SolventQuality Determines the Size and InterfacialBehavior of Soluble Asphaltene Nanoaggregate Clusters

Soluble asphaltene nanoaggregate clusters near the onset point of precipitation have been shown to stabilize water-in-oil (W/O) emulsions resulting in flow assurance problems in the oil and gas industry. Broadly, it has been shown that factors including size, shape, and heterogeneity of model colloidal particles influence the stability of W/O emulsions. However, the size and structure of soluble asphaltene nanoaggregate clusters across the full range of solvent quality are unknown. Here, we study the influence of solvent quality on the size and structure of soluble asphaltene nanoaggregate clusters and the model asphaltene molecule VO-79 using small-angle X-ray scattering (SAXS). We observe that the radius of gyration <i>R</i>_g 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, <i>D</i>_f ≈ 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. The 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.