Thermodynamic investigation of nanoplastic aggregation in aquatic environments.

In this study, the aggregation behavior of polystyrene nanoplastics (PS NPs) in the absence or presence of oppositely charged particulate matters is systematically investigated for a wide range of electrolyte conditions. Herein, we used isothermal titration calorimetry combined with time-resolved dynamic light scattering to provide kinetic and thermodynamic insights into the NP aggregation. The thermodynamic profiles of homoaggregation and heteroaggregation were fit using an independent site and two independent sites models, respectively, demonstrating different interaction modes of both aggregation processes. We found that the contribution of solvation entropy was significant and variable in most cases, and this thermodynamic parameter was a large determinant of the thermodynamics of NP aggregation. Furthermore, the stability of PS NPs in natural water matrices was found to be correlated with ionic strength and the content of natural colloids (e.g., metal oxides and clay particles). These results point to the importance of considering the role of thermodynamic variables when studying the fate of NPs within various environmental conditions.

Entropy-Driven Aggregation of Titanium Dioxide Nanoparticles in Aquatic Environments.

The aggregation process of engineered nanoparticles (ENPs) is important in assessing their fate and transport in the environment. Here, we present the application of isothermal titration calorimetry (ITC) in studying the thermodynamics of ENPs' aggregation in aqueous solutions containing monovalent (NaCl) and divalent (CaCl2) electrolytes, natural organic matter, and hematite natural NPs, which enables us to elucidate their interaction mechanism. The free energies for the aggregation of TiO2 at different solution conditions were dominated by large favorable entropy, presumably because of the expulsion of bound water molecules to the solution upon complexation. The copresence of humic acid and Ca2+ facilitated aggregation for both homo- and heterosystems through intra- or intermolecular bridging, leading to the formation of more compact aggregates. We believe that this ITC strategy can be successfully used to characterize the interaction details between ENPs and various environmental components in ambient water systems.