Thermodynamic properties of mixtures of linear and branched alkanes with 1,2-dibromoethane and tetrahydronaphthalene. Part 2.—Free energies and entropies of mixing

Abstract

Free energies of mixing have been measured by light scattering on seventeen systems at 300 K and two systems at 308, 318 and 328 K. The mixtures were branched and linear alkanes, C₆ to C₁₆, with 1,2-dibromoethane (DBE) and tetrahydronaphthalene (THN). The three branched alkanes were 2,2,4-trimethylpentane (br-C₈), 2,2,4,6,6-pentamethylheptane (br-C₁₂) and 2,2,4,4,6,8,8-heptamethylnonane (br-C₁₆). The Flory–Huggins combinatorial entropy is more appropriate than the regular solution entropy to calculate G^E from ΔG_M. For the linear alkanes, G^E and TS^E increase with chain length from 900 and 541 J mol^–1(n-C₆) to 1130 and 963 J mol^–1(n-C₁₆) for the DBE systems. G^E is ∼150 J mol^–1 higher with the br-alkanes. G^E for THN is also higher for the branched alkanes but only ∼400 J mol^–1. The higher TS^E values for the linear alkanes are associated with the long chain disordering entropy which was calculated to be respectively 477, 403 and 292 J mol^–1 for n-C₁₆, n-C₁₂ and n-C₈. The value of TS^E(disorder) appears to be >H^E(disorder). However this better solvent quality may be because the linear alkane has, owing to its shape, a larger combinatorial entropy than the compact branched alkane of the same volume. The value of TS^E(disorder) diminishes with temperature as H^E(disorder). TS^E(disorder) compares reasonably well with the values obtained by viscosities with SnBut₄ as order breaker. The causes for the large values of TS^E for DBE are undoubtedly the same as those for H^E previously discussed in Part 1.