Is surface pressure a measure of interfacial water activity? Evidence from protein adsorption behavior at interfaces

The tension at the air-water interface is fundamentally due to the excess standard chemical potential of water in the interfacial region. Thus, a reduction in surface tension as a result of adsorption of an amphiphilic molecule, that is, surface pressure, must a priori be viewed as a reduction in the standard chemical potential of surface water molecules and not due to a compensating lateral pressure exerted by the adsorbed film. On the basis of this simple premise, the dynamics of reduction of surface tension during adsorption of proteins at the air-water interface has been analyzed in terms of reaction of interfacial water molecules with protein and/or their displacement from the interfacial phase by protein molecules. The concept developed assumes that reduction in interfacial water activity occurs as a result of two molecular processes, namely anchoring of the protein to the interface and subsequent concentration-dependent two-dimensional aggregation of the adsorbed protein. Experimental adsorption data of 20 proteins were analyzed using this theory. The theory successfully explained surface pressure evolution in terms of the number of water molecules affected per protein molecule during anchoring (n(R)) and surface aggregation (n(agg)) stages. The fitted nR values of 20 proteins showed a linear relationship with M-2/3, confirming the intuitive expectation that the number of water molecules displaced during the anchoring step is roughly proportional to the cross-sectional area of a globular protein molecule. The ratio n(agg)/n(R), which reflects the propensity of a protein to aggregate at the interface, showed a linear dependence on the interfacial shear viscosity of adsorbed protein films. The theory accounts for thermodynamic changes in the interfacial region in terms of physical processes taking place as a result of protein adsorption.