Thermal Behavior of Even Chain Length Lithium n-Alkanoates

Abstract

The thermal phase sequences and thermal histories of a homologous series of even chain length lithium n-alkanoate, LiCnH2n-1O2 (LiC8-18), have been determined by differential scanning calorimetry. The phase changes accompanying heating these compounds above the melt are dependent on chain length. One intermediate phase is observed between the lamellar crystal and isotropic melt in the phase sequence for LiC10-12: and two for LiC14-18: All the phase sequences are enantiotropic. Additionally, cooling and reheating runs are in good agreement with those of fresh samples. The premelting transitions, in the long chain length compounds, are preceded by an exothermic peak on heating fresh samples. This transition disappears on cooling and reheating the samples. It is probably due to the physical state of the melt in contact with the sample holder or from molecular structure factors, and not a liquid crystalline phase as proposed by an earlier study. The premelting transitions, in the long chain compounds, are ascribed to rotation of the methylene groups, in the crystalline lattice, resulting in a change in conformation from the nearly all-trans the more disordered plastic crystal phase, which is stable over a narrow temperature range. The thermodynamic data indicate that the melting processes are different for short and long chain adducts arising from subtle changes in their molecular structures.