Thermodynamic and structural analyses of the solid phases in multi-alkane mixtures similar to petroleum cuts at ambient temperature

Structural analyses were carried out by X-ray diffraction, at ambient temperature, on multi-alkane samples whose mole fraction distribution shows a shape of the ‘exponential decreasing’ type, as observed in petroleum cuts. Nine samples, whose normal-alkane number varies from 15 to 23, have been studied with mole fraction continuous distributions of normal-alkanes going from C22–C36 to C14–C36: each mixture differs from the previous sample by the addition of a lighter n-alkane. At the solid state, the multi-alkane solid samples C22–C36 and C21–C36 are two-phase, C20–C36 to C15–C36, three-phase, and the broader distribution C14–C36, four-phase. In these polyphase solid systems, whose heaviest n-alkane is always C36, the average composition of the heavy and middle phases are constant and their structure are isostructural to the β′ ordered intermediate solid solution, observed in n-alkane binary or ternary molecular alloys; the mean carbon atom number of the light phase decreases as the global average carbon atom number of the synthetic mixtures in relation to the addition of light n-alkanes and its structure simultaneously evolves from the β′ ordered intermediate solid solution towards the β-RI(Fmmm) and the α ‐ RII ( R 3 ¯ m ) disordered solid solutions, observed in pure n-alkanes: the light n-alkane added between each distribution intercalates itself into the structure whose molecule stacking period (thickness) is compatible with its own carbon chain length, in order to reduce the molecular gaps.