In-situ study of the solid–solid phase transitions occurring in real diesel wax crystalline systems using differential scanning calorimetry and high-resolution synchrotron X-ray powder diffraction

Abstract

Differential scanning calorimetry and variable temperature high-resolution X-ray powder diffraction using synchrotron radiation are used to identify the solid-state phase transitions occurring in real diesel wax systems and correlated with the structural packing of these systems. The waxes are found to undergo three solid-state phase transitions. The first involves a partial transition of some of the n-alkane fraction from the stable low temperature orthorhombic structure in which the waxes pack with four molecules per unit cell and space group Fmmm to the high temperature rotator phase. The second, previously unidentified, transition reflects this phase separation behaviour within the multi-homologous wax mixtures probably associated with the aggregation of highly ordered lamellae of similar chain lengths which are able to maintain the low temperature orthorhombic phase over a higher temperature range before transforming into the rotator phase. The final transition reflects the transformation of the rotator phase to the molten state. A molecular scale packing model consisting of aggregates of similar n-alkane chain lengths and aggregates of differing chain lengths is proposed to account for this phenomenon.