Spectroscopic studies of the interaction of methyl orange with cationic alkyltrimethylammonium bromide surfactants

The spectral signature of the azo dye methyl orange (MO) exhibits a considerable sensitivity to the polarity of the dye's environment. We have used this dependence of absorption wavelength on microenvironment to investigate the 1:1 ion pair complexes formed from electrostatic interaction of a series of cationic alkyltrimethylammonium bromide (C(n)TAB) detergents with the anionic organic probe in aqueous solution At surfactant concentrations far below the critical micelle concentration, MO shifts in absorbance lambda(max) from 462 nm in aqueous solution to 367-377 nm upon complex formation. Nonlinear least-squares deconvolution of the absorbance spectra reveals a previously unreported significant blue shift in lambda(max) with increasing surfactant alkyl chain length. Our time-dependent spectrophotometric studies of the MO-C,TAB complex systems reveal, also for the first time, that a subsequent spectral shift to shorter wavelengths (lambda(max) = 348-350 nm) occurs for those alkyl chain lengths greater than n = 14. These observations are consistent with an aggregation of the surfactant-dye pair after initial complex formation. Increased hydrophobic interaction of the au, ring of the probe with additional neighboring surfactant alkyl chains to achieve a final equilibrium state is hypothesized to account for the MO absorbance spectral shift. (C) 1998 Academic Press.