Interfacial properties of a model polyampholyte studied by surface force measurements, ESCA, and ellipsometry

The adsorption of a model polyampholyte, i.e., [acrylamide]-[sodium 2-(acry lamido)-2-methylpropanesulfonate]-[2-(methacryloyloxy)ethyltrimethylammoniu m chloride] ([AM]-[NaAMPS]-[MADQUAT]) at mica and silica surfaces was inves tigated by surface force measurements, electron spectroscopy for chemical a nalysis (ESCA), and ellipsometry. It was found that the adsorption isotherm of this polyampholyte on mica was of the high affinity type, yielding an a dsorbed amount at saturation of 5.5 +/- 0.5 mg/m(2). By lowering the electr on escape angle in the ESCA measurements (making the analysis more surface sensitive), it was found that the negatively charged groups in the polyamph olyte are concentrated in the outer part of the adsorbed layer, i.e., away from the negatively charged mica surface. From ellipsometry studies with si lica, it was found that the adsorption was relatively fast, reaching satura tion after about 1000 s on adsorption from a 20 ppm solution. From both ell ipsometry and ESCA it was found that the effect of electrolyte on the adsor bed amount of preadsorbed polyampholyte is relatively minor at concentratio ns up to 10 mM. At higher electrolyte concentrations, however, some desorpt ion of the polyampholyte occurs. Surface force measurements showed that the extension of the polymer layer normal to the mica surface was small compar ed to the radius of gyration of the polymer in bulk. Upon addition of elect rolyte the adsorbed layer was found to contract.