Voltammetric Manifestation of the Ultraslow Dynamics at the Interface between Water and an Ionic Liquid

The ultraslow relaxation (on the order of minutes) of the electrical double-layer structure, related to a change in the phase-boundary potential across the interface between water (W) and the ionic liquid (IL) trioctylmethylammonium bis(nona-fluorobutanesufonyl)amide ([TOMA(+)][C(4)C(4)N(-)]) (Y. Yasui et al., J. Phys. Chem. B. 2009, 113, 3273), appears to be invisible in the transfer of tetrapropylammonium ions across the [TOMA(+)][C(4)C(4)N(-)]vertical bar W interface, provided that the charging current, which shows an unusual dependence on the voltage scan rate, is subtracted to obtain the faradaic current. This counterintuitive observation can be explained by the differences in the timescales of the fast and slow components of the relaxation dynamics of the electrical double layer on the IL side (ms and min). In contrast, the effect of the slow dynamics becomes surfaced in ion-transfer voltammetry when the ion is surface-active. The transfer of pentadecafluorooctanoate across the [TOMA(+)][C(4)C(4)N(-)]vertical bar W interface is irreversible, which is attributable to the self-inhibition of pentadecafluorooctanoate ions transferred to the IL phase. This process is likely to be affected by the ultraslow structural change of the IL side of the interface.