Ionization of cationic micelles: Effect of the detergent structure

Abstract

The critical micelle concentration (CMC) and ionization degree a of micelles of alkyltrimethylammonium bromides (C mH 2m+1N+(CH 3) 3Br − with m = 8, 10, 12, 14, and 16), dodecyldimethylalkylammonium bromides (C 12H 25(C m′H 2m′+1)N+(CH 3) 2Br −, with m′ = 1, 2, 3, 4, 8, and 10), and tetradecyltrialkylammonium bromides ( C 14H 29N+(C m″H 2m″+1) 3Br −, with m″ = 1 to 4) have been measured at 25°C using a bromide ion specific electrode and/or electrical conductivity. The results show a decrease of CMC and a upon increasing m for the alkyltrimethylammonium bromides, and a decrease of CMC and an increase of a upon increasing m′ and m″ for the second and third series of compounds. The changes of a have been correlated with the changes of surface areas per head group, calculated on the assumption of spherical micelles with a hydrophobic core radius, and a volume per hydrophobic moiety given by Tanford's equations ( J. Phys. Chem. 78, 2649 (1974)). It is shown that in the third series the available surface area per head group at the micelle surface becomes smaller than the intrinsic surface area of the head group if m″ > 2. In this case the micelle aggregation number becomes smaller than that for a spherical micelle uniformly filled with alkyl chains.