Viscoelastic Properties of Produced Water Emulsions

Understanding viscoelastic properties of fluid interfaces leads to improved oil-water separation strategies. This study provides interfacial tension and dilatational viscoelasticity data for cyclohexane in contact with synthetic produced water (PW) or DI water containing the anionic surfactant sodium dodecyl sulfate (SDS). Pendant drop tensiometry with oscillatory dilatational rheology was applied over SDS concentrations of 0.001-1 g/L and frequencies of 0.1-0.5 Hz. The PW-cyclohexane system exhibited interfacial tension (44.4 ± 0.32 mN/m) lower than that of the corresponding DI water-cyclohexane interface (47.9 ± 1.26). The apparent critical micelle concentration (CMC) at the PW-cyclohexane interface (∼0.1 g/L) was more than an order of magnitude lower than in DI water-cyclohexane (∼1.94 g/L), reflecting the strong influence of dissolved ions on surfactant adsorption. Interfacial dilatational measurements showed a clear maximum in the complex modulus |E*| (∼32 mN/m at 0.01 g/L SDS in PW), more than twice the maximum observed for DI water. The phase angle also increased rapidly with SDS concentration, indicating an earlier transition toward viscous-dominated interfacial behavior in PW. Time resolved droplet size distributions for oil-in-PW emulsions demonstrated that increasing SDS concentration produced smaller initial droplets and progressively reduced droplet growth over 24 h, consistent with enhanced reduction of coalescence at higher surfactant levels. Together, these results show that PW chemistry fundamentally modifies surfactant CMC, interfacial rheology, and droplet-scale emulsion behavior, providing fundamental insight into how water composition influences surfactant-based interfacial behavior relevant to oil-water separation and PW treatment.