Dilational rheology of protein films adsorbed at fluid interfaces

This report aims at (i) presenting a quantitative interpretation of interfacial dilational moduli (vertical bar E vertical bar) for four proteins at three different interfaces and (ii) identifying the main parameters responsible. The proteins were adsorbed from aqueous solution against air, n-tetradecane and sunflower seed oil, as a function of protein concentration and adsorption time. Experimentally, a dynamic drop tensiometer is a convenient instrument to generate the required sinusoidal oscillations for compression/expansion of interfaces (Benjamins et al., 1996 [1]). Theoretically, a simple two-dimensional solution model with a constant molecular area of the protein described the data only at fairly low pressures. Much better agreement over the entire elastic range was found with a recent extension of the model. This extension accounted for adsorbed proteins adopting smaller molecular areas with increasing surface pressure. Three factors dominated the values of the dilational modulus: (i) rigidity of protein molecules, (ii) degree of interfacial non-ideality and (iii) tension of the clean interface (Benjamins et al., 2006 [2]). The last factor is clearly of great relevance to food emulsions. For each protein at different interfaces, the elasticity increased with the enthalpy parameter (H-S) of the equation of state. Elasticity and H-S both increased with the clean-interface tension, gamma(0), i.e., with decreasing polarity of the interface (Benjamins et al., 2006 [2]; Fainerman et al., 2003 [3]). The elasticity of the different proteins also increased with increasing rigidity of the molecules, indicating a lower compressibility of the molecular area at the interface. Pure viscosities were never observed in our experience. However, viscoelastic behaviour was found at high pressures, i.e., in densely packed surfaces. The measured viscous phase angles strongly decreased at still higher pressures, indicating that the active relaxation mechanism slowed down with increasing molecular packing density. Specific kinetic models are yet to be developed for such mechanisms. (C) 2010 Elsevier Ltd. All rights reserved.