Influence of the alkyl tail length on the anionic surfactant-PVP interaction

The effect of the surfactant tail length on the interaction between sodium alkylsulfates (CnOS, n = 6, 8, 10, 12) and poly(vinylpyrrolidone) (PVP) in aqueous solution has been investigated by electron paramagnetic resonance (EPR)spectroscopy employing TEMPO-choline (TC) as a spin probe. Experimental evidence show that all of the considered surfactants molecularly interact with PVP. However, the cooperative behavior of the surfactant molecules when self-aggregating onto the polymer strongly increases with the surfactant tail length. In fact, in the case of C6OS, the TC EPR parameters indicate that surfactant monomers randomly associate with the polymer chain. In the case of C8OS, formation of surfactant-polymer clusters occurs simultaneously to free micellization. In the case of C12OS and C10OS, the nitrogen isotropic hyperfine coupling constant of TC (<AN>) shows that formation of surfactant-polymer clusters occurs. The correlation time (τC) of the nitroxide in the same systems shows that electrostatic repulsion among the clusters, formed on the PVP macromolecules, favors a broadening of the polymer coil and a stiffening of its chain. The average number of surfactant molecules participating in each cluster adsorbed onto the polymer, as determined by fluorescence quenching measurements, is much higher for C12OS than for C10OS.