The state of a paramagnetic probe in solutions containing mixed micelles of anionic and nonionic surfactants from data on nuclear magnetic relaxation

The state of Mn2+ ions in solutions of binary mixtures of anionic (sodium d ecyl, dodecyl, and tetradecyl sulfates) and nonionic (Tween 40 and Triton X -100) surfactants at 298 K was studied by the nuclear magnetic relaxation m ethod. It was found that an increased spin-lattice relaxation rate of water protons is caused by formation of mixed surfactant aggregates. The method proposed previously for describing the micellization of anionic surfactants was extended to mixed aggregates containing a nonionic surfactant. A mathe matical simulation revealed the formation of mixed micelles of alkyl sulfat es and nonionic surfactants of two types: with 1 : 1 and 1 : 2 ratios. In t he latter case, the mixed micelles are incapable of binding counterions, wh ich is probably caused by the blocking of head sulfate groups by the ethyle ne oxide chains of the nonionic surfactants. The adequacy of the used appro ach was supported by the method of competing reactions between iminodiaceti c acid and mixed micelles of sodium dodecyl sulfate and Tween 40 for bindin g with Mn2+ cations.