Thermodynamic effects of orientational order in chain-molecule mixtures. Part 2.—Temperature-dependence of heats of mixing of branched and normal alkane mixtures

Abstract

Heats of mixing, and excess heat capacities derived therefrom, have been obtained between 25 and 45°C at a constant concentration for squalane + n-C₆ and between 25 and 75°C for squalane + n-C_n, n= 8, 10, 12, 14 and 16 and for 2,2,4-trimethylpentane + n-C₁₆, 2,2,4,4,6,8,8-heptamethylnonane + n-C₁₆ and squalane + heptamethylnonane. The concentration dependence has been studied at several temperatures for squalane + n-C₈ and + n-C₁₆, the heat per unit segment volume being symmetrical in segment fraction. Negative Δh_M occur for squalane + n-C₆ and + n-C₈, with c^E _p negative and increasing in magnitude with temperature. The results are consistent with the large difference in thermal expansion between the components and are predicted by the Prigogine–Flory theory. For squalane + n-C₁₄ and + n-C₁₆, trimethylpentane + n-C₁₆ and heptamethylnonane + n-C₁₆, Δh_M is strongly positive but c^E _p is large and negative, sharply decreasing in magnitude with temperature. This behaviour is at variance with current theory which predicts c^E _p≈ 0. It may be explained in terms of an orientational order in higher n-C_n liquids which is destroyed on mixing with the branched alkanes and which decreases with temperature. Squalane + n-C₁₀ and + n-C₁₂ give intermediate results, combining effects of orientational order at low temperature, and free volume at high. Squalane + heptamethylnonane gives negative Δh_M, independent of temperature, in accord with theory indicating the lack of orientational order in these branched components. Literature values of Δh_M and Δs_M for hexane isomers + n-C₁₆ also suggest orientational order in n-C₁₆.