Adsorption isotherm and equation of state for β-Lactoglobulin layers at the air/water surface

The adsorption behaviour of β-Lactoglobulin (BLG) at the air/water surface and the visco-elastic properties of the BLG adsorption layers are discussed in terms of quasi-equilibrium adsorption isotherms and equation of state gained by comparing a model with a set of experimental data obtained by bubble profile analysis tensiometry. The presented simultaneous characterization of the adsorption isotherm and the dilational visco-elasticity under dynamic and equilibrium conditions is based on measurements of the dynamic surface pressure and its response to sinusoidal bubble area variations. The thermodynamic model was applied to the set of experimental data under the assumptions of monolayer compressibility, surface dimerization and development of a secondary layer. The effect of the bubble surface age on the adsorption isotherm and equation of state, and also on the visco-elastic characteristics is discussed and it allows concluding that the existence of a bilayer structure, which is formed beyond a certain protein adsorption, is responsible for the observed relaxation processes.