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 monito red in equilibrium conditions by neutron reflectivity. It was observed that for a bulk concentration of 100 mg/L, the amount of protein adsorbed per u nit surface increases from 2.8 to 4.4 mg/m(2) when the ethanol concentratio n 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 p rotein and alcohol are added than when a single substance is in the solutio n. The addition of protein has an effect when the alcohol concentration is less than 20%. These results are consistent with the occurrence at the inte rface of a protein network leaving a surface fraction available for ethanol . A thermodynamic model has been developed using scaling law arguments to m odel the surface pressure and dilational modulus measurements. It introduce s an exponent which is characteristic of the solvent "quality" and of the s tructure of the interfacial layer. The results are interpreted as showing t hat ethanol modifies the solvent properties, the interactions between the p rotein and the solvent, and the structure of the adsorption layer. The main transition seems to occurr at 6% ethanol. (C) 1998 Academic Press.