problems, such as erosion and catalytical mechanisms at oxide surfaces, and incorporation of interfacial water onto charged cell membranes.

Keywords: Second harmonic generation, Sum frequency generation, water interface, Hofmeister effect, Surface physical chemistry