Asphaltenes strongly adsorb to oil-water interfaces, forming viscoelastic films that result in solid-like mechanical properties and stabilize crude oil emulsions. It has been hypothesized that asphaltenes form the most stable 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. Here, we study the influence of solvent quality on the size and interfacial behavior of soluble asphaltene nanoaggregate clusters and the model asphaltene molecule VO-79 using small-angle X-ray scattering (SAXS) and the oscillating pendant drop method, respectively. We observe that the radius of gyration of soluble asphaltene nanoaggregate clusters is independent of solvent quality until the onset point of precipitation where it then decreases with decreasing solvent quality. Soluble asphaltene nanoaggregate clusters self-associate to form mass fractals with an approximate fractal dimension of 2. Interestingly, VO-79 does not show any evidence of nanoaggregate formation under all conditions tested. Lastly, we show that the interfacial tension (IFT) at the oil–water interface is initially dependent on solvent quality. Transport of soluble asphaltene nanoaggregate clusters from the bulk to the interface may be described as a diffusion-controlled process that is dependent on solvent quality. After aging, however, the IFT becomes independent of solvent quality, suggesting that emulsion stability depends on other factors beyond IFT, including the rheological properties of asphaltenes, their colloidal stability, and reorganization at the oil-water interface. Using the oscillating pendant drop method, we observe that the complex dilatational modulus of soluble asphaltene nanoaggregate clusters is dependent on solvent quality and increases with aging time. In contrast, VO-79 shows a relatively low complex dilatational modulus that is independent of time and solvent quality. Lastly, we observe that formation of an interfacial film that wrinkles upon droplet contraction, revealing that soluble asphaltene nanoaggregates can form a partially incompressible interfacial film.
