On the Applicability of the Regular Solution Theory to Asphaltene + Diluent Mixtures

The similarity of the Hildebrand or Hansen solubility parameter is frequently used in petroleum science as a measure of the compatibility of constituents and for interpreting and correlating properties of asphaltene + diluent mixtures. A partial specific volume at near infinite dilution and enthalpies of solution are sensitive measures of solute–solvent interactions derived from high precision density and calorimetry measurements for dilute mixtures. In this contribution, the partial specific volumes and enthalpies of solution of pyrene and various Athabasca and Maya asphaltenes at near infinite dilution on a mole fraction basis, in decane, toluene, 1-methylnaphthalene, quinoline, anisole, 2,6-lutidine, pyridine, methylene chloride, and tetrahydrofuran are reported over the temperature range of 20–80 °C. At 20 °C, these diluents possess solubility parameters ranging from 15 MPa0.5 (decane) to 22 MPa0.5 (quinoline). The properties of pyrene + diluent mixtures are used to illustrate the application and misapplication of the regular solution theory to such mixtures. Thermodynamic measurements, partial specific volume, and enthalpy of solution are shown to be independent of both Hildebrand and Hansen solubility parameter values. These results do not support the use of the solubility parameter or other simple solution thermodynamic concepts to describe asphaltene + diluent mixture behavior. The need for a more detailed description of physiochemical phenomena arising upon mixing asphaltenes with diluents is discussed.