Dilational Viscoelastic Properties of Water-Fuel Interfaces in Single and Binary Surfactant Systems

The interfacial rheology of a surfactant film surrounding a water-in-oil emulsion influences the stability of the emulsion significantly. In this research, we chose monoolein of molecular weight of 356.54 and pentaerythritol oleate (PETO-B) of molecular weight of 1193.93 as the surfactants and dissolved them in clay-treated ultralow sulfur diesel fuel separately and together to mimic single and binary surfactant systems. The water/fuel interfacial viscoelastic properties were then studied using the oscillation pendant drop method subject to different surfactant concentrations and oscillation frequencies. Surfactant coverage on the interface and the surfactant molecule migration rate were calculated to investigate the viscoelastic phenomenon of the surfactant film. The migration rate of surfactant in the bulk was found to be crucial for the surfactant to effectively compensate the interfacial tension (IFT) gradient caused by interfacial area expansion and compression. Faster moving monoolein molecules could rapidly compensate the IFT gradient as oscillation was triggered, resulting in much higher elastic modulus in comparison to PETO-B molecules with lower migration rate. Higher surfactant concentration also produced faster migration rate resulting in a higher film elastic modulus. Lower oscillation frequency promoted the formation of a film with lower elasticity and higher viscous modulus, while higher oscillation frequency led to lower film viscous modulus. The calculated occupying area per surfactant molecule on the interface indicated that monoolein had a much tighter arrangement at the interface than PETO-B. The binary surfactant system was constructed with monoolein and PETO-B with various weight ratios. The interfacial rheology of such a system was determined by the faster migrating surfactant that built the fundamental framework of the film and adding PETO-B to monoolein-containing fuel could therefore decrease the interfacial elastic modulus and, hence, the emulsion's stability, to benefit emulsified water separation.