Studies on Mesogenic Disc-like Molecules II. Heat Capacity of Benzenehexa-n-heptanoate from 13 to 393 K

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Abstract
The heat capacity of benzene-hexa-n-heptanoate, C6(OCOC6H13)6, with a purity of 99.94 mole per cent has been measured with an adiabatic-type calorimeter in the range from 13 to 393 K. Five phase transitions were found at 129 (Phase IV - III), 222.80 (III - II), 230.81 (II - I); 353.79 (I - “discotic” mesophase) and 359.28 K (“discotic” mesophase - isotropic liquid). The enthalpy and entropy of these transitions were determined. The infrared spectra recorded in the range 4000-30 cm−1 remarkably depended on temperature. Characteristic features of the “discotic” mesogen have been elucidated as follows; (i) The mesogen exhibits a rich solid polymorphism accompanied by a large amount of transition entropy. (ii) The heat capacity is smaller in the mesophase than in the adjacent crystalline and isotropic liquid phases. (iii) Short-range order effect of the mesophase still persists in the isotropic liquid phase. (iv) The transition entropy at clearing point is large compared with those of classical liquid crystals and melting entropies of plastic crystals. It is, however, very small portion of the cumulative transition entropy. (v) The infrared spectrum recorded at the mesophase is substantially the same as that at the isotropic liquid. (vi) As far as the transition entropy and the infrared spectra are concerned, paraffinic moieties in the molecule are highly disordered in the mesophase. (vii) A sequential phase transitions in the solid state can be regarded as a successive phase transition concerning the conformational melting of the paraffinic moieties. As a conclusion, the present thermodynamic study suggests that the “discotic” mesophase should be regarded as being situated between typical liquid crystals of rod-like molecules and typical plastic crystals of globular molecules.