APPLICABILITY OF GIBBS LAW TO PROTEIN ADSORPTION-ISOTHERMS

Protein adsorption at interfaces is the primary phenomenon involved in foaming and emulsifying properties. However, our understanding of the relationships between the structure of proteins and the properties of their adsorbed layers is poor. A set of experimental isotherms of proteins adsorbed at the air/water interface was obtained some years ago from surface pressure and surface concentration measurements (D.E. Graham and M.C. Phillips, J. Colloid Interface Sci., 70 (1979) 415). The correct interpretation of these data should provide a basis of the understanding of the mechanisms of protein adsorption. Current models of protein adsorption give good fits to experimental isotherms. However, in order to be valid, these models must satisfy the fundamental thermodynamic Gibbs' law of interfacial adsorption. Using Gibbs' law, we have calculated the surface excess concentration GAMMA(e) from the surface pressure isotherm of beta-casein. There is a possible fit between the calculated and experimental values of GAMMA(e) at bulk protein concentrations below 10(-3) g l-1) and a strong discrepancy at bulk concentrations between 10(-2) and 1 g l-1 We tried to discover the reasons for such difficulties in the experimental procedures used to measure the surface pressure and surface concentration. The conclusion is that current models of protein adsorption do not give a thermodynamically acceptable view of this phenomenon.