In Spectroscopic measurements. In situ UV/vis absorption spectra in total internal reflection mode (TIR-UV/vis) were recorded at the polarisable L|L interface using the setup described in Fig. and experimental methodology described previously by Molina-Osorio et al. In this optical setup, the light source passes through the organic EDL and is reflected at the L|L interface at an obtuse angle. As a result, the TIR-UV/vis absorbance spectra may contain signals from species either in the EDL on the organic side of the L|L interface, adsorbed on the L|L interface, or a combination of both. A silanised 4-electrode electrochemical cell was used to ensure that L|L interface was entirely flat. To ensure TIR conditions, lenses, a diaphragm, and mirrors were used to focus the light source onto the L|L interface at an angle of incidence of 75°. The latter is greater than the critical angle at an aqueous|TFT interface, determined to be ca. 70° previously. With lenses, the reflected light was focused onto an optical fibre connected to the spectrometer. Neutral density filters were used to reduce the intensity of the light. The heating of the interfacial region was reduced with hot mirrors that cut wavelengths above 700 nm. Under TIR conditions, the penetration depth of the evanescent field on the aqueous side of the L|L interface is ca. 200 nm. The L|L interface was established 30 min prior to the start of the in situ measurements to decrease any baseline drift during the TIR-UV/vis measurements. Furthermore, before the addition of 1 mL of oxidant to the aqueous phase, that same volume was removed from the blank aqueous electrolyte solution (used to obtain the "blank" spectra), so that the position of the L|L interface remained constant.