In situ observation of the surfactant-induced displacement of protein froma graphite surface by atomic force microscopy

Atomic force microscopy has been used to visualize, in real time, the break down and removal of a beta-lactoglobulin film from a graphite surface, by t he addition of the nonionic surfactant Tween 20 to the liquid cell of the m icroscope. The initial stage of surfactant adsorption onto the graphite sur face is seen to involve the nucleation of small surfactant domains within t he protein network. These surfactant domains expand, compressing the protei n network. The reduction in surface area covered by protein is compensated by an increase in the thickness of the protein film. Eventually, at suffici ently high surface concentration of surfactant, the protein network fractur es allowing release of protein for displacement from the surface. The displ acement mechanism observed at the graphite surface has been compared with t he displacement by Tween 20 of a spread beta-lactoglobulin protein layer fr om an air-water interface. In both cases a similar "orogenic" mechanism of displacement has been observed. The present studies provide a molecular mod el for the cleaning of protein films from surfaces.