Micellar aggregation of alkyltrimethylammonium bromide surfactants studied by electron paramagnetic resonance of an anionic nitroxide

The self-aggregation process of five alkyltrimethylammonium bromide surfactants [CH3(CH2)(n-1)N(CH3)(3)Br (n = 6,8,10,12,16), C(n)TAB] in aqueous solution at pH = 7 has been studied by electron paramagnetic resonance (EPR) spectroscopy by employing 3-carboxy-PROXYL in its deprotonated form [2,2,5,5-tetramethyl-3-carboxypyrrolidinyloxy sodium salt, CP-] as a spin probe. In all the considered systems, the nitrogen isotropic hyperfine coupling constant of CP-, A(N)), decreases and the correlation timed tau(C), increases with increasing surfactant molality. Concerning the surfactants with long hydrophobic tails (n = 8,10,12,16), both (A(N)) and tau(C) present a slope change corresponding to the critical micellar composition, c.m.c. In these cases the tau(C) increase can be interpreted in terms of a reduction of the spin probe mobility determined by the strong electrostatic interaction between the CP- charge and the cationic micelles' surface. Particularly, the tau(C) values show that the microviscosity experienced by CP- in C(16)TAB micelles is much higher in respect to that found in the other micellar systems, thus suggesting a different structural organization of the aggregates' surface. The (A(N)) decrease can be ascribed to a partial embedding of the NO moiety of CP- in the outer part of the micellar hydrophobic core. The CP- affinity for the micellar pseudo-phase has been estimated by evaluating the distribution coefficient, K-d, of the spin probe between the micelles and the aqueous medium. The K-d value increases with the length of the surfactant hydrophobic chain. Because of the short hydrophobic tail, the C(6)TAB aqueous mixtures exhibit a peculiar behaviour: (A(N)) smoothly decreases and tau(C) of CP- increases with increasing surfactant molality without any abrupt slope change. This experimental evidence suggests that C(6)TAB association can be described in terms of a gradual change, with increasing surfactant molality, from solvent-mediate to direct surfactant-surfactant interactions.