Probing the Real and Imaginary Dielectric Response of the Electric Double Layer Using Surface Plasmon Resonance Nanostructures

Surface plasmon resonance (SPR) provides a sensitive optical probe of electric double layers (EDLs), yet conventional EC-SPR measurements typically report only resonance-angle shifts (Δθ), offering limited insight into the coupled dielectric and structural response of the interface. Here, we use plasmon-resonant gold grating nanostructures to simultaneously measure both Δθ and reflectance modulation (ΔR) in aqueous electrolytes and ionic liquids over a wide temperature range. By incorporating the full complex refractive index into finite-difference time-domain (FDTD) simulations, we reproduce the experimentally observed voltage-induced changes in both Δθ and ΔR, something not achievable with models that neglect optical absorption in the EDL. This integrated experiment-theory approach allows us to extract quantitative information about local dielectric screening, interfacial absorption, and the spatial extent of the perturbed EDL. Using known temperature-dependent dielectric functions of water and ionic liquids, our FDTD model accurately captures the thermal evolution of Δθ in concentrated aqueous solutions, consistent with an increase in the effective Debye screening length at elevated temperatures. In contrast, the same model fails to reproduce the anomalous temperature and voltage response of the ionic liquid [EMIM][TFSI], reflecting the breakdown of Debye-type screening in dense Coulomb fluids and the dominant role of ion–ion correlations and interfacial structuring. These results highlight that only by including the imaginary dielectric response and using full FDTD modeling can one quantitatively interpret EC-SPR measurements and distinguish classical screening behavior from the nonclassical electrostatics of ionic liquids.