23Na relaxation in SDS and SDES-polymer systems

Na-23 NMR longitudinal relaxation rates (R-1 = 1/T-1) measurements were carried out, at a magnetic field of 9.4T, on surfactant and surfactant-polymer solutions (surfactant: sodium dodecylsulfate (SIDS), sodium decylsulfate (SIDES); polymer: poly(ethylene oxide) (PEO) MW 20,000, poly(vinyl pyrrolidone) (PVP) MW 24,000). The sharp increase of R-1 on aggregation allows to determine the CMCs in pure surfactant solutions and the critical aggregation concentrations (CACs) in the presence of polymer. Sodium relaxes at the same rate in the vicinity both of pure surfactant micelles and of the mixed aggregates. The observed R, are different because of the different ionization degrees. A peculiar feature of the SDS-PEO system is the strong increase of sodium relaxation rate at concentrations higher than 8 mmol kg(-1). This is not due to the saturation of the polymer, but rather indicates an abrupt change in the aggregates. The Na-23 relaxation has been complemented by C-13 NMR spectra, in particular for the SDS-PEO system. They support the view that, in the range between the CAC and the polymer saturation, modifications of the mixed polymer-surfactant aggregates occur. (C) 2004 Elsevier B.V. All rights reserved.