Amphitropic liquid crystals. Part 1.—Effect of a thermotropic mesogen on lyotropic mesomorphism, and of a surfactant on thermotropic mesomorphism. The C16EO8–5-CB–water system

Abstract

The mesomorphic properties of a ternary system consisting of a thermotropic mesogen (pentylcyanobiphenyl; 5-CB) and a lyotropic system (octaethyleneglycol hexadecyl ether–water; C₁₆EO₈–H₂O i. e. 3,6,9,12,15,18,21,24,-octaoxatetracontan-1-ol) have been studied using optical microscopy, differential scanning calorimetry, nuclear magnetic resonance spectroscopy and low-angle X-ray diffraction. The two types of mesophase are immiscible. Addition of the surfactant to 5-CB reduces the nematic–isotropic transition, until above ca. 30 wt.% no nematic phase is observed. Similar results were observed with other polyoxyethylene surfactants. The surfactant, C₁₆EO₈, forms a large range of lyotropic mesophases (lamellar, hexagonal, two types of cubic and nematic). Up to 12% 5-CB was solubilised in the lamellar phase, whilst only ca. 5% dissolved in the hexagonal phase and lower concentrations still could be incorporated into the other mesophases. X-Ray and ²H₂O NMR measurements indicate that 5-CB swells the alkyl-chain region of the surfactant, rather than adsorbing at the headgroup–water surface.