Surface charge regulation upon polyelectrolyte adsorption, hematite, polystyrene sulfonate, surface charge regulation: Theoretical calculations and hematite-poly(styrene sulfonate) system

The charge regulation of a mineral surface upon adsorption of a strong polyelectrolyte is studied theoretically and experimentally. Self-consistent-field calculations were done to evaluate the charge characteristics of a model oxide surface in the absence and presence of a linear strong polyelectrolyte. The calculations show high affinity adsorption isotherms for the polyelectrolyte at different pH. At the adsorption plateau the surface charge is overcompensated by the charge of the adsorbed polyelectrolyte. The increase in surface charge upon polyelectrolyte adsorption is substantial. For a bare surface a similar adjustment would require a pH change by about three units. At a given pH and salt concentration the proton co-adsorption ratio increases with the adsorbed polyelectrolyte charge till the charge compensation point is reached and then it decreases again.The measured adsorption isotherms of linear poly(styrene sulfonate) (PSS) on hematite do not show a high affinity character. This might be due to fast flocculation and a non-equilibrium polymer conformation at the surface. At pH 7 the adsorption plateau is reached and the surface charge is overcompensated by the adsorbed polyelectrolyte charge. The adsorption of PSS increases the surface charge, similarly as in the calculations. The increase of the surface charge with increasing PSS adsorption is about linear up to the charge compensation point and decreases beyond the charge compensation point.