Fluidity and Order in the Hydrocarbon‐Water Interface of Synthetic and Biological Micelles as Determined by Fluorescence Polarization

A fluorescence polarization technique which was previously described was Utilized for the evaluation of microviscosities, {article}{empty}{document}η{document} , transition temperatures and degrees of order in the hydrocarbon‐water interface of a series of micellar systems. The systems studied were micelles of compounds with a hexadecyl chain attached to different polar heads, and liposomes of lecithin and lecithin‐cholesterol (∼ 2:1). 1‐Dimethylamino naphthalene 5‐sulfoamide (DNSA) was found to dissolve in a well‐defined region of the micelles studied and was employed as the fluorescence probe. The DNSA inclusion region possesses an apparent dielectric constant in the range of 28–40, inferred from the emission maximum of the embedded DNSA, and was therefore interpreted to be the hydrocarbon‐water interface. In all systems studied the value of {article}{empty}{document}η{document} falls in the narrow range of 17–40 centipoises at 27°C. From the change of {article}{empty}{document}η{document} with temperature in the range of 0–60°C, phase transitions at 37, 33, 29 and 19°C were detected for micelles of cetyltrimethylammonium bromide, tetramethylammonium palmitate, cetylbetaine and egg lecithin, respectively. No phase transition was observed for lecithin‐cholesterol liposomes. Fluorescence polarization spectra of DNSA dissolved in the micelles indicated that a certain degree of order prevails in their hydrocarbon‐water interface.