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 theological behavior of concentrated toluenein-water emulsions stabilized by globular protein (bovine serum albumin, BSA, C = 5 x 10(-5) M) at 22 degrees 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 nortionic surfactant to 1 X 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. (c) 2006 Elsevier B.V. All rights reserved.