Voltammetry of immobilised microdroplets containing p-chloranil on basal plane pyrolytic graphite electrodes

Abstract

The electron transfer properties of p-chloranil (2,3,5,6-tetrachloro-1,4-benzoquinone, TCBQ) were investigated in both homogeneous and heterogeneous media. For the homogeneous study, the electrochemical reduction of TCBQ was carried out in different aprotic solvents (namely: benzonitrile (BN), N,N,dimethylformamide (DMF), propylcyanide (PrCN) and dimethylsulfoxide (DMSO)) and revealed two successive one-electron reductions according to a quasi-reversible EE mechanism. For the heterogeneous study, cyclic voltammetry with basal plane pyrolytic graphite electrodes modified with microdroplets of benzonitrile/TCBQ was employed. The droplets were found to be randomly dispersed with a degree of overlapping and average diameters of 5 μm giving the microdroplets individual volumes of ca. 33 fL. The redox processes within the electrically insulating microdroplets were shown to be very sensitive to the nature and concentration of ions in the surrounding aqueous phase as, in order to retain electroneutrality within the unsupported oil phase, electric field-induced migration of ions likely occurs during the Faradaic current flow. Depending on the lipophilicity of the aqueous electrolyte cation uptake into or electrochemical generated anion expulsion from the organic phase containing the electroactive specie TCBQ was induced electrochemically. Alkali metal cation uptake into the microdroplet environment was not observed. However less hydrophilic tetraalkylammonium cations NR₄⁺ (R⁺ = Bu and Pe) inserted. Proton insertion into the oil phase was also shown to occur as the current|voltage shifted to more positive potentials, making the reductive process more facile, as the pH of the buffer solution was decreased. The higher efficiency of proton insertion as compared with Group I cations insertion was explained in terms of the formation of strong O–H covalent bonds which outweighs the ion phase transfer thermodynamics. Finally, the cross-phase electron transfer across the benzonitrile|water interface was examined when the TCBQ microdroplets were purposely made conductive by addition of a hydrophobic, nonpartitioning electrolyte in the oil phase. Again, the resulted voltammetry was found to change depending on the identity and concentration of the salt dissolved in the surrounding aqueous environment.