Molecular recognition in aggregates formed by asphaltene and resin molecules from the Athabasca oil sand

The conformation of lowest energy of an asphaltene molecule of the Athabasc a sand oil was calculated through molecular mechanics. The molecule has a c omplex globular shape with small internal cavities. This shape resulted mos tly from the existence of polymethylene bridges connecting the aromatic reg ions. Molecular aggregates formed with the asphaltene and with nine resins from the same oil, and with n-octane and toluene, were also studied. The re sins showed higher affinities for the asphaltene than toluene and n-octane and also exhibited a noticeable selectivity for some of the external sites of the asphaltene. This selectivity based on the molecular recognition of t he site depends on the fit between the resins and the site of the asphalten e. The selectivity explains why resins of one oil may not solubilize asphal tenes from other crudes. An analysis of the changes in the enthalpic and en tropic contributions to the free energy showed that both contributions shou ld be considered when the stability of the asphaltene and resin molecular a ggregates is examined.