The polyliquid crystalline EPR spectra of nitroxide spin probes and their interpretation

Abstract

The EPR spectrum of 5-doxyl stearic acid dissolved in the randomly oriented lamellar G phase of sodium decanoate, decamol, and water was recorded at room temperature and the spectral shape was analyzed using relaxation theory. The spectrum was calculated as the superposition of fast-motion spectra coming from differently oriented domains characterized by a linewidth dependent on the angle formed by the director and the magnetic field. It is shown that a quantitative interpretation of the spectral shape requires the introduction of the effect of nonquantization of the nitrogen nuclear spin along the magnetic field, the m = 0 line being especially sensitive to the inclusion of this effect. The fourth rank order parameter P 4 for the spin probe was also obtained and found to be lower than the value predicted by Maier-Saupe theory on the basis of P 2 . A semiquantitative treatment of the polyliquid crystalline spectrum is also given providing simple explanations of the relevant spectral features.