We investigated the binding of different ions to the charged surface of TiO₂ nanoparticles. Using isothermal titration calorimetry (ITC), we measured selected thermodynamic parameters of the binding of ions from different salts to charged nanoparticle surfaces and the aggregation of nanoparticles at different pH values. The salts NaCl, KCl, and NaBr were investigated. For NaCl, pH values of 2; 4,80; and 10 were studied, while KCl was investigated at pH 10 and NaBr at a pH of 4,86. The largest thermal effect was measured for NaBr, followed by KCl, NaCl at pH 4,80, NaCl at pH 2, and finally NaCl at pH 10. The integrated ITC signal was additionally analyzed using model functions to determine the thermodynamic parameters ∆H⊖, ∆S⊖, ∆G⊖, K_b and \tau.
The experimental results were further investigated using constant-pH Monte Carlo simulations within a coarse-grained model in the NVT ensemble. When comparing the NaCl and KCl systems, the simulation results qualitatively agreed with the trends observed in the ITC measurements, whereas for the NaBr system, the simulations could not reproduce the experimental measurements. In comparisons among the NaCl systems, however, the simulation results were inconsistent with the experimental measurements. Several possible interpretations of these findings were proposed, primarily based on the hydration and dehydration of ions and nanoparticle surfaces, electrostatic screening and changes in the effective nanoparticle charge, as well as limitations of the applied model. We conclude that the selected model is primarily suitable for comparing ions whose hydrated-ion radii differ sufficiently and for systems in which the pH is as far as possible from the point of zero charge of the nanoparticles.
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