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

Abstract

Enthalpies of mixing of dibromoethane (DBE) and tetrahydronaphthalene (THN) were measured with branched and linear alkanes at 298 K. These were three highly branched alkanes 2,2,4-trimethylpentane, 2,2,4,6,6-pentamethylheptane, 2,2,4,4,6,8,8-heptamethylnonane and five linear alkanes from C₆ to C₁₆. Enthalpies of mixing at 308, 318 and 328 K for DBE with branched and linear hexadecane and at 328 K for THN with branched hexadecane are also presented. With THN, the maxima in the enthalpies of mixing at 298 K are generally <700 J mol^–1 and, surprisingly the differences between the mixtures with normal and branched alkanes as the second component are only tens of J mol^–1 and are either positive or negative although THN might have been expected to act as an order-breaker. H^E values with THN may be explained either by the existence of some order in THN and/or by an exothermic contribution from the condensation effect. With the DBE mixtures, a disordering contribution to the enthalpies of mixing can be discerned, although the total enthalpies are large (2000 J mol^–1). The H^E_disorder whose values are respectively 367, 208 and 128 J mol^–1 at 298 K disorder for n-C₁₆, n-C₁₂ and n-C₈, agree with those found for other order-breaker molecules and diminish with temperature, as does the orientational order of the long-chain alkanes. The origin of the large enthalpies for DBE with the alkanes is discussed in terms of an unfavourable interaction of the bromine atom with the CH₂ groups, a change in the trans–gauche equilibrium of DBE in going from the pure state to the solution and the effect of non-central forces in DBE.