Equilibrium and dynamics of adsorption of mixed β-casein/surfactant solutions at the water/hexane interface

The adsorption of mixtures of beta-casein and the anionic surfactant sodium dodecyl sulphate and the cationic dodecyl trimethyl ammonium bromide were studied at the water/hexane interface by drop profile analysis tensiometry and compared with those obtained for the water/air interface. The obtained interfacial tension isotherms suggest a gradual replacement of the protein molecules from both liquid interfaces with increasing surfactant concentration and a fixed amount of protein. The mechanical properties of the interfacial layers were probed by dilational rheological investigations. The results show, that although the equilibrium state is described by the existing theoretical models adequately, the quantitative analysis of the dilational visco-elasticity data requires further refinement of the corresponding model. (C) 2009 Elsevier B.V. All rights reserved.