Effect of composition on freezing points of model hydrocarbon fuels

A study was made of the effect of composition on the freezing points (crystallization) of model hydrocarbon jet fuel type mixtures. Solutions of higher n-alkanes (C12–C17) in several solvents were emphasized. Freezing points (Tm) of solutions of single alkanes were found to conform with the Van't Hoff equation. From the slopes and intercepts of plots of concentration (In X) versus 1 T m , heats (ΔHm) and entropies (ΔSm) of fusion and extrapolated freezing points of pure alkanes (Tm,o) were derived. For isoparaffinic solvents the derived Tm,o values were in good agreement with the literature. For ΔHm and ΔSm, only the even carbon numbered alkanes exhibited values similar to literature data. The behaviour of the odd numbered alkanes can be explained on a basis of solid-phase transitions. For alkanes in other solvents (particularly aromatics), there is sometimes substantial disagreement between derived and literature data depending on the solvent. For mixtures of two alkanes in a single solvent, significant changes or reversals in slope were observed for 1 T m plotted against In X for one of the two alkanes, suggesting interaction between the solutes. At C16 concentration range of 0.2 to 2%, the addition of C13 can depress the freezing point, but at C16 concentrations ⪢ ≈4% changes in X (C13) had negligible effect on T5.