Evaluation of the adsorption trends of a low molecular-weight polyelectrolyte with a site-binding model

Abstract

The site-binding model is very useful for describing the adsorption of ions and small ionized molecules. It has been slightly modified to include 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 polyacrylic acid onto titanium dioxide as an example. The triple layer model is used to relate charge densities to interfacial potential profiles. Comparison between adsorption trends and the surface layer composition 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 screening due to cation condensation makes it easier. Calculations of the pH change in the solution, due to adsorption, are also made by comparing ionization ratios of both the surface and polymer units in the adsorbed layer and before adsorption. Trends in electrokinetic potentials as a function of the solution's parameters are evaluated assuming the identity of the shearing surface and the inner boundary of the diffuse layer. All data compare well with experimental values. The very good agreement betwen the experiment and model calculations supports the fact that (small) polyelectrolyte molecules adsorb essentially flat on the surface.