Polyelectrolyte adsorption layers studied by streaming potential and particle deposition

Adsorption of a cationic polyelectrolyte, polyallylamine hydrochloride (PAH), having a molecular weight of 70,000 on mica was characterized by the streaming potential method and by deposition of negative polystyrene latex particles. Formation of PAH layers was followed by determining the apparent zeta potential of surface ζ   as function of bulk PAH concentration. The zeta potential was calculated from the streaming potential measured in the parallel-plate channel formed by two mica plates precovered by the polyelectrolyte. The experimental data were expressed as the dependence of the reduced zeta potential ζ/ζ0ζ/ζ0 on the PAH coverage ΘPAHΘPAH, calculated using the convective diffusion theory. It was found that for the ionic strength of 10−2 M, the dependence of ζ/ζ0ζ/ζ0 on ΘPAHΘPAH can be reflected by the theoretical model formulated previously for surfaces covered by colloid particles. The electrokinetic measurements were complemented by particle deposition experiments on PAH-covered mica surfaces. A direct correlation between the polymer coverage and the initial deposition rate of particles, as well as the jamming coverage, was found. For ΘPAH>0.3ΘPAH>0.3 the initial deposition rate attained the value predicted from the convective diffusion theory for homogeneous surfaces. The initial deposition rates for surfaces modified by PAH were compared with previous experimental and theoretical results obtained for heterogeneous surfaces formed by preadsorption of colloid particles. It was revealed that negative latex deposition occurred at surfaces exhibiting negative apparent zeta potential, which explained the anomalous deposition of particles observed in previous works. It was suggested that the combined electrokinetic and particle deposition methods can be used for detecting adsorbed polyelectrolytes at surfaces for coverage range of a percent. This enables one to measure bulk polyelectrolyte concentrations at the level of 0.05 ppm.

Surfaces modified by adsorbed polyelectrolyte layers can be characterized by streaming potential and particle deposition studies. The results obtained can be interpreted in terms of theoretical model developed for particle covered surfaces. It was shown that these methods are sensitive to the very low polyelectrolyte coverage not attainable by other techniques.Figure optionsDownload as PowerPoint slide