Effect of pH and the extent of micellization on the redox behavior of non-ionic surfactants containing an anthraquinone group

Cyclic voltammetry has been employed for the study of aqueous electrochemistry of the surfactants, α-(anthraquinonyloxyhexyl)-ω-hydroxy-oligo(ethylene oxide) (ACPEG) and α-anthraquinonyl-ω-hydroxy-oilgo(ethylene oxide) (APEG), which have wide differences in surface activity. Potential–pH diagrams have been constructed and the various features of the diagrams have been analyzed in the light of the change in solution equilibria and the difference in the extent of micellization. The redox potentials of the surfactants have been found to exhibit strong pH dependence. The electrode reaction involves two-electron reduction of anthraquinone (AQ) to its dianion (AQ2−), which is highly sensitive to the pH of the solution. At controlled pH, potential–pH plots allow the establishment of the values of the ionization constants for dihydroanthraquinone (AQH2) and its monoanion (AQH−) as pKa1=7.83 and pKa2=11.38, respectively. Under unbuffered conditions, the effective pH close to the electrode surface controls the potential of the electrode process. The changeover from the H+-available to the H+-depleted electrode process gives rise to a sudden jump in potential. In highly alkaline solutions, AQ forms an adduct with hydroxyl ion, which causes a linear decrease in the potentials with increase in pH. The different extent of micellization results in a difference in the peak current and the half wave potentials (E1/2) for ACPEG and APEG but causes no significant change in the shapes of the E1/2–pH diagrams. This has been explained in terms of the disruption reaction of the micelles, preceding the electrochemical reaction.