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.