Binary mixtures of nCB and nOCB liquid crystals. Two experimental evidences for a smectic A-nematic tricritical point

Modulated differential scanning calorimetry (MDSC) has been used to obtain specific heat measurements on octyloxycyanobiphenyl (8OCB), octylcyanobiphenyl (8CB) and nonyloxycyanobiphenyl (9OCB) liquid crystals and on several binary mixtures of 8OCB + 9OCB and 8CB + 9OCB. The order of the mesophase transitions, smectic A(SmA) to nematic(N) and N to isotropic (I) on pure components and on binary mixtures has been studied and, in addition, both binary mesophase diagrams have been built. For each set of mixtures, there exists a tricritical point (TCP) composition at the SmA N transition. The TCP compositions have been found to be X-9OCB = 0.63 for the system 8OCB + 9OCB and X-9OCB = 0.42 for the system 8CB + 9OCB. In addition, the specific heat critical exponents (alpha) through second order SmA to N transition have been obtained between 8OCB(or 8CB) and the TCP composition for the two sets of binary mixtures. When these alpha-values were plotted against the normalised nematic range, a common and uniform crossover trend was found. Indeed, this fact suggests a uniform behaviour in terms of the McMillan ratio for the nCB and nOCB series of liquid crystals. Moreover, for these compounds there exists a common value of (T-AN/T-NI)(TCP) of 0.99 and also a value of (T-AN/T-NI)(3D-XY) of about 0.96.