Isothermal titration calorimetry as a powerful tool to quantify and better understand agglomeration mechanisms during interaction processes between TiO2 nanoparticles and humic acids

TL;DR

This study uses isothermal titration calorimetry to analyze the thermodynamics and mechanisms of TiO2 nanoparticle interactions with humic acids, revealing enthalpically and entropically driven agglomeration processes.

Contribution

It demonstrates the effectiveness of calorimetry in quantifying interaction parameters and elucidates mechanisms like charge inversion and van der Waals forces in nanoparticle-humic acid agglomeration.

Findings

Strong TiO2-humic acid interactions are thermodynamically favorable.

High humic acid concentrations promote homoagglomeration.

Charge inversion leads to large agglomerates via patch and bridging mechanisms.

Abstract

The association processes between engineered TiO2 nanoparticles and Suwannee River humic acids are investigated by isothermal titration calorimetry and by measuring the exchanged heat during binding process allowing the determination of thermodynamic (change of enthalpy, Gibbs free energy and entropy) and reaction (binding affinity constant, reaction stoichiometry) parameters. Our results indicate that strong TiO2-Suwannee River humic acid interactions are entropically and enthalpically favorable with exothermic binding reactions and that the mixing order is also an important parameter. High humic acid concentrations induce homoagglomeration ("self" assembly) and are shown to favor an enthalpically driven association process. Light scattering techniques are also considered to investigate the influence of TiO2 surface charge modifications and agglomeration mechanisms. Patch and bridging mechanisms are found to result into the formation of large agglomerates once charge inversion of TiO2-humic acid complexes is achieved. Moreover van der Waals interactions are also found to play a significant role during interaction processes due to the amphiphilic character of humic acids.