Polarity effects of asphaltene subfractions on the stability and interfacial properties of water-in-model oil emulsions

Asphaltenes can adsorb at the interface between crude oil and produced water and form a defensive interfacial layer to stabilize emulsions. This paper details the effects of the polarities of asphaltene subfractions on emulsion stability and interfacial properties. Asphaltene subfractions with different polarities were first obtained by a solvent-precipitation method, followed by the characterization of their molecular structures and dispersive states in the model oil. The results show that the subfractions with lower polarity contain a higher proportion of aliphatic groups, which benefits oil compatibility, thus leading to a better dispersive state and weaker aggregation tendency. Then, the stabilities of the emulsions prepared with these subfractions were explored at different concentrations. The emulsion stability worsens as polarity weakens at 500 ppm. However, at a concentration of 5000 ppm, the emulsion stability increases first with a polarity decrease, reaches a peak, and then decreases. The adsorption behavior and interfacial structural strength are detected through dynamic interfacial tension and dilational viscoelastic experiments. The subfractions with lower polarities exhibit better dispersion in the model oil, allowing for higher rates of absorption. The dilational modulus and emulsion stability depend on the migration rate and surface coverage of the subfractions and share similar trends with changes in polarity. With an increase in the polarity of the subfractions, the surface coverage and migration rate have reverse contributions on the dilational modulus and emulsion stability. Additionally, the contributions become more obvious after asphaltene aggregation.