Effect of solution pH on the adsorption of BLG at the solution/tetradecane interface

Using drop profile analysis tensiometry the adsorption dynamics and the equilibrium equation of state of beta-lactoglobulin (BLG) at the water/tetradecane (W/TD) interface are studied at pH 3, 5 and 7. The data are well described by a thermodynamic adsorption model using almost identical model parameters for all three pH values except for the surface activity coefficient. The surface pressure isotherms at the water/air (W/A) surface exhibit much steeper run than interfacial pressure at the W/TD interface for any of the studied cases at pH 3, 5 and 7, and the calculated adsorption isotherm data point at smaller adsorbed amounts for these pH. This seems to be in contrast to the much larger interfacial pressure changes reached at high BLG concentrations at the W/TD interface, which are almost three times higher than those at the W/A surface. The observations can be explained by a strong interaction between BLG and the oil molecules at the interface. The dynamic interfacial tensions can be adequately described by a mixed adsorption model, assuming a diffusional transport of the protein molecules in the aqueous bulk phase and an adsorption mechanism which assumes a change of the adsorption activity parameter in dependence of the interfacial coverage. (C) 2016 Elsevier B.V. All rights reserved.