Electrochemically driven ion insertion processes across liquid/liquid boundaries: Neutral versus ionic redox liquids

The oxidation and rereduction of the redox liquids para-N,N,N',N'-tetrahexy lphenylenediamine (THPD) and para-N,N,N'-trihexylphenylenediamine (TriHPD) associated with anion and proton insertion and expulsion are studied as a f unction of the proton concentration in aqueous NaClO4 electrolyte. Voltamme tric, in situ UV/vis-spectroelectrochemical, and quartz crystal microbalanc e techniques are employed. The biphasic acid-base equilibria of the redox l iquids involving protonation and simultaneous anion transfer from the aqueo us phase are shown to exhibit only small deviation from ideal behavior and well-defined biphasic dissociation constants, pK(A,biphasic) have been dete rmined. However, the protonation of the bulk redox liquids is shown to be d ominated by intermolecular rather than intramolecular factors. In particula r, the ability of THPD to undergo bulk protonation by HClO4 is higher (pK(A 2,biphasic) = 5.1) compared to that of TriHPD (pK(A2,biphasic) = 3.9); this is opposite to the behavior predicted on the basis of the estimated values for the aqueous protonation equilibrium constants, pK(A2) = 7.5 +/- 0.5 an d pK(A2) = 8.8 +/- 0.5 for THPD and TriHPD, respectively. Further, the elec trochemically driven deprotonation occurs irrespective of protonation const ants at essentially the same potential for both materials. The three-phase junction electrode\redox liquid\aqueous electrolyte for the initiation of t he anion and proton insertion-electrochemical reactions is shown to be the key to processes observed for neutral redox liquids, whereas ionic redox li quids show reactivity independent of the three-phase junction due to suffic ient ionic bulk conductivity.