Interfacial behavior of beta-lactoglobulin at a stainless steel surface: An electrochemical impedance spectroscopy study

The electrochemical impedance spectroscopy technique was used to investigat e the interfacial behavior of beta-lactoglobulin at an austenitic stainless steel surface over the temperature range 299 to 343 K at an open circuit p otential. The electrode/electrolyte interface and corresponding surface pro cesses were successfully modeled by applying an equivalent-electrical-circu it approach. A charge-transfer resistance value was found to be very sensit ive to the amount of adsorbed protein (surface concentration), thus indicat ing that the adsorption of the protein (i) was accompanied by the transfer of the charge, via chemisorption, and (ii) influenced the mechanism and kin etics of the corrosion reaction. This was also apparent from the large decr ease in the corrosion activation energy (16 kJ mol(-1)) caused by the adsor ption of the protein. Adsorption of beta-lactoglobulin onto the stainless s teel surface at an open circuit potential resulted in a unimodal isotherm a t all the temperatures studied and the adsorption process was described wit h a Langmuir adsorption isotherm. From the calculated Gibbs free energies o f adsorption it was confirmed that beta-lactoglobulin molecules adsorb stro ngly onto the stainless steel surface. The enthalpy and entropy values indi cated that the molecule partially unfolds at the surface upon adsorption. T he adsorption process was found to be entirely governed by the change in en tropy. (C) 2000 Academic Press.