AOT and pluronic F68 coadsorption at fluid/fluid interfaces: A continuous-flow tensiometry study

Efficient emulsifier surfactants reduce oil/water interfacial tensions to low values within a fraction of a second. Unfortunately, in many cases, these surfactants do not provide high coalescence resistance due to the poor structural-mechanical (viscoelastic) strength of the adsorption layer. The emulsifier interfacial layer can be fortified by including an emulsion stabilizer, usually of higher molecular weight, that imparts higher interfacial elasticity and provides long-term emulsion stability. Unfortunately, the mechanisms of emulsifier and stabilizer coadsorption at the fluid/fluid interface are not well understood. To attack this problem, we study mixed adsorption of an anionic surfactant, AOT, a well-known emulsifier, and a nonionic polymeric surfactant, Pluronic F68, at the mineral oil/water interface by applying continuous-flow tensiometry. Adsorption/desorption of AOT is instantaneous. Pluronic F68, however, exhibits a strong desorption barrier and is practically irreversibly adsorbed at the air/water and mineral oil/water interfaces when washed with water. When AOT solutions are used for washout of the Pluronic F68 adsorbed layer, complete displacement of polymeric surfactant from the oil/water interface is observed at AOT concentrations above the critical micelle concentration. When adsorption occurs from mixed AOT/Pluronic F68 solutions, however, the polymeric surfactant successfully competes with ACT for the interface, but only at higher concentrations where the singly adsorbed polymer adopts a hairpin conformation at the interface. Optimized AOT/Pluronic F68 surfactant mixtures are very efficient for mineral-oil-in-water emulsification. The emulsions show long-term stability and a unique immunity against dilution.