Surface activity and redox behavior of nonionic surfactants containing an anthraquinone group as the redox-active site

Abstract

Two new nonionic surfactants, α-anthraquinonyloxyhexyl-ω-hydroxy-oligo(ethylene oxide) (ACPEG) and α-anthraquinonyl-ω-hydroxy-oligo(ethylene oxide) (APEG), were synthesized. The fundamental interfacial behavior of these surfactants at the air/water interface has been investigated by means of surface tensiometry to provide an insight into the relationship between the structure of the hydrophobic moiety and the surfactant properties, with specific emphasis on the anthraquinone linkage. Aggregation numbers of the surfactants have been determined from static light scattering measurements. At a constant hydration of the ethylene oxide chain, the increase in the hydrophobic chain length in ACPEG raises its hydrophobic interaction and results in enhanced aggregation and significant variation in the interfacial and micellization properties compared to APEG. The electrochemical behavior of the surfactants has also been studied in 0.16 M NaCl aqueous solutions and in 0.1 M tetrabutylammonium perchlorate acetonitrile solutions at the interface of a glassy carbon electrode. A difference in the extent of aggregation has a pronounced effect on the cyclic voltammetric behavior of the surfactants in aqueous solution. In organic media, on the other hand, the redox process depends only on the molecular geometry of the monomeric species. A comparison of the electrochemical responses in the two phases has been made to explain the distinctive features of the redox properties of the surfactants.