Possibility of isostructural condensed states of matter in the D phase of ANBC and the cubic mesophase of BABH: heat capacity of 4 '-n-octadecyloxy-3'-nitrobiphenyl-4-carboxlic acid, ANBC(18)

Heat capacity measurements have been made on ANBC(18) at temperatures from 8 to 490 K by adiabatic calorimetry. All known phases were detected. The te mperatures, enthalpies and entropies of transition were determined for the phase transitions observed. On the basis of the entropy of transition to th e SmC phase from the DI or cubic phases, it is pointed out that the D phase of ANBC and the cubic phase of BABH might be identical in nature. It is sh own that the arrangement of 'molecular' cores has a higher degree of order in the isotropic (D and cubic) phases than in the SmC phase, whereas the te rminal alkoxy chains are more disordered in the isotropic phases than in th e SmC phase. The degrees of disorder in the D and cubic phases relative to the SmC phase are very similar in terms of the entropy of transition per me thylene group. The inverted phase sequence in ANBC (SmC --> D on heating) a nd BABH (cubic --> SmC) can be accounted for in terms of the competing role s in the entropy between the molecular core and the chains.