Thermodynamic Properties of the n-Alkanes C19H40 to C26H54 and Their Binary Phase Diagrams

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The shapes of the C22H46-C24H50 and C23H48-C24H50 binary phase diagrams were analyzed. In the C22H46-C24H50 binary system the increased stability of the binary compounds with increasing temperature can be explained by the much larger heat capacity and entropy of the binary compounds compared to that of the components C22H46 and C24H50. In the C23H48-C24H50 system this effect is much less pronounced. The measured enthalpy data of n-alkanes C19H40 to C24H50 and of the binary system C22H46-C24H50 were analyzed to obtain the ‘excess’ heat capacity per atom of carbon {[Cp/(Rm)]-3} (Rm being the number of carbon atoms). The ‘excess’ heat capacity per carbon atom is the value of the heat capacity above the Debye high temperature value of 3R. At low temperatures (below 280 K) one is in the Debye temperature θD region. At higher temperatures the large ‘excess’ heat capacity of the solids explains the movements in the carbon chains. In the liquid the excess heat capacity is small and corresponds numerically to the anharmonic vibrations in low melting metals. In contrast to metals, where the difference in heat capacity between liquid and solid below the melting point is positive Cp(L-s)>0, in the alkanes studied it is strongly negative Cp(L-s)≪0. This explains the shape of the binary phase diagrams C22H46-C24H50, C24H50-C26H54, C22H46-C23H48 and C23H48-C24H50.