Fibers and Other Aggregates of ω-Substituted Surfactants

The aggregation in dilute aqueous solutions of several ω-substituted single-chain quaternary ammonium surfactants has been studied by Krafft temperature and critical aggregation/micelle concentration measurements and by cryo-transmission electron microscopy: (16-carboxyhexadecyl)trimethylammonium bromide (1a); (16-methoxycarbonylhexadecyl)trimethylammonium bromide (1b); (17,18-dihydroxyoctadecyl)trimethylammonium bromide (1c). Octadecyltrimethylammonium bromide (2) was included for comparison. The aggregation of 1a was also studied by Raman spectroscopy. Surfactant 1a at pH 6.8 forms ribbonlike fibers with lengths of up to 1.2 μm and cross sections of 3−4 nm × 12−16 nm; at pH 2.2, ribbon to rodlike fibers; and at pH 11.5, wormlike aggregates. Surfactants 1b and 1c at pH 6.8 form rod-shaped aggregates. The Raman spectroscopy results indicated that 1a is not in a fully extended conformation within its fibers at pH 6.8. Tentative structural models have been proposed for the aggregates of surfactants 1.