Temperatures and Enthalpies of Solid−Solid and Melting Transitions of the Odd-Numbered n-Alkanes C21, C23, C25, C27, and C29

Very high purity samples of normal pentacosane and heptacosane are made by synthesis: the procedures for the C25 and C27 synthesis and the original conditions of their purification, particularly using the extraction by supercritical carbon dioxide, are described. Measurements of temperatures and enthalpies of the solid−solid transitions and of the melting transition were carried out by differential scanning calorimetry on the C25 and C27 synthetic samples and the odd-numbered homologous n-alkanes (from C21 to C29). The results show the importance of the effect of the impurities, particularly for the δ and γ crystal−crystal transition temperatures of C25 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.