Dynamic properties of β-casein/surfactant adsorption layers

The dynamic surface elasticity of mixed solutions of β-casein and surfactants of different chemical nature was measured as a function of surface age and surfactant concentration with the aim to elucidate the influence of non-ionic interactions between the components on the dynamic surface properties. Small admixtures of the amphiphilic non-ionic polymer poly(N-isopropylacrylamide) (PNIPAM) did not influence the dynamic surface elasticity and the surface properties of the solution were entirely determined by the adsorbed protein. The corresponding kinetic dependencies of the dynamic surface elasticity have two local maxima. The increase of the polymer concentration resulted in the appearance of a third local maximum in the surface elasticity corresponding to the polymer adsorption. In this case the height and position of the second peak in the kinetic curve of the surface elasticity were changed noticeably indicating interactions between the components. The increase of the surfactant hydrophobic chain length in the case of small additions of cationic surfactants of low molecular weight increased the hydrophobicity of the formed protein/surfactant complex and also led to changes of the shape of the dynamic surface elasticity. The application of the scanning probe microscopy indicated different mechanisms of the β-casein displacement from the liquid surface by the surfactants of low and high molecular weights, correspondingly.