Insights into Na+/Ca2+ Ion Exchange on Poly(4-styrenesulfonate) Anion in Bulk Solution and Adsorbed on TiO2 Nanoparticles

This study investigates Na+/Ca2+ ion exchange on poly(4-styrene sulfonate) anion (PSS) chains both in solution and PSS adsorbed onto TiO2 nanoparticles to gain deeper insights into the behavior of surface-bound polyelectrolytes and evaluate their potential as ion exchangers. Nanoparticle characterization was performed using electrokinetic light scattering (ELS) and a particle charge detector. PSS adsorption on TiO2 nanoparticles was evaluated by spectrophotometry and ELS. Subsequent ion exchange was investigated via isothermal titration calorimetry, ion-selective electrode potentiometry, and dynamic light scattering (DLS). The Na+/Ca2+ exchange on PSS chains was found to be an endothermic process, indicating that the favorable binding and rehydration effects are insufficient to compensate for the strongly unfavorable Ca2+ ion dehydration. In solution, the Na+/Ca2+ exchange on PSS is more pronounced than in the TiO2/PSS system. Conformational changes in the PSS polymer and surface modifications of the nanoparticles as a function of Ca2+ ion concentration were indirectly investigated using DLS. The calculated Na+/Ca2+ ion-exchange capacity was 2.99 mmol/g for PSS in solution and 0.26 mmol/g for the TiO2/PSS system. These findings suggest that adsorbing PSS onto TiO2 nanoparticles substantially decreases the ion-exchange capacity, probably due to the conformational constraints imposed by surface immobilization. However, potential practical implementation will require a comprehensive evaluation of the ion selectivity and regeneration efficiency of these PSS-based materials.