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

Abstract

The self-aggregation process of five alkyltrimethylammonium bromide surfactants [CH₃(CH₂)_n−1N(CH₃)₃Br (n = 6,8,10,12,16), C_nTAB] 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 time, τ_C, increases with increasing surfactant molality. Concerning the surfactants with long hydrophobic tails (n = 8,10,12,16), both 〈A_N〉 and τ_C present a slope change corresponding to the critical micellar composition, c.m.c. In these cases the τ_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 τ_C 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 〈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 C₆TAB aqueous mixtures exhibit a peculiar behaviour: 〈A_N〉 smoothly decreases and τ_C 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.