NMR studies in surfactant and polymer-surfactant systems: Micelle formation of sodium ω-phenyldecanoate and interaction with poly(ethylene oxide)

Abstract

The micellization process of sodium ω-phenyldecanoate and the interaction between this surfactant and poly(ethylene oxide) (PEO) in aqueous solution have been studied, using 1H NMR spectroscopy. The critical micelle concentration (CMC) and the micellar aggregation number of sodium ω-phenyldecanoate are found to be 0.0121 ± 0.0001 m and 26 ± 1, respectively. Upon micellization, large upfield (low frequency) chemical shifts are observed for the aromatic protons and the methylene protons near the terminal phenyl group, while the shifts of protons attached to methylene carbons near the headgroup are very small. These observations imply that the terminal phenyl group is on average located in the micellar interior. In the presence of PEO, two critical concentrations are observed, corresponding to, respectively, the formation of a polymer-surfactant aggregate and the coexistence of the polymer-surfactant aggregate and surfactant micelle. The first critical concentration, X 1, is at 0.00885 ± 0.00004 m and is independent of the PEO molecular weight and concentration. The second critical concentration, X 2, is dependent on PEO concentration. The binding ratio, ( X 2 − CMC)/ C PEO, is 0.44 ± 0.01 ω-phenyldecanoate ion per monomer unit of PEO. In the PEO/ω-phenyldecanoate system large 1H aromatic ring current-induced shifts are observed for the surfactant protons near or on the terminal phenyl group and for the PEO protons. Very small shifts are observed for the surfactant protons near the head group. These observations suggest that the terminal phenyl group and most of the PEO are in close proximity, and on average in the interior of the polymer-surfactant aggregate.