Effect of micelle formation on the absorption spectra of a functionalized detergent with the anthraquinone moiety

Abstract

Measurements of the absorption spectra of aqueous solutions of (2-anthraquinonylmethyl)dodecyldimethylammonium (AQD⁺) bromide have been performed. From the concentration dependence of the molar absorption coefficient at λ_max= 330 nm (ε_app), the c.m.c. and the molar absorption coefficient of AQD⁺ participating in its micelle formation were calculated by assuming a micellar pseudo-phase model. The change in ε_app above the c.m.c. was found to be due to interactions of the anthraquinone moieties, by studying the effect of the addition of hexadecyltrimethylammonium bromide (CTAB) to the AQD⁺ micellar solution. The effect of the addition of CTAB to the AQD⁺ monomer solution was also examined, but the spectra of the AQD⁺ monomer were little affected by the addition of CTAB. Measurements of the absorption spectra of aqueous solutions of the monomeric AQD⁺ and sodium dodecylsulphate (SDS) point to the formation of a stable ion-pair complex between them. On further addition of SDS, the complex disappeared above the c.m.c. of SDS. This amphiphatic anthraquinone derivative serves as a probe for determining the c.m.c. of SDS in the absence and presence of electrolyte because the molar absorption coefficient at 290 nm changes by a factor of 7 above and below the c.m.c. of the host surfactant.