Interfacial shear rheology of asphaltenes at oil–water interface and its relation to emulsion stability: Influence of concentration, solvent aromaticity and nonionic surfactant

The interfacial shear rheology of asphaltene film at toluene–heptane/brine interface has been studied. The oscillation mode of a rheometer utilizing a biconical geometry has been employed to determine film structure and mechanical strength of the interfacial layer, as a function of interface age, solvent aromaticity and asphaltene concentration. With 60% toluene–40% heptane as oil solvent, a concentration threshold around 2–5 g/L asphaltenes was found for the formation of gel-like structure at oil/brine interface, after aging for 6–7 h. This concentration is close to the concentration threshold for the formation of stable W/O emulsion. Further increase in concentration from 5 to 10 g/L gives similar interfacial modulus values, indicating that a saturation state of the interfacial layer is reached. At a given asphaltenes concentration 5 g/L, the storage modulus decreases when increasing the proportion of toluene and vanish with 100% toluene, however, much higher modulus was obtained for the same asphaltene concentration in 30% toluene which has flocculation inside. In addition, when nonionic surfactant coexist with asphaltenes in the bulk phase, only viscous interface was detected, which may attribute to the effect of surfactant for displacing asphaltenes at the interface and inhibiting asphaltene cluster formation in the bulk phase.