Thermotropic ionic liquid crystals. V. Deuterium NMR study of sodium n-alkanoates

Deuterium NMR spectra and spin-lattice relaxation times T₁ were obtained as functions of temperature for α-deuterated sodium n-alkanoates (n_c = 4-7) in the smectic A and "isotropic" liquid phases. The temperature dependence of the orientational order parameter is found to disagree with the predictions of McMillan's and Woo's theories for the smectic A phase. Reorientational models are presented to explain the experimental order parameters in terms of the structure of the anhydrous solid or the lyotropic gel phase of these compounds. The same models are used in the analysis of reorientational contributions to the ²D T₁ in the mesophase; order director fluctuations are also estimated to contribute significantly to the relaxation times. The ²D T₁ temperature dependence in the mesophase resembles that of the ²³Na T₁ measured previously, implying shared relaxation mechanisms for the two nuclei. Estimated correlation times for ²D relaxation in the isotropic phase provide evidence for the persistence of local structure characterized by motion on a time scale of tens of picoseconds.