A thermophysical study of the melting process in alkyl chain metal n-alkanoates: The thallium (I) series

The peculiar thermal behavior of the thallium(I) n-alkanoates series (consi sting in several transitions between polymorphic and mesomorphic phases) in comparison with other metallic n-alkanoates series is stated. The allowanc e of highly accurate adiabatic heat capacity data permits a study of the CH 2 contributions to the lattice heat capacity curve at low temperature. More over, in this series an anomalous gradual enhancement of the lattice heat c apacity has been interpreted from vibrational spectroscopy results as a non cooperative effect due to the internal hindered rotation of the alkyl chain (formation of gauche defects, even in the solid state). The thermodynamics of the "stepwise melting process" from the totally ordered solid at low te mperature to the isotropic liquid is based on a revised lattice heat-capaci ty curve. This was used to evaluate the energy and entropy not only of the clear first order transitions present in the series but also of the describ ed noncooperative effect. The CH2 enthalpy and entropy contribution for thi s series is estimated and a comparison with the published values for other series is carried out. Moreover, the texture of the mesophases is revealed by polarized light microscopy. (C) 1999 American Institute of Physics. [S00 21-9606(99)51132-X].