Comparing foam and interfacial properties of similarly charged protein–surfactant mixtures

The foam stability of protein–surfactant mixtures strongly depends on the charge of the protein and the surfactant, as well as on their mixing ratio. Depending on the conditions, the mixtures will contain free proteins, free surfactants and/or protein–surfactant complexes. To be able to compare different protein–surfactant mixtures, generic knowledge about the occurrence of each of these states and their relative contribution to foam stability is essential. In this work, the foam stability and interfacial properties of bovine serum albumin (BSA) mixed with sodium dodecyl sulphate (SDS) as well the binding of SDS to BSA as are studied at different molar ratios (MR). A comparison is made with ß-lactoglobulin (BLG) mixed with SDS. Both proteins and SDS are negatively charged at pH 7. The foam stability in the presence of small amounts (up to MR 1) of SDS is half the value of the pure protein solutions. The foam stability for both protein surfactant mixtures reaches a minimum at MR 20. A further increase of the MR leads to an increase of foam stability. The foam stability of BLG–SDS at MR >20 follows the foam stability of pure SDS solutions at equivalent concentrations, while BSA–SDS mixtures have an offset and begin to increase from MR >50. This behaviour was also reflected in the surface pressure and complex dilatational elastic moduli, and could be linked to the binding of the surfactant to the proteins. Both proteins bind SDS at high and low affinity binding sites. BSA's high affinity binding sites have a binding stoichiometry of 5.5 molSDS/molprotein, and BLG's high affinity binding site has a stoichiometry of 0.8 molSDS/molprotein (determined by isothermal titration calorimetry). Binding to the low affinity binding sites, occurs with a binding ratio, leading to an accumulation of free surfactants. While the basic mechanisms underlying the foam properties of mixed systems are not explained in detail by this approach, the foam stability plots of both protein surfactant mixtures could be superimposed using the concentration of free SDS