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-1SO3Na (n = 6, 8, 10)] has been studied by EPR spectroscopy by employing TEMPO-choline [4-(N,N-dimethyl-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 const. (AN) and the correlation time (τC) of the label on the surfactant molality have been analyzed. In order to allow a correct interpretation of the exptl. evidence a preliminary study on the factors influencing the EPR spectrum of TC in soln. has been performed. EPR spectra of TC in various solvents show that the AN value increases with increasing solvent polarity and, esp., H-bonding ability. The exptl. values have been compared with those obtained by a composite ab initio computational approach, in which AN is detd. by a suitable combination of post-Hartree-Fock and d. functional calcns. Solvent effects are modeled by using the polarizable continuum model (PCM) and, for solvents with H-bonding ability, by including a few explicit solvent mols. The exptl. 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 aq. soln. has been also investigated, finding that the AN value is almost const., whereas τC increases with the electrolyte molality. In surfactants' aq. soln., both AN and τC of TC, plotted as a function of the surfactant molality, show a slope change, corresponding to the crit. micellar compn. (c.m.c.). The τC increase can be interpreted in terms of a redn. of the label mobility detd. by the strong electrostatic interaction between the TC pos. charge and the anionic micelles' surface. The AN 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 τC and AN 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 behavior 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 estd. by evaluating the distribution coeff., Kd, of the spin label between the micelles and the aq. medium. The Kd value increases with the length of the surfactant hydrophobic chain.