EVALUATION OF THE ADSORPTION TRENDS OF A LOW-MOLECULAR-WEIGHT POLYELECTROLYTE WITH A SITE-BINDING MODEL

The site-binding model is very useful for describing the adsorption of ions and small ionized molecules. It has been slightly modified to in clude multi-site adsorption of larger molecules such as oligomers and low molecular weight polyelectrolytes. We describe alterations of the classical model and the results of calculations for adsorption of poly acrylic acid onto titanium dioxide as an example. The triple layer mod el is used to relate charge densities to interfacial potential profile s. Comparison between adsorption trends and the surface layer composit ion as a function of pH and ionic strength demonstrates the prominent influence of ions binding in the adsorption process. The site-binding model makes it easy to simulate the ions displacement associated with polyelectrolyte adsorption. Strongly bound electrolyte anions prevent polyacrylic acid from adsorbing, and, in contrast, electrostatic scree ning due to cation condensation makes it easier. Calculations of the p H change in the solution, due to adsorption, are also made by comparin g ionization ratios of both the surface and polymer units in the adsor bed layer and before adsorption. Trends in electrokinetic potentials a s a function of the solution's parameters are evaluated assuming the i dentity of the shearing surface and the inner boundary of the diffuse layer. All data compare well with experimental values. The very good a greement betwen the experiment and model calculations supports the fac t that (small) polyelectrolyte molecules adsorb essentially flat on th e surface.