Thermodynamic Study of Heptane plus Silicone Mixtures. Excess Volumes and Enthalpies at 298.15 K

Excess molar volumes V(E) and partial molar enthalpies and excess enthalpies H(E) of binary mixtures of heptane + linear (hexamethyldisiloxane, octamethyltrisiloxane, decamethyltetrasiloxane, dodecamethylpentasiloxane) and cyclic (octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane) silicone oligomers have been determined at 298.15 K. Both excess properties have small values (|V(E)| < 0.13 cm(3).mol(-1), H(E) < 0.23 kJ.mol(-1)) for linear silicones, exhibiting a monotonic increase with an increasing number of dimethylsiloxane units. Higher V(E) and H(E) data are instead observed in the case of cyclic molecules, with a larger value exhibited by octamethylcyclotetrasiloxane. Partial molar volumes at infinite dilution of silicones in heptane are markedly larger than those of n-alkanes of the same size. Likewise, their enthalpies of solvation in heptane are less negative than those of hydrocarbons. Cyclic silicones display partial molar volumes lower than corresponding linear compounds, and their solvation enthalpies are slightly more negative. Both partial molar volumes and enthalpies of solvation of silicones are found to be additive, thus enabling the prediction of these properties for unstudied molecules. The additivity of partial molar volumes was found to also hold for a short silicone polymer, thus indicating an open structure of the polymeric chain in hydrocarbon solution, that is, equally exposed to the solvent as the shorter oligomers.