Rheological properties of concentrated emulsions stabilized by globular protein in the presence of nonionic surfactant

The influence of a nonionic surfactant (polyoxyethylene(20)sorbitan monooleate, Tween 80) on the rheological behavior of concentrated toluene-in-water emulsions stabilized by globular protein (bovine serum albumin, BSA, C = 5 × 10−5 M) at 22 °C have been studied using different methods of shear rheology. The systems under investigation are non-Newtonian liquids while the yield stress is observed at low shear stresses. The dependencies of the yield stress and shear viscoelasticity on the total concentration and surfactant/protein molar ratio were discussed. The increase in concentration of a nonionic surfactant leads to the decrease in viscosity and suppression of emulsion elasticity. A sharp jump in the rheological properties appears at a minimal concentration of the added Tween 80, whereas elasticity practically disappears, viscosity abruptly decreases and emulsions become unstable against coalescence in approaching concentration of the nonionic surfactant to 1 × 10−3 M. The observed effects can be attributed to the gradual displacement of a high-molecular-mass component (protein), which is responsible for the emulsion elasticity, from the stabilizing (interfacial) layers by the nonionic surfactant.