Molecular mechanics of aggregates of asphaltenes and resins of the Athabasca oil

An analysis of the effects of an almost continuous chemical distribution of asphaltenes and resins on the molecular recognition processes occurring in crude oil indicates that their aggregates will have a broad distribution b oth in the chemical composition and in the strength of the intermolecular i nteractions responsible for the aggregation. Then, crude oil cannot be desc ribed just as a sol formed by solid asphaltene particles dispersed by resin s or as a simple micellar system of asphaltene and resin molecules. The mol ecular aggregates may vary from solid particles formed by asphaltenes and r esins to loosely bound micelles with quite short lifetimes. These different aggregates may coexist within the crude oil and many will exchange compone nts with others. The entropic contributions to the changes in free energy u pon aggregation were also discussed. Molecular mechanics calculations showe d that a model asphaltene aggregate from Athabasca exhibits stronger intera ctions with its resins than with solvents such as toluene and n-octane. The resins showed a considerable selectivity for the different adsorption site s of the asphaltene aggregate. This selectivity was stronger than that foun d for the solvent molecules, indicating that it is enthalpically more favor able for them to form aggregates with the asphaltenes. The selectivity may also help to el:plain the specificity of some resins that are able to dispe rse only the asphaltenes of certain types of crude oils while failing to do the same for others.