THERMODYNAMICAL ANALYSIS OF ROTATION OF SPIN PROBES IN A SODIUM DODECYL-SULFATE MICELLE

ESR spectra of five spin probes in water, dodecane, and a sodium dodecyl sulfate (SDS) micelle were measured at various temperatures. Linearity was observed between the logarithm of rotational correlation time (<c) and 1/T in all systems. The <c value was analyzed by the theory of absolute reaction rate. The activation enthalpy (ΔF≠.) of hydrophobic probes in the micelle was between those in water and dodecane, suggesting that the core was liquid-like. On the contrary, ΔF≠ of hydrophilic probes was greater than that in water. The activation free energy (ΔF≠) of the probes in the micelle was about 1 kcal/mol greater than that in water. This increase of ΔF≠ in the micelle was attributed to the small activation entropy (ΔS≠) in the case of hydrophobic probes in contrast to that of hydrophilic probes, in which a large ΔF≠ was responsible for the increase. The position of the probes in the micelle was discussed on the basis of solubility in water. ΔH≠, and hyperfine coupling constants. © 1979, American Chemical Society. All rights reserved.