An analysis of the solid phase behavior of the normal paraffins

Abstract

A set of best values for t he te mperatures a nd enthalpies of fusion alld transition for the n-paraffins is presented. From a n a nalysis of these data a general qualitative t heory of the phase behavior of the n-paraffins is developed. Four di stinct. crystal structures-h exagonal, t riclinic, monoclinic, and orthorhombic-describe t he solid phases of all n-paraffins with more than nine carbon atoms in the cha in. The la tter t wo structures beco me equivalent at longer chain lengt hs. Odd-even d ifferences are resolved in terms of reasonable diffet'ences in end group packing, a nd t he smooth in crease in melting a nd transition t emperature " 'ith increasing chain length is attributed to a dec rease in t he r atio of end groups to ch ain gro ups. Double t ransitions a re predicted for several pure n-paraffins a bove n-C34H 70 Impurity effec ts are isolated from the pure n-para ffin properties a nd disc ussed. The equat ion, TM (O K ) = 414.3 (n-1. 5) ! (n + 5.0) is presented as a correct d escript ion of the m elting te mperatures (T M ) of all n-paraffins above n-C44H IlO Suffi cient data t o p ermit an acc urate extrapolation of the enthalpie and entropies of fu sion to the infinite-c bain limit are not avail a ble.