Dynamic Surface Tension and Surface Dilatational Elasticity Properties of Mixed Surfactant/Protein Systems

We present the study on dynamic surface tension and surface dilatational elasticity properties of dilute aqueous systems of pentaglycerol fatty acid esters (pentaglycerol monostearate, C18G5, and pentaglycerol monooleate, C18:1G5), whey protein, sodium caseinate, and mixed surfactant and protein at room temperature. The adsorption kinetics at the air-liquid interface has been studied by bubble pressure tensiometer and the oscillation bubble (rising drop) method. It has been shown that the dynamic surface tension curve basically presents two-regions; namely induction region and rapid fall region. During the induction time the adsorption is the diffusion-controlled process of amphiphilic surfactant or protein molecules from the bulk of the solution to the interface. Whey protein and sodium caseinate showed longer induction time ∼10000 ms compared to the surfactant systems, where induction time was estimated to be ∼1000 ms. However, in both the protein and surfactant systems, the induction time goes on decreasing with increasing the concentrations. The similar behavior was observed in the mixed system, and lower surface tension values were observed at higher concentrations. The fitting of the experimental data to the theoretical equation shows the presence of two relaxation mechanisms of widely different time scale for the adsorption of surfactant or protein molecules at the interface. The relaxation time strongly varies with the concentrations following the power law, and at fixed concentration it was the highest for whey protein and the lowest for C18:1G5 system. The surface dilatational elasticity determined within the frequency range of ∼0.1 to 1 cycle/s supports the dynamic surface tension data.