Effect of Ethanol on the Structure and Properties of beta-Casein Adsorption Layers at the Air/Buffer Interface

The adsorption of beta-casein at the air-solution interface has been monitored in equilibrium conditions by neutron reflectivity. It was observed that for a bulk concentration of 100 mg/L, the amount of protein adsorbed per unit surface increases from 2.8 to 4.4 mg/m2 when the ethanol concentration in the bulk changes from 0 to 20% (v/v). Surface pressure measurements on aqueous solutions indicate that the surface pressure is higher when both protein and alcohol are added than when a single substance is in the solution. The addition of protein has an effect when the alcohol concentration is less than 20%. These results are consistent with the occurrence at the interface of a protein network leaving a surface fraction available for ethanol. A thermodynamic model has been developed using scaling law arguments to model the surface pressure and dilational modulus measurements. It introduces an exponent which is characteristic of the solvent "quality" and of the structure of the interfacial layer. The results are interpreted as showing that ethanol modifies the solvent properties, the interactions between the protein and the solvent, and the structure of the adsorption layer. The main transition seems to occurr at 6% ethanol. Copyright 1998 Academic Press.