Adsorption characteristics of mixed monolayers of a globular protein and a non-ionic surfactant

Experimental and theoretic studies of the adsorption behaviour for the mixture of globular protein (human serum albumin (HSA)), and non-ionic surfactant (decyl-dimethyl-phosphine-oxide C10DMPO) are performed. The experimental results for the mixtures agree well with a theoretical model which assumes significant differences between the partial molar areas of the protein and the surfactant, and takes into account large unbound charge of the protein molecules. An anomalous surface tension increase of the mixtures at low surfactant concentrations was found experimentally and explained on the basis of a thermodynamic model. The concentration range at which a comparable coverage of the mixed surface layer by protein and surfactant molecules appears is shown to be quite narrow.