Water Dynamics in the Electric Double Layer at the Lipid Interfaces: Water in the Diffuse Layer is Dynamically Different from Bulk Water.

Charged interfaces are ubiquitous; therefore, their molecular-level understanding is crucial. At such interfaces, an electric double layer (EDL) is formed, and the EDL is considered to consist of the compact Stern layer (SL) adjacent to the interface and the diffuse Gouy-Chapman layer (DL) extending into the bulk. Despite this well-established picture, the properties of each layer, particularly their dynamic aspects, remain poorly understood. Here, we employ two-dimensional heterodyne-detected vibrational sum-frequency generation (2D HD-VSFG) spectroscopy to investigate the hydrogen-bond dynamics in the SL and DL at two oppositely charged lipid/water interfaces. By varying the salt concentration of the solution, we are able to examine the structure and dynamics of water in the SL and the DL separately. We find that the major hydrogen-bond dynamics of not only SL but also DL are markedly different: The dynamics of the DL water are ultrafast (∼0.1 ps) at the positively charged DPTAP interface, whereas it is significantly slower (∼1 ps) at the negatively charged DPPG interface. These results demonstrate that the properties of the water in both SL and DL are significantly affected by the nature of the lipid at the water surface, challenging the widely accepted view that water in the DL is merely bulk-like.