Interfacial Properties of a Model Polyampholyte Studied by Surface Force Measurements, ESCA, and Ellipsometry

Abstract

The adsorption of a model polyampholyte, i.e., [acrylamide]−[sodium 2-(acrylamido)-2-methylpropanesulfonate]−[2-(methacryloyloxy)ethyltrimethylammonium chloride] ([AM]−[NaAMPS]−[MADQUAT]) at mica and silica surfaces was investigated by surface force measurements, electron spectroscopy for chemical analysis (ESCA), and ellipsometry. It was found that the adsorption isotherm of this polyampholyte on mica was of the high affinity type, yielding an adsorbed amount at saturation of 5.5 ± 0.5 mg/m2. By lowering the electron escape angle in the ESCA measurements (making the analysis more surface sensitive), it was found that the negatively charged groups in the polyampholyte are concentrated in the outer part of the adsorbed layer, i.e., away from the negatively charged mica surface. From ellipsometry studies with silica, it was found that the adsorption was relatively fast, reaching saturation after about 1000 s on adsorption from a 20 ppm solution. From both ellipsometry and ESCA it was found that the effect of electrolyte on the adsorbed amount of preadsorbed polyampholyte is relatively minor at concentrations up to 10 mM. At higher electrolyte concentrations, however, some desorption of the polyampholyte occurs. Surface force measurements showed that the extension of the polymer layer normal to the mica surface was small compared to the radius of gyration of the polymer in bulk. Upon addition of electrolyte the adsorbed layer was found to contract.