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_1N(CH3)3Br (n = 6,8,10,12,16), CnTAB] 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_, hANi, decreases and the correlation time, tC , increases with increasing surfactant molality. Concerning the surfactants with long hydrophobic tails (n ¼ 8,10,12,16), both hANi and tC present a slope change corresponding to the critical micellar composition, c.m.c. In these cases the tC 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 tC values show that the microviscosity experienced by CP_ in C16TAB 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 hANi 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, Kd , of the spin probe between the micelles and the aqueous medium. The Kd value increases with the length of the surfactant hydrophobic chain. Because of the short hydrophobic tail, the C6TAB aqueous mixtures exhibit a peculiar behaviour: hANi smoothly decreases and tC of CP_ increases with increasing surfactant molality without any abrupt slope change. This experimental evidence suggests that C6TAB association can be described in terms of a gradual change, with increasing surfactant molality, from solvent-mediate to direct surfactant–surfactant interactions.