Temperature dependence of motion of a spin probe in aerosol ot reversed micelles

Abstract

The ESR spectra of a spin probe, 2,2,6,6-tetramethyl-4-piperidone N-oxide, in Aerosol OT reversed micelles in heptane solutions were measured at various temperatures. The rotational correlation time ( τ c ) at room temperature became smaller with an increase in the ratio, R, of the number of water to Aerosol OT molecules. However, the situation was reversed at low temperature, which suggests that at low temperature the reversed micelle became rigid and the motion of the probe was strongly hindered within the micelle. Spectral changes at low temperature revealed a phase separation in micelles with large R (= 33), in which part of the water was expelled from the micelle. In this sample, exchange of the location of the probe between the water pool and the shell formed by the surfactant molecules affects the line shape and the width of the spectral lines. The relationship between the reversed micelle structure and hyperfine coupling constants as well as that of the internal rotation of the probe in the micelle is discussed in detail.