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 pro teins adsorbed at the air/water interface was obtained some years ago from surface pressure and surface concentration measurements (D.E. Gra ham and M.C. Phillips, J. Colloid Interface Sci., 70 (1979) 415). The correct interpretation of these data should provide a basis of the und erstanding 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 thermo dynamic Gibbs' law of interfacial adsorption. Using Gibbs' law, we hav e calculated the surface excess concentration GAMMA(e) from the surfac e pressure isotherm of beta-casein. There is a possible fit between th e calculated and experimental values of GAMMA(e) at bulk protein conce ntrations below 10(-3) g l-1) and a strong discrepancy at bulk concent rations between 10(-2) and 1 g l-1 We tried to discover the reasons fo r such difficulties in the experimental procedures used to measure the surface pressure and surface concentration. The conclusion is that cu rrent models of protein adsorption do not give a thermodynamically acc eptable view of this phenomenon.