Adsorption behavior of non-ionic demulsifiers at the oil/water interface stabilized by asphaltenes: Experiments, adsorption kinetics, and mechanisms

Abstract

Formation of emulsions containing asphaltenes is both undesirable and unavoidable in petroleum production. Chemical demulsifiers, such as non-ionic surfactants, have long been applied as the cheapest and most effective method of breaking emulsions by altering the oil/water interfacial properties. In this study, the association state of the asphaltenes, the dehydration rate, and the interfacial rheology under various demulsifier concentrations were used to analyze the contributions of non-ionic demulsifiers to the breaking of the asphaltene-containing emulsions. Additionally, the dynamic interfacial tension (DIFT) data over time were used to investigate the adsorption kinetics of the asphaltenes and the demulsifier at the oil/water interface. Results indicate that liquid paraffin enhanced the association of the asphaltene molecules, while the non-ionic demulsifier rendered asphaltene aggregate sizes smaller. The addition of the demulsifiers reduced the interfacial strength, leading to an unstable interface. The oil-soluble demulsifiers reduced IFT more effectively than the water-soluble ones, due to their faster migration rate and low interfacial tension gradient to the oil–water interface. The adsorption process of the asphaltenes, influenced by the demulsifier concentration, could be divided into three stages: a short-term diffusion-controlled process, a transition process, and a slow descent process (referred to as Regimes I, II, and III). In Regime I, the diffusion coefficient of asphaltenes increased with the demulsifier concentration, indicating that the demulsifier aided the dispersing of the asphaltenes in the solution. The equilibrium IFT and the maximum surface excess concentration of the asphaltenes in the long-term adsorption process decreased with the demulsifier concentration, indicating that the asphaltene-containing emulsions generally became unstable after adding the demulsifiers. Our results offer valuable insights into the adsorption kinetics and the mechanisms of alterations by the asphaltenes and demulsifiers of the oil/water interface, with implications for various interfacial phenomena (e.g., emulsion stability) involving surfactants in oil production, transportation, and processing.