Interfacial viscosity and ionic reorientation probed using electrochemical surface plasmon resonance at the gold electrode interface of ionic liquids

Abstract

The interfacial dielectric relaxation of ionic liquids (ILs) at the gold electrode interface has been investigated using electrochemical surface plasmon resonance (SPR), by analyzing the SPR response to the potential step for four ILs with the two cations, trioctylmethylammonium and 1-butyl-3-methylimidazolium, and three amide anions, bis(fluorosulfonyl)amide, bis(trifluoromethanesulfonyl)amide, and bis(nonafluorobutanesulfonyl)amide. For all the four ILs, the SPR response to the potential step exhibits a fast relaxation process mainly ascribable to the ionic reorientation, followed by an ultraslow relaxation process ascribable to the ionic translation, with the opposite directions of SPR shift to each other. The ultraslow relaxations by the positive potential steps are always significantly slower than those by the negative steps. The average time constants of the ultraslow relaxation process and the amplitudes of the fast relaxation process are evaluated by the fitting with a multiple exponential function and are compared with the measured bulk viscosity and the dipole moments of each ion. The interfacial viscosity, estimated from the time constants of the ultraslow relaxation process, is several orders of magnitude higher than the bulk viscosity, and the viscosity increase is larger for the ILs composed of smaller ions. The amplitude of the fast relaxation, reflecting the ionic reorientation, is moderately correlated with the dipole moments of counter-ions on the electrode at the pre-step potential.