Thermodynamics of Mixtures Containing Oxaalkanes. 4. Random Mixing and Orientational Effects in Ether plus Alkane Systems

Molar excess enthalpies, H-m(E), and volumes, V-m(E), of linear or cyclic ether + n-alkane systems have been discussed in terms of the effective dipole moment, (mu) over bar, of the ether, differences of standard enthalpies of vaporization of the ether and of the isomeric alkane, the number and relative positions of the oxygen atoms, the shape of the ether, and the relative size of the mixture compounds. The mentioned solutions have also been studied using the Flory model and the Kirkwood-Buff formalism. Both theories provide consistent results. At 298.15 K, the random mixing hypothesis is a good approximation for mixtures including linear or cyclic monoethers or linear diethers. Orientational effects become stronger in solutions with 2,5,8-trioxanonane, 2,5,8,11-tetraoxadodecane, or 2,5,8,1 1,14-pentaoxapentadecane. In the case of 1,3-dioxolane mixtures, this type of effect is more relevant than in systems with 1,4-dioxane. This is supported by W-shaped C-p,m(E) curves, large variations of the Flory interaction parameter, X-12, with the oxaalkane concentration, and local mole fractions of the ether-ether type that are higher than the bulk ones, particularly at lower x(1) values. The latter means that orientational effects are more important at this condition, and this is confirmed by large X-12 variations with x(1) in the region of low x(1) values. At equimolar composition. X-12 values of solutions containing 2,5,8,11-tetraoxadodecane or cyclic ethers remain nearly constant with the number of C atoms of the alkane, which reveals that systems with such oxaalkanes are similar from an interactional point of view. From values of the internal excess energies at constant volume, U-Vm(E), it is shown that interactions between like molecules are usually overestimated. Nevertheless, the general trends observed are independent of the H-m(E) or U-Vm(E) data considered. Structural effects are present in mixtures with components that differ largely in size (e.g., dipropyl ether + hexadecane or 2,5,8,11,14-pentaoxapentadecane + hexane).