THERMODYNAMICS OF BINARY-MIXTURES CONTAINING OXAALKANES .4. CYCLIC DIETHERS AND ACETALS + N-ALKANES OR + CYCLOHEXANE

The data available in the literature for the excess Gibbs energies, GE, excess enthalpies, HE, activity coefficients at infinite dilution, γ∞i, and excess heat capacities, CEP, of cyclic diethers (1,4-dioxane) or acetals (1,3-dioxane, 1,3-dioxolane and alkyl-substituted 1,3-dioxolanes) + n-alkanes or + cyclohexane are examined on the basis of the DISQUAC group contribution model. The interaction parameters, quasi-chemical and dispersive, of the O/CH2 contact in these dioxacycloalkanes are compared with the interaction parameters determined previously for linear mono- or diethers and acetals and with cyclic monoethers. As a rule, in the non-substituted cyclic molecules, the O groups are sterically less hindered by adjacent alkyl radicals than in the parent linear molecules; consequently the quasi-chemical parameters are relatively large. As expected, substitution of 1,3-dioxolane in the 2-position with alkyl groups produces a decrease in the quasi-chemical parameters. The dispersive parameters of cyclic monoethers are larger than the dispersive parameters of the parent linear molecules, especially for oxolane (and other small-ring cyclic monoethers), probably because of electron delocalization (ring strain). There is little difference between the dispersive parameters of the O/CH2 contact in cyclic dioxaalkanes (1,3-dioxolane, 1,3-dioxane, 1,4-dioxane) and the parent linear ethers or acetals. 1,4-Dioxane can be described with the same quasi-chemical parameters as oxane. In cyclic acetals, 1,3-dioxolane and 1,3-dioxane, the quasi-chemical parameters differ slightly from those of oxolane and oxane, respectively. Using structure-dependent parameters, the model describes consistently GE, γ∞i and HE of the investigated mixtures. A more careful examination shows, however, that the experimental HE curves of dioxacycloalkane + alkane mixtures are somewhat flatter than calculated with DISQUAC. Accordingly, the CEP curves are W-shaped. Probably because of stronger non-randomness effects, these effects are more pronounced in cyclic dioxaalkanes than in linear ones. Moreover, the well-known, rather unusual, dependence of HE of dioxacycloalkane + n-alkane mixtures on the chain length of the n-alkane indicates the occurrence of complex conformational changes in these mixtures, impossible to represent with DISQUAC. © 1990.