Density Measurements of Alloys Exhibiting the Re-Entrant Nematic Phase

Abstract

Measurements of the density, p, of mixtures of 60CB (cyanohexyloxybiphenyl) and 80CB (cyanooctyloxybiphenyl) are presented as a function of temperature and composition. In a certain range of temperature, T, and composition, C, these mixtures exhibit the re-entrant nematic phase, p (T) is not linear but exhibits an approach to linearity at lower temperatures. Consequently the isobaric compressibility is evaluated only in the vicinity of the smectic A phase and is found to be β_S =7.71 ± 0.06 × 10⁻⁴ g/cc/°C for the smectic A phase and β_N = 8.03 ± 0.15 ± 10⁻⁴ gms/cc/°C for the nematic and reentrant nematic phases. 1/p is found to depend linearly on C implying the partial intercalation of molecules in neighboring layers of the smectic A phase to ensure that, when alloyed, 60CB and 80CB occupy no more than their volume in pure compounds. Finally, very precise measurements in the vicinity of the smectic A re-entrant nematic transition show an anomalously large pretransitional increase in the density of the smectic A phase.