Novel Thermotropic Liquid Crystals without a Rigid Core Formed by Amphiphiles Having Phosphonium Ions

Abstract

Thermotropic liquid-crystalline behavior of phosphonium salts, which are structurally simple amphiphiles with positively-charged phosphorus atoms and without rigid cores, was evaluated by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffractometry. The phosphonium salts were found to show a smectic A (SA) phase in which a homeotropic structure was formed spontaneously. It is significant that the phosphonium salts possess an advantageous feature as thermotropic liquid crystals and show a stable liquid-crystalline phase and a simple phase transition behavior in comparison with commonly available ammonium analogs. For example, dimethylditetradecylphosphonium chloride showed the highest SA to isotropic (I) phase transition temperature (TSI, 187 °C) and the SA phase in the widest temperature range (119 °C on heating) among the cationic amphiphiles examined in the present study, while dimethylditetradecylammonium chloride with the same structure except the cationic part showed the lowest TSI (70 °C) and the narrowest liquid-crystalline temperature range (22 °C on heating).