Dynamic surface properties of adsorbed protein solutions: BSA, casein and buttermilk

The dynamic surface properties of three widely different protein systems are investigated using different experimental techniques. These techniques are (i) static drop experiments which allow description of the adsorption of proteins at a clean surface, (ii) experiments at a constant rate of dilatation where the area is continuously expanded and a steady state surface tension is obtained, (iii) modulus experiments where a surface in equilibrium is subjected to small periodic disturbances, (iv) stress relaxation experiments which are theoretically similar to moduli experiments, and (v) to a lesser degree, aspiration and sweeping experiments. The results are described by a process involving two steps: (i) diffusion of proteins to the surface and (ii) unfolding of adsorbed protein molecules at the surface. Only native protein molecules are exchangeable, while the unfolded protein molecules are not. From these results the diffusion relaxation times and reaction relaxation times are obtained.