Surface shear viscosity and protein-surfactant interactions in mixed protein films adsorbed at the oil-water interface

Abstract

The surface shear viscosity of protein films adsorbed from pure or mixed β-lactoglobulin + cationic gelatin aqueous phases have been studied at the n-hexadecane—water interface (5 mmol/ dm 3 bis-tris-propane buffer, pH 7.0, 2 × 10 −3 wt% protein). The effect of the introduction of nonionic surfactant Tween 20 (polyoxyethylene (20) sorbitan monolaurate) and anionic surfactant SLES 2EO (sodium lauryl ether sulphate) into the subphase below the 1-day-old pure or mixed protein films has been investigated. The addition of Tween 20 to individual β-lactoglobulin or gelatin films brings the surface viscosity rapidly down to zero, but not so for the mixed β-lactoglobulin + gelatin film. The addition of SLES 2EO to the gelatin film produces a molecular rearrangement and a gradual decrease of the surface viscosity. The effect of oil-soluble surfactant C 12E 2 (diethylene glycol monododecyl ether) on the surface shear viscosity of adsorbed β-lactoglobulin has been studied. A small amount of C 12E 2 induces a large increase in surface viscosity. Synergistic adsorption effects are proposed between the oppositely charged β-lactoglobulin and gelatin molecules and between the oil-soluble surfactant C 12E 2 and the β-lactoglobulin molecules.