Using the Optical Probe Methyl Orange To Determine the Role of Surfactant and Alcohol Chain Length in the Association of 1-Alkanols with Alkyltrimethylammonium Bromide Micelles

We have used the optical probe methyl orange to characterize the interaction of primary alcohol additives with micelles assembled from a series of alkyltrimethylammonium bromide (C n TAB) surfactants in aqueous solution. The anionic methyl orange (MO) interacts electrostatically with the cationic C n TAB headgroups to reveal micellar properties through variations in the probe's absorption wavelength maximum. The sensitivity of the absorbance of methyl orange to its microenvironment is interpreted to reflect the nature of the association of 1-alkanols with C n TAB micelles, an association dependent on both the surfactant and alcohol chain lengths. From the observed effect of alcohol addition on the methyl orange absorption λmax, we propose three distinct roles for 1-alkanols: cosolvent at the micellar surface, coaggregate within the micellar palisade layer, and cosurfactant via extension of the alcohol into the micellar core. Our suggestions for the surfactant and 1-alkanol chain length criteria that determine the role of alcohol additives within C n TAB micelles are presented.