Electrochemical impedance spectroscopy study of the adsorption behaviour of α-lactalbumin and β-casein at stainless steel

At all the temperatures studied, adsorption of α-LA and β-CN onto the stainless steel surface was successfully described with a Langmuir adsorption isotherm. ΔGADS0 values for α-LA and β-CN (−54 and −55 ± 0.5 kJ mol−1 at 298 K, respectively) show that the molecules have a strong affinity for the stainless steel surface. The ΔHADS0 (15.6 and 3.6 ± 0.5 kJ mol−1) and ΔSADS0 (236 and 192 ± 1 J K−1 mol−1) values for α-LA and β-CN, respectively, indicated that α-LA partially unfolds at the surface upon adsorption and for both proteins the adsorption process is governed entropically. A comparison is made for the first time of the thermodynamic values for adsorption of four proteins on the same stainless steel surface under the same experimental conditions, which show remarkable differences due to their conformational stability emphasizing the importance of these thermodynamic measurements in understanding the complex protein/metal interactions.