Pretransitional phenomena in pure states and in a binary mixture of axially and transversàlly dipolar rodlike molecules of near-identical structures

Abstract

Mesophase induction generates strong smectic pretransitional fluctuations in the first three phases of a thermotropic polymorphism: Isotropic → Cholesteric → Chiral Smectic-A → Chiral Smectic-C → Chiral Hexatic Smectic → Crystal, observed to occur in a binary mixture comprising 80% cholesteryl oleate (CO) and 20% cholesteryl oleyl carbonate (COC), by weight. Both the pure esters show the same thermotropic phase sequence, e.g., Isotropic → Cholesteric → Smectic-A * → Crystal, and they are structurally identical except for an extra oxygen atom in the first chain position and an extra methylene group in the fatty chain of the carbonate. Both oleate and oleyl carbonate crystals have unit cells belonging to the primitive monoclinic Bravais lattice but different space groups, e.g., P2 1 and Pc, respectively. Smectic fluctuation is observed in the axially dipolar oleate because of a contrast factor, operative between the background and the fluctuations. In transversally dipolar oleyl carbonate such a mechanism is not operative, and fluctuations are observable neither in X-ray diffraction nor in static dielectric permittivity measurements. Within the constraint of a mixture of axially and transversally dipolar molecules, the fluctuations are a manifestation of the competition between the inherent tendency of1 the rods to stay parallel and the propensity of the dipoles to align anti-parallel.