Reentrant nematic transitions in cyano-octyloxybiphenyl (8OCB)

Owing to the large dipole associated with its nitrile bond, 8OCB (cyano-octyloxybiphenyl) tends to associate as antiparallel pairs. The number of pairs, $N$, increases with increasing mole fraction, $x_{6\mathrm{OCB}}$, of a shorter homolog, 6OCB (cyanohexyloxybiphenyl). The stability of the nematic and smectic-$A$ phase depends upon $N$: increasing $N$ favors the nematic and decreasing $N$, the smectic-$A$ phase. When $N$ saturates, the smectic-$A$ phase no longer exists and the nematic reenters. Experimental results which support the above are presented. (1) Density measurements of the 8OCB-6OCB system yield two main results. (a) The density $\rho$ is not linear in temperature at high temperatures but becomes so at low temperatures. (b) $\left(\frac{1}{\rho }\right)\propto x_{6\mathrm{OCB}}$. The first indicates saturation in the number of associations at low temperatures. The second implies that the volume of the mixture is simply the sum of the volumes of the two components. An investigation of the temperature dependence of (1b) indicates that the saturation effect of (1a) is associated with the reentrant nematic phase. (2) Using the results of (1b), we estimate $N$ from lattice-spacing measurements of the smectic-$A$ phase of the mixtures. Even though $N$ is large in 8OCB (∼95% of the molecules are paired at any one time) it increases with the addition of 6OCB. All the molecules are paired when $x_{6\mathrm{OCB}}\sim 37$%. (3) In the pressure-temperature phase diagram, the maximum pressure at which the smectic-$A$ phase is stable, $P_M$, decreases as $N$ increases. Since pressure tends to stabilize the nematic at the expense of the smectic-$A$ phase, this is consistent with an increase in $N$ with pressure. $P_M$ is found to decrease linearly in the 6OCB-8OCB scavenging parameter, $x_2=2\mathrm{}{x}_{6\mathrm{OCB}}{x}_{8\mathrm{OCB}}-{\left(x_{6\mathrm{OCB}}\right)}^2$. $x_2$ is proportional to the difference between the number of 6OCB-8OCB and 6OCB-6OCB pairs. These results, as well as others, are discussed in terms of a Landau theory which assumes that the nematic and smectic-$A$ order parameters are coupled, and that the coupling depends upon $N$, temperature, and pressure.