Smectic mesophases from dihydroxy derivatives of quaternary alkylammonium salts

A series of N,N-di(2-hydroxyethyl)-N-methyl-N-alkylammonium bromides (alkyl chain from dodecyl to octadecyl) was synthesized and characterized. Its thermotropic liquid-crystal behaviour was studied by differential scanning calorimetry, polarizing optical microscopy, infrared spectroscopy and X-ray diffraction. Two smectic phases were identified, an ordered smectic T and a disordered smectic A phase. Both phases result from an alternate periodic stacking of polar sublayers of ionic head groups and apolar sublayers of alkyl chains in a disordered conformation. In the low-temperature smectic T phase, the ionic head groups are arranged inside the polar sublayers in a particularly dense fashion, a tetragonal crystal lattice. In the high-temperature smectic A phase, the ionic head groups are arranged in a disordered fashion with normal packing density.