Thermodynamic investigation of the thermotropic cubic mesogen, 1,2-bis(4-n-octyloxybenzoyl)hydrazine

The molar heat capacity of the thermotropic cubic mesogen 1,2-bis (4-n-alkoxybenzoyl) hydrazine, BABH(8) for short, with a purity of 99.43 mol% has been precisely measured with an adiabatic calorimeter at temperatures between 14 and 480 K. The enthalpy and entropy gained at each phase transition across the phase sequence of [crystal(2) --> crystal(1) --> cubic mesophase --> SmC --> isotropic liquid] have been determined. The existence of a solid-to-solid phase transition with a fairly large entropy change seems to be necessary for the alkyl moieties attached to both sides of the molecule to play the role of 'solvent' in the cubic mesophase. On the basis of curvature elasticity considerations, the small energy difference between the cubic and SmC phases is favourably accounted for in terms of the jointed-rod micelles model. The reason for the immiscibility of BABH(8) with the cubic D mesogen, 4'-n-hexadecyloxy-3'nitrobiphenyl-4-carboxylic acid is discussed in terms of the large difference in their molecular size and of 'structure breaking' arising from the admixture of heterogeneously hydrogen-bonded materials.