N NQR and Orientational Ordering in Smectic Liquid Crystals

Liquid crystals are systems which flow like liquids while retaining the mechanical and optical anisotropy of crystals. On a microscopic level they are characterized by molecular orientational order accompanied by a partial or complete translational disorder of the molecular positions (1~3) • Thermotropic liquid crystals are formed upon heating many anisotropic organic compounds, while lyotropic liquid crystals are obtained by mixing amphiphilic compounds with water. Lyotropic liquid crystals are especially important in biology in view of their role in the structure of cell membranes. Both thermotropic and lyotropic materials may exhibit more than one liquid crystalline phase. Smectic phases, whose specific property is the arrangement of the molecules into parallel layers, have been most intensively investigated during the last few years. Studies by means of miscibility relations (4-6), X-ray diffraction (7~15). nuclear magnetic resonance (16-21), nuclear quadrupole resonance,