On the molecular basis of aggregation and stability of Colombian asphaltenes and their subfractions

Vacuum residue (VR) is the heaviest fraction obtained by fractional distillation of the crude oil. Crude oil and its fractions, such as VR, can be separated in its components saturates, aromatics, resins, and asphaltenes (SARA fractions). Asphaltenes are of particular importance for up- and down-stream processes due to their molecular complexity, high heteroatom content, and strong tendency to self-aggregate. To evaluate the molecular characteristics of asphaltenes responsible for aggregation, we used a fractionation method based on mixtures of toluene and acetone to separate asphaltenes, isolated from a heavy Colombian residue, into different solubility subfractions. In this contribution, we show correlations between elemental composition and average molecular properties of subfractions and their solubility behavior. In addition, we were able to differentiate asphaltenes into 'unstable' and 'stable' subfractions in toluene solution by an analytical centrifugation method. From the stability analysis, we demonstrated that less soluble subfractions (consisting of more aromatic, heavier, and more condensed asphaltenes) are more prone to flocculation, and hence, they are less stable than more soluble subfractions. The results of colloidal characterization prove that less soluble subfractions form large fractal structures of small nanoaggregates, whereas the most soluble subfraction form highly stabilized dispersions of non-flocculating nanoaggregates. Results obtained in this work are expected to provide insights regarding the importance of solubility-based fractionation method as a basis for understanding asphaltene aggregation in heavy crude oils.