Orientational order in the lamellar G phase of the sodium decanoate-n-decanol-water system

Abstract

We have measured the angular variation of the electron resonance spectra of three nitroxide spin probes dissolved in the lamellar G phase of the sodium decanoate-n-decanol-water system. The angular dependence of the line positions is analysed to yield the second rank orientational order parameter for the spin probe. The analysis of the linewidths is not so straightforward and it is shown that it is insufficient to employ a theory which assumes the nuclear spin to be quantized parallel to the magnetic field. Correct analysis of the linewidths yields the fourth rank order parameter as well as rotational correlation times relating to the motion of the probe. The order parameters for the rigid spin probe, derived from cholestane, are in agreement with a simple theory for the single particle orientational energy. In contrast the results for the two flexible probes, derivatives of stearic acid, are not accounted for by this simple theory, in conflict with previous contentions.