The system of methyl orange and the cationic alkyltri-The spectral signature of the azo dye methyl orange (MO) exhibits methylammonium bromide (C n TAB) surfactants, particua considerable sensitivity to the polarity of the dye's environment. larly cetyltrimethylammonium bromide (CTAB, n Γ
16), We have used this dependence of absorption wavelength on microenhas been the focus of numerous spectroscopic studies (1, 3vironment to investigate the 1:1 ion pair complexes formed from 12). These investigations have revealed several key changes electrostatic interaction of a series of cationic alkyltrimethylammonin the absorption spectra of aqueous solutions of methyl ium bromide (C n TAB) detergents with the anionic organic probe in orange as surfactant is added. Initially, a decrease in the aqueous solution. At surfactant concentrations far below the critical absorbance of free methyl orange in neutral solution at 463 micelle concentration, MO shifts in absorbance l max from 462 nm in nm is accompanied by a new absorption band at 375 nm for aqueous solution to 367-377 nm upon complex formation. Nonlinear least-squares deconvolution of the absorbance spectra reveals a pre-those [C n TAB] values below the critical micelle concentraviously unreported significant blue shift in l max with increasing surtions (cmc) (9). A slight dependence of the l max of this factant alkyl chain length. Our time-dependent spectrophotometric new absorption band on the surfactant tail length has been studies of the MO-C n TAB complex systems reveal, also for the first observed (377 nm for n Γ
12 and 375 nm for n Γ
16). On time, that a subsequent spectral shift to shorter wavelengths (l max Γ
further addition of surfactant at concentrations leading to 348-350 nm) occurs for those alkyl chain lengths greater than n Γ
micelle formation, the absorption l max of MO shifts to 431 14. These observations are consistent with an aggregation of the nm, independent of alkyl chain length.
surfactant-dye pair after initial complex formation. Increased hy-
The 375-nm absorption band at surfactant concentrations drophobic interaction of the azo ring of the probe with additional below the cmc is attributed to 1:1 MO-surfactant ion pairs.
neighboring surfactant alkyl chains to achieve a final equilibrium
The electrostatic interaction of the MO-surfactant ion pair state is hypothesized to account for the MO absorbance spectral shift. and the change in the degree of water hydration of MO have