Micellar aggregation of sulfonate surfactants studied by electron paramagnetic resonance of a cationic nitroxide: an experimental and computational approach

The micellization process of three sulfonate surfactants [CH3(CH2)(n-1)SO3Na (n = 6,8,10), CnSO3Na] has been studied by electron paramagnetic resonance (EPR) spectroscopy by employing TEMPO-choline [4-(N,Ndimethyl-N-(2-hydroxyethyl))ammonium-2,2,6,6-tetramethylpiperidine-1-oxyl chloride, TC) as a spin label. The dependence of both the nitrogen isotropic hyperfine coupling constant (<A(N)>) and the correlation time (tau(C)) of the label on the surfactant molality have been analysed. In order to allow a correct interpretation of the experimental evidence a preliminary study on the factors influencing the EPR spectrum of TC in solution has been performed. EPR spectra of TC in various solvents show that the <A(N)> value increases with increasing the solvent polarity and, especially, H-bonding ability. The experimental values have been compared with those obtained by a composite ab initio computational approach, in which <A(N)> is determined by a suitable combination of post-Hartree Fock and density functional calculations. Solvent effects are modelled by using the polarizable continuum model (PCM) and, for solvents with H-bonding ability, by including a few explicit solvent molecules. The experimental and computed values are in good agreement, confirming the reliability of the adopted computational strategy. The effect of the ionic strength on the EPR spectrum of TC in NaCl and Na2SO4 aqueous solution has been also investigated, finding that the <A(N)> value is almost constant, whereas tau(C) increases with the electrolyte molality. In surfactants aqueous solution, both <A(N)> and tau(C) of TC, plotted as a function of the surfactant molality, show a slope change, corresponding to the critical micellar composition (c.m.c.). The tau(C) increase can be interpreted in terms of a reduction of the label mobility determined by the strong electrostatic interaction between the TC positive charge and the anionic micelles surface. The <A(N)> decrease can be ascribed to the embedding of the NO moiety of TC in the outer part of the micellar hydrophobic core. By comparing the data collected for the different surfactants, it can be seen that the variation of both tau(C) and <A(N)> upon micellization increases with the surfactant chain length. This evidence can be interpreted in terms of an increasing strength of the TC-micelle surface interaction, and of an increasing hydrophobic behaviour of the outer part of the micellar core in which the NO moiety of TC is solubilized. The TC affinity for the micellar pseudo-phase has been estimated by evaluating the distribution coefficient, K-d, of the spin label between the micelles and the aqueous medium. The K-d value increases with the length of the surfactant hydrophobic chain.