Dilatational rheology of protein plus non-ionic surfactant films at air-water and oil-water interfaces

The dilatational rheology of adsorbed films of beta-lactoglobulin has been studied at the air-aqueous solution interface and the n-tetradecane-aqueous interface using a Langmuir trough apparatus employing a pulse change in interfacial area. The effect of the addition of the non-ionic surfactant C12E6 to films adsorbed from 10(-3) wt% beta-lactoglobulin at pH 7 and 30 degrees C was also examined. The surfactant appears to bind to beta-lactoglobulin with a low molar ratio (one surfactant molecule per protein molecule), but this and/or co-adsorption has significant effects on the dilatational moduli. Films exhibit higher dilatational moduli at the oil-water interface compared to the air-water interface and show a stronger dependence on the surfactant concentration at the oil-water interface. At around equimolar ratios of protein and surfactant there is a slight maximum in the dilatational moduli, more noticeable at the oil-water interface, though separate measurements also indicated a slight enhancement of foam stability in this region. At higher surfactant concentrations there is a more marked decrease in the dilatational moduli to similar values for both types of interface. Moduli measured at strains up to at least 5% appear to be considerably non-linear. (C) 1998 Elsevier Science B.V. All rights reserved.