Biradical Spin-Labeled Micelles

A biradical spin-label, N,N′-di-[4-(1-oxyl-2,2,6,6-tetramethylpiperidyl)] urea, has been used for studying the dynamic structure of a micelle, sodium dodecyl sulfate (SDS). Analysis of the ESR spectra of the label has shown that the biradical was incorporated in the micelle. The tumbling motion of the label in the micelle was not strongly inhibited, suggesting a dynamic character of the micelle structure. The exchange rate of the biradical between the micelle and the water phases was slower than 107sec−1. The partition coefficient expressed as the ratio of the concentration of the biradical in the micelle to that of the biradical in the water phase was approximately 6×105 in the SDS concentration lower than 2×10−2M. The biradicals form aggregate in the micelle, when they are present in excess in the solution.