Thermal Behavior of n-Alkanes from n-C32H66 to n-C80H162, Synthesized with Attention Paid to High Purity

Abstract

The general principle for synthesizing higher n-alkanes of the highest purity is described briefly. Thermal behavior of the synthesized n-alkanes from n-C32H66 to n-C80H162 were examined by means of DSC, small- and wide-angle X-ray scattering and optical microscopy. All the crystalline n-alkanes except for n-C80H162, when crystallized from solution, showed transitions to high-temperature monoclinic modifications (Mh01, h=1 or 2) before the rotator transition or the melting, irrespective of the number of carbon atoms (n) being odd or even and the difference in room-temperature modifications of even n-alkane. One feature of the transition was its irreversibility. The odd n-alkanes of n-C33H68, n-C37H76, and n-C45H92 showed also another type of solid-solid transition below the transition mentioned above. Both types of the transitions were thermally proved to be of the first-order. Although no simple functional forms were generally given to the relationships between the transition temperature and n, the temperatures for the transition of orthorhombic higher n-alkanes to the Mh01 phase increased with n, reaching about 90°C for n-C69H140. The transition to Mh01 was accompanied by a morphological change on the crystal surface, in the form of characteristic striations parallel to the bs subcell axis. This suggests that the solid-solid transition proceeds successively from a nucleating site with a staggering of molecules in the chain direction. If the slow rate of the process and complicated behavior of the Mh01 phase are taken into account, the kinetical view point of the phase transition leads to the working hypothesis that the molecular motion of the long chains in the n-alkane crystals, which consists of rotational and longitudinal motions or a flip-flop motion, takes place only in this transformation process.