Liquid crystals comprising hydrogen-bonded organic acids - II. Heterodimers in mixed mesogenic acids

Nematic mesophase stability, as determined by the excess mesophase range in mixtures of organic acids (supramesogen derived from para-substituted phen yl, 4-substituted cyclohexane and bicyclo[2.2.2] octane carboxylic acids), is increased in mixtures of disparate species-mixed acids with differing su bstituent sizes (different supramesogen tail lengths) and different acid ty pes (aliphatic and aromatic supramesogen cores). These trends enable one to estimate contributions to mesophase stability stemming from tail disparity and core differences, contributions that appear to carry over into analogo us covalent mesogens. IR measurements of solid mixtures of acids suggest th at the heterodimer acids population dominates, and deuterium NMR studies of labelled acids in the nematic phase emphasize the dynamic character of thi s class of supramesogens: dimer lifetimes are shorter than microseconds.