Influence of the headgroup molecular structure on the anionic surfactant-PVP interaction studied by Electron Paramagnetic Resonance of a cationic nitroxide

In the present work the mixtures water–sodium decylsulfate (C10OS)-poly(vinylpyrrolidone) (PVP) and water–sodium decylsulfonate (C10S)–PVP have been investigated by electron paramagnetic resonance (EPR) spectroscopy employing 4-(N,N-dimethyl-N-(2-hydroxyethyl)) ammonium-2,2,6,6-tetramethyl-piperidine-1-oxyl chloride (TEMPO-choline, TC) as spin probe. TC cations condense on the surface of the aggregates formed by anionic surfactants, acting as counterions, and fit their NO moiety in the outer shell of the hydrophobic core. In water–C10OS–PVP mixtures the nitrogen isotropic hyperfine coupling constant of TC (〈AN〉), reported as a function of the surfactant molality at constant PVP composition, shows two breakpoints: the former (critical aggregation concentration, c.a.c.) corresponds to the formation of surfactant–polymer clusters while the latter (c2) corresponds to the formation of free surfactant micelles. The trend of the correlation time (τC) of the nitroxides in the same system shows that the electrostatic repulsion among the clusters formed onto the PVP macromolecules favours a broadening of the polymer coil and a stiffening of its chain. In contrast, in water–C10S–PVP mixtures no surfactant–polymer interaction could be detected. The effect of the molecular structure of the surfactant headgroup on the surfactant–polymer interaction has been discussed in terms of charge density distribution. In order to further highlight the importance of the headgroup molecular structure on the surfactant association behaviour, the micellisation process of the sodium alkylsulfates (CnOS, n = 6,8,10) has been investigated by the same experimental approach, and the results have been compared with those, previously reported (A. M. Tedeschi, G. D'Errico, E. Busi, R. Basosi and V. Barone, Phys. Chem. Chem. Phys., 2002, 4, 2180, ref. 46), relative to the sodium alkylsulfonates (CnS). CnOS micelles are more ordered and compact with respect to the CnS ones. For the former class of surfactants the electrostatic interaction with TC is stronger and the hydrophobic behaviour of the TC solubilization site is higher.