Probing Molecular Interactions of an Asphaltene Model Compound in Organic Solvents Using a Surface Forces Apparatus (SFA)

摘要

Studies on the molecular interaction mechanisms of asphaltenes in organic solvent have not reached a widely accepted conclusion, mainly because of a poor definition of asphaltene molecules and lack of accurate information on the molecular structure. In this study, N-(l-hexyIheptyl)-N/-(5-carboxylicpentyl) perylene-3,4,9,10-tetracarboxylic bisimide (CSPe) of the polyaromatic core with a proper molecular weight and heteroatoms in its structure was used as a model compound of asphaltenes in an attempt to understand interaction mechanisms of molecular aggregation in organic solvents. A surface forces apparatus (SFA) was used to directly measure the molecular interactions of C5Pe in toluene and heptane. For the interactions between two model day (mica) surfaces across a CSPe-in-toluene solution, the repulsion observed between the adsorbed CSPe molecules was shown to be of a steric origin. The force-distance profiles at short separation distances under high compression force during approaching were well-fitted with the Alexander- de Gennes (AdG) scaling theory, while the weaker repulsive forces at lower compression force regime over longer separation distances can also be fitted with the AdG model using an independent set of fitting parameters, indicating the presence of possible secondary brush structures of the CSPe molecules in toluene. For interactions of pre-adsorbed C5Pe films (CSPe versus mica and CSPe versus CSPe), no significant adhesion was detected in toluene, while strong adhesion was measured in heptane. The comparison of the results obtained with the model compound CSPe and native asphaltenes shows that CSPe behaves quantitatively different from the real asphaltenes in the context of contact angle and interaction force profiles. However, there are qualitative similarities in terms of intermolecular forces, indicating that the polar components in real asphaltene molecules play an important role in determining their interfacial activities.