Interactions of an azobenzene-functionalized anionic amphiphile with cationic amphiphiles in aqueous solution

Interactions between the azobenzene-containing amphiphile methyl octyl orange (MOO) and n-alkyltrimethylammonium bromides (C(n)TAB, n = 1, 6, 8, 10, 12, 16) in aqueous solution have been studied using UV-vis spectroscopy. Interactions occur at concentrations below the critical micelle concentrations of the individual cationic surfactants (n = 8, 10, 12, 16) as indicated by a ca. 80 nm blue shift of the main absorption band of MOO. In addition, aggregation of MOO with C(6)TAB, which is a hydrotrope rather than a surfactant, was also observed. The critical aggregation concentration (cac) of MOO determined by surface tension experiments was 0.68 mM. The cac's of aqueous mixtures of C(n)TAB and MOO (1: 1) were considerably lower than those of the individual surfactants. The absorption spectrum of MOO at concentrations above the cac in the 1: 1 mixtures was also blue shifted with respect to the absorption spectrum of MOO in aqueous solution. Vesicular aggregates formed in equimolar aqueous mixtures of C(n)TAB and MOO were characterized by means of cryo- and negative staining transmission electron microscopy. Vesicles are small and range in diameter from 8 to 15 nm although some vesicles having a diameter between 30 and 80 nm were also observed. Vesicle size distributions were confirmed using dynamic light scattering. Vesicle sizes decreased upon increasing the chain mismatch, and ultimately, vesicles transformed into micelles. Micelles were also formed in a 1:1 aqueous mixture of C(6)TAB and MOO.