The concentration dependence of adsorption from a mixture of β-lactoglobulin and sodium dodecyl sulfate onto methylated silica surfaces

Abstract

The adsorption from a mixture of SDS and β-Lactoglobulin, 1:5 ( w w ), onto a methylated silica surface was studied in situ by ellipsometry. The amounts adsorbed from different concentrations of the mixture, at pH 7, were compared with those adsorbed from the corresponding pure SDS and β-lactoglobulin solutions. At high concentrations of the mixture, where the CMC of SDS is approached or exceeded, the adsorbate was probably dominated by SDS, indicated by similar kinetics and amounts adsorbed as for SDS solution alone. The amount adsorbed increased, in this concentration range, when the system was rinsed with buffer solution. This was probably due to an exchange between SDS and β-lactoglobulin when the system was diluted. At intermediate concentrations of SDS and β-lactoglobulin, the amounts adsorbed from the mixtures increased and reached a maximum. This maximum was observed both before and after rinsing. Before rinsing the adsorbate might have been a mixture of SDS and β-lactoglobulin while after rinsing probably only β-lactoglobulin remained. At low concentrations, larger amounts were adsorbed from the mixture than from β-lactoglobulin solution alone. In this concentration range rinsing caused minor desorption, indicating that SDS is co-adsorbed with the protein, even in those cases were no adsorption from a pure SDS solution was seen. This indicated that the binding of SDS to β-lactoglobulin at low concentrations is stronger than to the silica surface and that this binding facilitated the adsorption of protein.