Thermodynamic effects of orientational order in chain-molecule mixtures. Part 1.—Heats of mixing of globular and normal alkanes

Abstract

Heats of mixing at 25°C have been obtained calorimetrically for alkane mixtures where component 1 is cyclohexane, 2,2-dimethylbutane or n-hexane and component 2 is a normal-C_n, n= 6,8,12 or 16, or a corresponding branched-C_n: 2,2-dimethylbutane, 2,2,4-trimethylpentane, 2,2,4,6,6-penta-methylheptane or 2,2,4,4,6,8,8-heptamethylnonane. For the three n-C_n series, the molar heats and heats per unit volume are positive and increase rapidly with n. With the branched-C_n, cyclohexane gives smaller positive heats decreasing with n, while n-hexane and 2,2-dimethylbutane show increasingly negative heats. The magnitudes and variations of the heats, and of the derived X₁₂ parameters are inconsistent with the usual interpretation of positive ΔH_M in n-alkane mixtures, i.e. an energetic weakness of methyl(end)-methylene (middle) interactions. The results are explained by energetic effects associated with a correlation of molecular orientations (CMO) in the pure components and the mixture. The CMO, studied optically by Bothorel and collaborators, is highly sensitive to molecular shape, increasing with n for pure n-C_n liquids, remaining zero for the branched-C_n, and being destroyed on mixing with spherical molecules such as cyclohexane and 2,2-dimethylbutane. Parameters (J), characterizing the CMO in the component liquids, are used to give a quantitative interpretation of the X₁₂ and ΔH_M values.