The surface force technique was employed to investigate the adsorption
of positively charged lysozyme onto negatively charged mica surfaces
in 10(-3)M NaCl at pH 5.6 at lysozyme concentrations ranging from 0.00
2 to 0.2 mg/mL. At equilibrium the adsorbed lysozyme nearly neutralize
s the surface charge of the mica at all bulk lysozyme concentrations i
nvestigated. Prior to charge neutralization the decay length of the lo
ng-range force is consistent with the electrostatic double-layer force
predicted by the DLVO theory. At low concentration, 0.002 mg/mL, a de
nsely packed side-on oriented layer adsorbs on the mica surface. Above
0.02 mg of lysozyme/mL, a rather thick layer is adsorbed onto the sur
face. It consists of an inner, strongly bound layer of both side-on an
d end-on adsorbed proteins and an outer layer of weakly adsorbed prote
ins. An adhesion force is established upon contact of the adsorbed pro
tein layers. The force measured between one lysozyme-coated surface an
d one bare mica surface is attractive at short separations. It was dem
onstrated that, at a concentration of 0.02 mg/mL, lysozyme adsorbs ''i
rreversibly'' with respect to dilution with 10(-3) M NaCl.