Effect of micelle formation on the absorption spectra of a functionalized detergent with the anthraquinone moiety
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 λmax= 330 nm (Îµapp), 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 Îµapp 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.