A REVERSIBLE MODEL FOR ASPHALTENE COLLOIDAL ASSOCIATION AND PRECIPITATION

摘要

It was soon recognized that asphaltenes in crude oil are dispersed in the form of colloidal aggregates. Classical descriptions often involve peptizing resins adsorbed at the surface, preventing flocculation. These are indeed transcriptions of the classical steric stabilization of conventional colloids. However asphaltenes have intriguing solubility properties. And none of the many interpretations proposed so far is capable to account consistently for all these properties. So in the present paper, we first list up all robust experimental features redundantly present in the current literature on asphaltene solubilization, and then we build up a new description demanding that it should not be contradicted by any one of the listed facts. In our description, asphaltene aggregation and precipitation are distinct steps of a completely reversible process. Aggregation proceeds from specific strong interaction sites located at the periphery of the asphaltene molecules: they drive the reversible association in 2D sheets, which is consistent with reported neutron scattering data. Precipitation eventually occurs, determined by Van der Waals attractions between aggregates, when the quality of the solvent shifts beyond the limits of a characteristic range of solubility parameter, which depends on the molecular composition of the 2D sheets. Our model provides non-trivial predictions for the solubility behavior of mixed asphaltenes and resins (co-solubilization). The agreement between the behaviors of oil fields asphaltenes in laboratory tests and the predictions of the model is good. Following our analysis, the risk of precipitation is efficiently anticipated from the solvent quality at the onset of precipitation, detailed analytical data being of little relevance.