Calorimetric study of the incommensurate smectic-A phase in DB7OCN+ 8OCB

Abstract

Mixtures of heptyloxyphenylcyanobenzoyloxybenzoate (DB7OCN) and octyloxycyanobiphenyl (8OCB) exhibit partial bilayer smectic-A (SAd) and bilayer smectic-A (SA2) phases when the mole fraction of 8OCB is less than 0.24. For mixtures containing larger amounts of 8OCB, an incommensurate smectic-A (SAic) phase is observed between SAd and SA2. The heat capacity variation Cp(T), determined with an ac calorimetric technique, is qualitatively different for samples going directly from SAd to SA2 and those exhibiting the SAd-SAic-SA2 sequence. The direct SAd-SA2 results indicate a continuous supercritical evolution of the SA2 phase. The results in the SAic range support the presence of a new phase, as indicated by X-ray studies, rather than a broad region of coexisting bulk SAd and SA2 phases.