Influence of seawater on interfacial Properties, foam performance and aggregation behaviour of Fluorocarbon/Hydrocarbon surfactant mixtures

Abstract

• Adding seawater lowers surface activity and adsorption kinetics of SDS/FS-50 mixtures. • Ca 2+ , Mg 2+ and K + have greater adverse effects on foaming ability than Na + in seawater. • Dual effects of salts on foam stability are observed at different surfactant concentrations. • Spontaneous transition from vesicle to micelle is observed after mixing SDS and FS-50. In this work, the influence of seawater on interfacial properties, foam performance and aggregation behaviour of fluorocarbon/hydrocarbon surfactant mixtures was systematically investigated. Two model foaming solutions containing different surfactants (nonionic APG, anionic SDS, and amphoteric FS-50) were prepared by deionized water and seawater, respectively. Multi-scale methods including surface tension measurements, foam ageing tests and cryogenic-transmission electron microscopy (cryo-TEM) were used. The results show that the addition of seawater reduces the equilibrium surface tension, foaming ability and adsorption kinetics of SDS/FS-50 mixtures. Among the counterions in seawater, Na + can lower the surface tension and slightly improve the foaming ability, while the presence of Ca 2+ , Mg 2+ and K + allows the formation of surfactant precipitate to weaken the foaming ability of the solution. It turns out that seawater has dual effects on foam stability and the defoaming/stabilizing effects of seawater are related to ionic surfactant concentrations. Besides, it is confirmed that the spontaneous transition from single-lamellar vesicle to rod-like micelle occurs after mixing FS-50 with SDS in the deionized water, while only multi-lamellar vesicles and surfactant crystals can be observed in the seawater. The aggregation behaviour strongly affects solution viscosity as well as foam performance. At high SDS concentrations, the surfactant precipitate/aggregate can inhibit the liquid drainage process and improve foam stability.