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

Abstract 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 C 22 –C 36 to C 14 –C 36 : each mixture differs from the previous sample by the addition of a lighter n -alkane. At the solid state, the multi-alkane solid samples C 22 –C 36 and C 21 –C 36 are two-phase, C 20 –C 36 to C 15 –C 36 , three-phase, and the broader distribution C 14 –C 36 , four-phase. In these polyphase solid systems, whose heaviest n -alkane is always C 36 , 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.