Towards a better understanding on agglomeration mechanisms and thermodynamic properties of TiO2 nanoparticles interacting with natural organic matter

TL;DR

This study investigates the thermodynamic mechanisms of TiO2 nanoparticle interactions with natural organic matter, revealing entropy-driven binding and proposing agglomeration mechanisms based on mixing order.

Contribution

It provides new insights into the thermodynamic properties and agglomeration mechanisms of TiO2 nanoparticles interacting with natural organic matter, using calorimetry and electrophoretic measurements.

Findings

TiO2-alginate interactions are entropy driven and exothermic.

Agglomeration mechanisms depend on mixing order: polymer bridging vs. individual coating.

Binding enthalpy correlates with zeta potential values.

Abstract

Interaction between engineered nanoparticles and natural organic matter is investigated by measuring the exchanged heat during binding process with isothermal titration calorimetry. TiO2 anatase nanoparticles and alginate are used as engineered nanoparticles and natural organic matter to get an insight into the thermodynamic association properties and mechanisms of adsorption and agglomeration. Changes of enthalpy, entropy and total free energy, reaction stoichiometry and affinity binding constant are determined or calculated at a pH value where the TiO2 nanoparticles surface charge is positive and the alginate exhibits a negative structural charge. Our results indicate that strong TiO2-alginate interactions are essentially entropy driven and enthalpically favorable with exothermic binding reactions. The reaction stoichiometry and entropy gain are also found dependent on the mixing order. Finally correlation is established between the binding enthalpy, the reaction stoichiometry and the zeta potential values determined by electrophoretic mobility measurements. From these results two types of agglomeration mechanisms are proposed depending on the mixing order. Addition of alginate in TiO2 dispersions is found to form agglomerates due to polymer bridging whereas addition of TiO2 in alginate promotes a more individually coating of the nanoparticles.