Unveiling microscopic structures of charged water interfaces by surface-specific vibrational spectroscopy

Charged water interfaces are ubiquitous and responsible for many important processes in nature and modern technologies, such as protein folding, electrochemistry, and photocatalysis. They appear in the form of an electric double layer that can be divided into two sub-layers. One is the so-called Stern layer composed of one to two hardly mobile, hydrogen-bonded, water monolayers next to the charged plane. The other is the diffuse layer, in which ions assuming the Poisson-Boltzmann distribution set up a dc field distribution. Being directly associated with the charged surface, the Stern layer directly controls the microscopic energy transfer and chemical reaction pathway at the interface. However, despite extensive studies on charged water interfaces, current knowledge on the microscopic structure of the Stern layer is still extremely limited. The difficulty lies in the inability of existing techniques to selectively extract structural information about the Stern layer in the presence of the diffuse layer. We now have developed a sum-frequency spectroscopy method that allows us to obtain the vibrational spectrum, and hence the microscopic structural information, of a Stern layer. Application of the method to a prototype lipid (fatty acid)-water interface reveals significant variation of its Stern layer structure upon deprotonation of the lipid headgroup. The measurement also yields a spectrum that characterizes the dc-field-induced sum-frequency generation from bulk water in general, and can help to deduce vibrational spectra of the Stern layer of other charged water interfaces. This unique ability of our method provides opportunities to gain better microscopic understanding of properties and functionality of charged water interfaces.