Thermodynamic Properties of the n -Alkanes C 19 H 40 to C 26 H 54 and Their Binary Phase Diagrams

The shapes of the C 22 H 46 -C 24 H 50 and C 23 H 48 -C 24 H 50 binary phase diagrams were analyzed. In the C 22 H 46 -C 24 H 50 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 C 22 H 46 and C 24 H 50 . In the C 23 H 48 -C 24 H 50 system this effect is much less pronounced. The measured enthalpy data of n -alkanes C 19 H 40 to C 24 H 50 and of the binary system C 22 H 46 -C 24 H 50 were analyzed to obtain the ‘excess’ heat capacity per atom of carbon {[ C p /( 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 θ D 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 C p ( L-s )>0, in the alkanes studied it is strongly negative C p ( L-s )≪0. This explains the shape of the binary phase diagrams C 22 H 46 -C 24 H 50 , C 24 H 50 -C 26 H 54 , C 22 H 46 -C 23 H 48 and C 23 H 48 -C 24 H 50 .