Adsorption of proteins at the solution/air interface influenced by added non-ionic surfactants at very low concentrations for both components. 1. Dodecyl dimethyl phospine oxide

• Proteins are highly surface active and adsorb at very low bulk concentrations.
• Surfactants lower the surface tension at much higher bulk concentration.
• The addition of smallest amounts of non-ionic surfactants change the adsorption behaviour of the proteins.
• The surface tension decrease for mixed solutions can be explained only by a change in the protein's surface activity.

The adsorption of proteins at liquid interfaces happens at rather low bulk concentrations due to their rather high surface activity. In contrast typical surfactants start to decrease the surface tension at bulk concentration in the range of mmol/l and reach a minimum value at about two or three orders of magnitude higher concentration. The two proteins studied here, β-lactoglobulin and β-casein, adsorb already remarkably at much lower concentrations, i.e. less than 1 μmol/l. When smallest amounts of a non-ionic surfactant are added to low concentrated protein solutions, changes in the surface tension are observed which cannot be explained by the existing theoretical models. An agreement with the experimental data can be achieved only when an increased surface activity of the protein due to the interaction with the surfactants is assumed.

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