Amphotropic Ionic Liquid Crystals with Low Order Parameters

Abstract

Methyl-tri-n-decylphosphonium salts (1P10A, where the counterion A is NO 3 - or the hydrate of bromide, Br - .H 2 O) have been shown to exhibit unique liquid-crystalline properties as neat compounds and with high concentrations (up to 20 wt %) of a small organic molecule, acetonitrile. These results indicate dual thermotropic-lyotropic (i.e., amphotropic) behavior. Optical micrographs of salts with even small amounts of acetonitrile are oily streak patterns. The low-angle peaks in X-ray diffractograms characteristic of the lamellar spacings in the smectic A phases of the neat salts are broadened but do not change their positions appreciably as the concentration of added acetonitrile is increased. The liquid-crystalline phases of both salts become oriented in strong magnetic fields. As a result, NMR experiments with the nitrate salt exhibited only one type of orientation for acetonitrile at concentrations up to ca. 10 wt %. However, two types of orientations with opposite signs of the order parameter were detected in the Br - .H 2 O salt at < 1 wt % acetonitrile. The acetonitrile proton line width decreases by a factor of 2 between 0.7 and 13 wt % in the nitrate salt. The dual thermotropic-lyotropic behavior and relatively low order parameter of the solute suggest that these amphiphilic salts might be useful for structural studies using NMR spectroscopy.