MOLECULAR MECHANICS STUDY OF THE SELECTIVITY IN THE INTERACTION BETWEEN SOME TYPICAL RESINS AND ASPHALTENES

Juan Murgich

Centro de Química, IVIC, Apartado 21827, Caracas 1020A, Venezuela

The use of Molecular Simulation in the study of aggregates of model asphaltene and resins is analyzed. The limitations of using these model molecules are pointed out. Molecular Mechanics calculations of aggregates of asphaltenes having a single aromatic region (continental) and others having smaller regions connected by alkyl chains (archipelago) are discussed in terms of the molecular recognition process present in petroleum. An analysis of the interactions showed that the van der Waals and the electrostatic forces (H bonds) were the most important. Stacking was the determining process in the aggregates of continental asphaltenes with unfavorable contributions from saturated rings and side chains. The steric interference of these groups limits the growth of the aggregates. Archipelago asphaltenes showed much more complex aggregates because some act as bridges between them. Tangling between archipelago asphaltenes molecules and aggregates may occur. This may influence the solubility of them and explain the long equilibration times of the dissolution found by GPC. The energy of interaction of asphaltenes with typical resins was calculated. Important differences were found for adsorption of the different resins. Only some were able to form strong aggregates with asphaltenes. Therefore, only a fraction of resins are active in the peptization process. Therefore, when the stability of a crude is analyzed, the active fraction of the resins should be considered instead of their total amount.