Carbon-13 NMR investigations of the orientational order in a columnar liquid crystal

We report a natural abundance carbon-13 nuclear magnetic resonance (NMR) investigation of benzene dissolved in a columnar liquid crystal formed by the carbohydrate mesogen octa-O-decanoyl-beta-cellobiose (Cel-II-10). The alignment of the mesophase in the magnetic field is sensitive to the field strength and to the thermal history of the sample. From carbon-13 line shapes, the director distribution functions and the molecular order parameter of the solute were estimated. In contrast to most columnar phases, Cel-II-10 aligns parallel to the magnetic field. The orientational order parameter of benzene is positive, which is expected for a columnar liquid crystal where the symmetry axis of the disk is aligned parallel to the column axis. However, an unexpected increase of the order parameter with increased temperature was observed. This behavior is probably a consequence of an exchange process between different solvation sites in the columnar mesophase and indicates that the population in the environment with higher order parameter increases when the sample is heated.