STRUCTURE AND DYNAMICS OF THERMOTROPIC LIQUID CRYSTALLINE AND SOLID PHASES OF SODIUM ALKANOATES (NMR).

摘要

The neat (smectic A) phase of anhydrous sodium alkanoates is studied by means of NMR, polarization microscopy, and X-ray diffraction. The domain sizes, ('23)Na spin-lattice relaxation times T(,1), and the ('23)Na quadrupolar coupling constants (QCC) in neat phase sodium 3- and 4-methylpentanoates are compared with results in other short-chain sodium alkanoates. The magnitude of the ('23)Na QCC and its dependence on the ionic lateral packing area are explained quantitatively by a quasi-crystalline model in which the cations and the carboxylate groups are arranged in a double-layered square array. The drop in the ('23)Na T(,1) at the clearing point in all salts is discussed. In a preliminary ('23)Na NMR study of sodium stearate, the sodium ion apparently experiences an effectively isotropic environment in the neat phase, while ('23)Na spectra in the superwaxy and subneat phases indicate a distribution of sodium sites.;Unit cell constants of solid short-chain Na alkanoates are determined from powder X-ray diffraction photographs. A correspondence between these cell constants and the neat phase long-spacings and ('23)Na QCC's is noted. The temperature dependence of the bilayer spacing in the alkanoates is determined over the range 25-300(DEGREES)C. Overall changes in bilayer spacings between the solid and the neat phase are found to be much smaller than in long-chain alkanoates.;Deuterium NMR spectra and T(,1)'s were obtained as functions of temperature for (alpha)-deuterated sodium n-alkanoates (n(,c) = 4-7) in the neat and "isotropic" liquid phases. 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 soaps. The same models are used in the analysis of reorientational contributions to the ('2)D T(,1) in the mesophase; order director fluctuations are also estimated to contribute significantly. The ('2)D t(,1) temperature dependence resembles that of the ('23)Na T(,1), implying shared relaxation mechanisms for the two nuclei. Estimated correlation times for ('2)D relaxation in the "isotropic" phase provide evidence for the persistence of local structure characterized by motion on a timescale of tens of picoseconds.