Competition Between Fibrinogen and a Nonionic Surfactant at Adsorption to a Wettability Gradient Surface

The competition between mixtures of fibrinogen and a non-ionic surfactant (C12E5) with respect to adsorption onto a wettability gradient solid surface was studied by the use of ellipsometry. The effects of surface hydrophobicity and surfactant association were investigated. Furthermore the effect of clouding of the surfactant was studied by performing measurements at temperatures above and below the cloud point. At all concentrations, the fibrinogen (0.02–0.40 mg ml−1) was preferentially adsorbed onto the hydrophilic part of the gradient surface. At surfactant concentrations above and around the CMC, the protein was inhibited from adsorbing by the surfactant at the hydrophobic as well as in the intermediate part (50° ⩽ contact angle ⩽ 80°) of the gradient. As the surfactant concentrations was further reduced the protein was able to compete and adsorb onto the whole or parts of the gradient surface. In the case of a surfactant concentration of two-fifths of the CMC, the competitive power of the surfactant increased with temperature and the surfactant could hinder protein adsorption over a larger interval of the gradient surface. These observations were also verified by in situ measurements on non-gradient surfaces. The competition can be explained by considering the main interactions between protein and surfactant with the surface. In this respect cooperation in the self-association of the surfactant seems to be of great importance. The use of gradient surfaces makes it possible to observe subtle changes in these interactions.