Different spin probe positions related to structural changes of nonionic microemulsions.

The ESR spectra of iodoacetamide, 5-doxyl stearic acid and FDNB derivative spin labels, in nonionic microemulsion systems, formulated with tetraethyleneglycoldodecylether in decane were measured in various amounts of water and surfactant content. At certain microemulsion compositions, the high field line of the ESR spectrum, splits into a doublet, suggesting that the spin probe locates in two different positions, i. e., the water core and the micromembrane which separates the aqueous from the oil phase. The method used for this study is based on i) the partition of spin labels between the microphases of the system expressed as the ratio of the high field peaks (H−1/H*−1) corresponding to the two different spin states; ii) the mobility of the spin labels in the dispersed phase expressed in terms of the degree of anisotropy (H1/H0); and iii) the polarity of the microenvironment of the label expressed in terms of the hyperfine coupling constant αn. The variation of these spectral parameters is discussed in connection with the corresponding structural state of the microemulsions. [ABSTRACT FROM AUTHOR]