The thermotropic liquid crystals formed by anhydrous sodium ricinoleate

Abstract

The polymorphism of sodium ricinoleate has been examined using differential scanning calorimetry, optical microscopy and low angle X-ray diffraction. There are three low temperature phases, separated by small transition enthalpies, where the molecules are packed in layers with little molecular disorder. At high temperatures (>160°C), two semi-molten mesophases are formed. Both have distinct optical textures under polarizing microscopy, that of the highest temperature phase being the typical fan pattern of a hexagonal phase. X-ray diffraction shows that this phase is comprised of hexagonally packed oblate spheroids arranged in layers according to an ABAB type pattern. It has not been reported previously for surfactants. The head groups form the cores of the spheroids with molten alkyl chains filling the remaining space. In the lower temperature mesophase, the head groups pack into rods joined in fours to give layers of a tetragonal array. The layers stack to give a body centred tetragonal structure. Again, the head groups form the rod cores, while alkyl chains fill the remaining space.