The structure and thermal behaviour of some long chain cerium(III) carboxylates

The even chain length cerium(III) carboxylates from the octanoate to the octadecanoate have been synthesised by metathesis. Thermogravimetry shows the presence of coordinated water for the short chain homologues, whereas the longer chain ones only contain adsorbed water. X-Ray diffraction and IR spectral measurements show that the solid phase has a lamellar, bilayer structure with planes of the cerium(III) ions coordinated to the carboxylate groups. The phase behaviour of the carboxylates has been studied by DSC and polarized-light microscopy. One or more mesophases are observed over the temperature range 70-120 degrees C and melting occurs between 130 and 150 degrees C. The textures observed on the polarizing microscope clearly show the anisotropic nature of the mesophases. Although the overall enthalpy and entropy of melting of these compounds increase with increasing chain length, the values are considerably lower than expected for complete fusion of the alkyl chains. Competition between melting of the chains and changes in the metal-carboxylate coordination region is the major factor responsible for the differences observed in the phase behaviour between the short and long chain derivatives.