Slow and Fast Dynamics at the Ionic Liquid/Gold Electrode Interface Separately Probed by Electrochemical Surface Plasmon Resonance Combined with Sequential Potential Pulse Techniques

Abstract

To investigate the potential dependence of the dynamics of the interface structure of an ionic liquid (IL), electrochemical surface plasmon resonance (ESPR) has been combined with normal pulse and differential pulse techniques at the gold electrode interface of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide. The fast response of SPR angle in 0.1 ∼ 0.2 s, originating from the orientation, distortion, and electronic polarization of ions in the first ionic layer, has been clearly observed, which is opposite to both the direction of the change in potential and of the slow response. This separate detection of the fast and slow responses has been realized based on the fact that the ionic rearrangement, which results in the slow response, does not proceed in a short potential pulse, especially for the positive pulse. The fast response, which reflects the interfacial dielectric constant, exhibits the potential-dependent dielectric saturation, i.e., a bell shape (or camel shape) potential dependence with a maximum around the potential of zero charge (PZC) and steep decrease at the potentials far from PZC. Molecular dynamics simulation explains that the decrease is caused by the strong electric field and ionic crowding in the first ionic layer which hinders the orientation and distortion of ions.