Thermotropic ionic liquid crystals. IV. Structural effects in sodium methylpentanoates

Thermotropic ionic liquid crystals sodium 4-methylpentanoate and sodium 3-methylpentanoate have been studied by polarization microscopy and NMR. The quadrupole coupling constant e2qQ/h of 23Na is obtained for the mesophase and the 23Na spin-lattice relaxation time T1 is reported as a function of temperature in both the mesophase and the isotropic melt. A model is presented for the arrangement of the polar groups in the mesophase of all short chain Na carboxylates—the smectic A bilayer consists of R–COO−Na+ ion pairs with interdigitated polar groups. The observed dependence of the defect size and e2qQ/h of 23Na upon the molecular structure is interpreted in terms of this model. A qualitative discussion of possible relaxation mechanisms contributing to the T1 of 23Na is presented.