Thermodynamic and structural analyses and mechanisms of the crystallisation of multi-alkane model mixtures similar to petroleum cuts

Thermodynamic and structural analyses were carried out by X-ray diffraction and differential thermal analyses as functions of the temperature on multi-alkane samples whose distribution of mole fractions shows a shape of the ‘exponential decreasing’ type, as observed in petroleum cuts. Nine samples, whose number of normal-alkanes, Cns, varied from 15 up to 23, were studied with continuous distributions of mole fractions going from the C22–C36 series to the C14–C36 series: C22 and C14 corresponding respectively to the first Cns of the two terminal series and C36 is always the last Cn of each series: each mixture differed from the previous sample by the addition of a lighter Cn−1. At the solid state and according to literature, the multi-alkane samples of C22–C36, C21–C36 and C20–C36 series are in a two-phase solid thermodynamic state, C19–C36 to C15–C36, three-phase, the broader distribution C14–C36, four-phase and the mixtures with numbers of Cns≤11, single-phase. Thus, the Cns crystallise in the solid phase, where the thickness of molecule layers is compatible with their own chain length. The results allow for extracting the recurring structural and thermodynamic properties for these types of mixtures during crystallisation, and to clarify the molecular mechanisms involved in the number of observed solid phases, in the order of their appearance and in their thermodynamic and structural behaviour during the course of cooling from the liquid state.