Appearance of a Liquid Crystalline Smectic A-Crystal G Tricritical Point in a Binary System of 7.O9O.7 Dimer and 12OCB Monomer

The phase diagram of a binary system consisting of a dimeric 7.O9O.7 and a monomeric 12OCB compound exhibiting Isotropic-Nematic-Smectic A-Soft Crystal G phase sequence has been reported. High-resolution modulated DSC measurement have been performed to characterize the nature of a rare smectic A (SmA)—soft crystal G (Cr-G) phase transition. Polarizing optical microscopy measurement was performed for identification of different mesophases and construction of the phase diagram. Primary objective is to search the possibility of tricritical point of a rare mesomorphic Sm-A-Cr-G phase transition by systematically varying the Smectic A width from 19 to 55 K. Such a large variation of the width of the SmA phase has been used for the characterization of the SmA-Cr-G transition in this work. Interestingly, the first order SmA-Cr-G transition (with ∆S/R = 1.92) in the pure compound 7.O9O.7 gradually gives way to a distinct second order SmA-Cr-G transition with increasing molar concentration of 12OCB due to reduction of the coupling strength between the orthogonal SmA and the tilted (hexagonal) Crystal G phase. The specific heat capacity peak also decreases linearly with increasing 12OCB concentration and starts diminishing near \({x}_{12OCB}=0.6,\) with a simultaneous disappearance of the phase shift (δ) peak indicating the existence of a tri critical point (TCP). It is observed that fluctuation driven enthalpy values only persists up to \({x}_{12OCB}=0.65\) and completely vanishes near \({x}_{12OCB}\) = 0.7. This binary mixture system presents a novel opportunity to explore SmA-Cr-G critical behavior in soft condensed matter systems.