An Analysis of the Solid Phase Behavior of the Normal Paraffins

A set of best values for the temperatures and enthalpies of fusion and transition for the n-paraffins is presented. From an analysis of these data a general qualitative theory of the phase behavior of the n-paraffins is developed. Four distinct crystal structures—hexagonal, triclinic, monoclinic, and orthorhombic—describe the solid phases of all n-paraffins with more than nine carbon atoms in the chain. The latter two structures become equivalent at longer chain lengths. Odd-even differences are resolved in terms of reasonable differences in end group packing, and the smooth increase in melting and transition temperature with increasing chain length is attributed to a decrease in the ratio of end groups to chain groups. Double transitions are predicted for several pure n-paraffins above n-C34H70. Impurity effects are isolated from the pure n-paraffin properties and discussed. The equation, TM(°K) = 414.3 (n−1.5)/(n + 5.0) is presented as a correct description of the melting temperatures (TM) of all n-paraffins above n-C44H90. Sufficient data to permit an accurate extrapolation of the enthalpies and entropies of fusion to the infinite-chain limit are not available.