Liquid crystal, binary mixtures 8CB+80CB:: critical behaviour at the smectic A-nematic transition

The experimental determination by means of DSC together with a thermodynamic analysis of the two-component phase diagram of octylcyanobiphenyl (8CB) + octyloxycyanobiphenyl (8OCB) has been performed. The thermodynamic consistency of the two-component system has been proved and similar excess Gibbs energies for mixtures in the isotropic, nematic and smectic phases have been obtained. In order to study the order of the nematic to smectic A phase transition, specific heat measurements obtained using modulated DSC have been made on three mixtures of 8CB + 8OCB as well as on the pure components. An evolution of the alpha-critical exponent has been found when the composition of the mixture changes between pure 8CB and pure 8OCB showing crossover behaviour between 3D-XY universal class and the tricritical point. As the 8OCB content in the mixture is further increased, the alpha-value decreases with the decrease in the McMillan ratio (T-NA/T-NI) giving rise to a value of about 0.96 at which the 3D-XY mode would be hypothetically reached. The tricritical point would be hypothetically reached at a McMillan ratio of about 0.99 and the extrapolated tricritical amplitude ratio A(-)/A(+) is estimated to be 1.6, a value very different from that obtained for the He-3-He-4 tricritical transition, but comparable with other experimental results reported in the literature.