Temperatures and enthalpies of solid-solid and melting transitions of the odd-numbered n-alkanes C-21, C-23, C-25, C-27, and C-29

Very high purity samples of normal pentacosane and heptacosane are made by synthesis: the procedures for the C-25 and C-27 synthesis and the original conditions of their purification, particularly using the extraction by supe rcritical. carbon dioxide, are described. Measurements of temperatures and enthalpies of the solid-solid transitions and of the melting transition wer e carried out by differential scanning calorimetry on the C-25 and C-27 syn thetic samples and the odd-numbered homologous n-alkanes (from C-21 to C-29 ). The results show the importance of the effect of the impurities, particu larly for the delta and gamma crystal-crystal transition temperatures Of C2 5 and C27. Several current data of the literature are also shown concerning the temperatures and the enthalpies of solid-solid and melting transitions of these five n-alkanes.