Compression/expansion rheology of oil/water interfaces with adsorbed proteins. Comparison with the air/water surface

Dynamic interfacial tensions and surface dilational moduli were measured for four proteins at three fluid interfaces, as a function of time and concentration. The proteins-beta-casein, beta-lactoglobulin, bovine serum albumin, and ovalbumin-were adsorbed from aqueous solution against air, n-tetradecane, and a triacylglycerol oil. The sinusoidal interfacial compression/expansion, at frequencies ranging from 0.005 to 0.5 Hz, was effected in a dynamic drop tensiometer suited to viscous oil phases. Generally, at interfacial pressures up to 15 mN/m, dilational moduli were purely elastic at frequencies from 0.1 Hz. In this elastic range, in-surface relaxation either was essentially completed or had not yet started within a time on the order of 10 s. Within this time span, protein exchange with the bulk solution was negligible. In cases where in-surface relaxation was completed in the imposed time, the moduli depended only on the equilibrium Pi(Gamma) relationship. We interpret these results in terms of a simple two-dimensional solution model, based on a Gibbs dividing surface, accounting for nonideal mixing to the first order with respect to both entropy and enthalpy. Interfacial mixing enthalpy is shown to have a major effect on the elasticity, with both quantities increasing in the sequence triacylglycerol < tetradecane < air. We also suggest a strong correlation between enthalpy and clean-interface tension that increases in the same order. At each interface, the enthalpy increases with increasing molecular rigidity: beta-casein < beta-lactoglobulin < bovine serum albumin < ovalbumin. Best agreement with the experimental data was obtained with a recently extended version of the model accounting for proteins adopting smaller molecular areas with increasing surface pressure. For interfacial pressures above 15 mN/m, the moduli were generally no longer purely elastic, with viscous loss angles ranging up to 36 degrees. In this range of high pressures, the moduli depended on relaxation mechanisms for which specific kinetic models must be developed.