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 co nsiderable sensitivity to the polarity of the dye's environment. We ha ve used this dependence of absorption wavelength on microenvironment t o investigate the 1:1 ion pair complexes formed from electrostatic int eraction of a series of cationic alkyltrimethylammonium bromide (C(n)T AB) detergents with the anionic organic probe in aqueous solution At s urfactant concentrations far below the critical micelle concentration, MO shifts in absorbance lambda(max) from 462 nm in aqueous solution t o 367-377 nm upon complex formation. Nonlinear least-squares deconvolu tion of the absorbance spectra reveals a previously unreported signifi cant 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 spe ctral shift to shorter wavelengths (lambda(max) = 348-350 nm) occurs f or those alkyl chain lengths greater than n = 14. These observations a re consistent with an aggregation of the surfactant-dye pair after ini tial complex formation. Increased hydrophobic interaction of the au, r ing of the probe with additional neighboring surfactant alkyl chains t o achieve a final equilibrium state is hypothesized to account for the MO absorbance spectral shift. (C) 1998 Academic Press.