Dilatational Response of Soluble Asphaltene Nanoaggregate Clusters at the Oil/Water Interface

Asphaltenes strongly adsorb to oil/water interfaces, forming viscoelastic films that confer solid-like mechanical properties and stabilize crude oil emulsions. It has been suggested that asphaltenes form the most stable crude oil emulsions close to their onset point of precipitation, where mixtures of soluble and insoluble asphaltenes are present in solution. Given their heterogeneity in chemical composition, molecular weight, and aggregation state, it remains challenging to identify the mechanisms that promote emulsion stability. In this study, we use the oscillating pendant drop method to investigate the dilatational rheological response of soluble asphaltene nanoaggregate clusters and the asphaltene model molecule violanthrone-79 (VO-79) as a function of solvent quality and aging time. We observe that the apparent complex dilatational modulus of soluble asphaltene nanoaggregate clusters is dependent on solvent quality and increases with aging time. We show that the apparent interfacial elastic modulus increases with aging time, whereas the apparent interfacial viscous modulus is independent of both solvent quality and aging time. These results support the long-standing hypothesis that insoluble asphaltenes hinder adsorption of soluble asphaltenes resulting in a decrease in elasticity. In contrast, VO-79 exhibits a relatively low apparent complex dilatational modulus that is independent of time and solvent quality. Lastly, we find that formation of an interfacial film that wrinkles upon droplet contraction under all solution conditions, revealing that soluble asphaltene nanoaggregate clusters can form a partially incompressible interfacial film. We hypothesize that soluble asphaltenes increase interfacial elasticity with decreasing solvent quality due to a decrease in cluster size which enhances filling efficiency and particle interactions akin to Pickering emulsions.