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 electrochemis try of the surfactants, alpha-(anthraquinonyloxyhexyl)-omega-hydroxy-oligo( ethylene oxide) (ACPEG) and alpha-anthraquinonyl-omega-hydroxy-oilgo (ethyl ene oxide) (APEG), which have wide differences in surface activity. Potenti al-pH diagrams have been constructed and the various features of the diagra ms 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 dian ion (AQ(2-)), which is highly sensitive to the pH of the solution. At contr olled pH, potential-pH plots allow the establishment of the values of the i onization constants for dihydroanthraquinone (AQH(2)) and its monoanion (AQ H(-)) as pK(a)(1) = 7.83 and pK(a)(2) = 11.38, respectively. Under unbuffer ed conditions, the effective pH close to the electrode surface controls the potential of the electrode process. The changeover from the H+-available t o the H+-depleted electrode process gives rise to a sudden jump in potentia l. In highly alkaline solutions, AQ forms an adduct with hydroxyl ion, whic h causes a linear decrease in the potentials with increase in pH. The diffe rent extent of micellization results in a difference in the peak current an d the half wave potentials (E-1/2) for ACPEG and APEG but causes no signifi cant change in the shapes of the E-1/2-DH diagrams. This has been explained in terms of the disruption reaction of the micelles, preceding the electro chemical reaction. (C) 2000 Elsevier Science S.A. All rights reserved.