Aggregation Kinetics of Higher-Order Fullerene Clusters in Aquatic Systems

The aggregation kinetics of nC60 and higher-order fullerene (HOF) clusters, i.e., nC70, nC76, and nC84, was systematically studied under a wide range of mono- (NaCl) and divalent (CaCl2) electrolytes and using time-resolved dynamic light scattering. Suwanee River Humic Acid (SRHA) was also used to determine the effect of natural macromolecules on nHOF aggregation. An increase in electrolyte concentration resulted in electrical double-layer compression of the negatively charged fullerene clusters, and the nC60s and nHOFs alike displayed classical Derjaguin-Landau-Verwey-Overbeek (DLVO) type interaction. The critical coagulation concentration (CCC) displayed a strong negative correlation with the carbon number in fullerenes and was estimated as 220, 150, 100, and 70 mM NaCl and 10, 12, 6, and 7.5 mM CaCl2 for nC60, nC70, nC76, and nC84, respectively. The aggregation mechanism (i.e., van der Waals interaction domination) was enumerated via molecular dynamics simulation and modified DLVO model. The presence of SRHA (2.5 mg TOC/L) profoundly influenced the aggregation behavior by stabilizing all fullerene clusters, even at a 100 mM NaCl concentration. The results from this study can be utilized to predict aggregation kinetics of nHOF clusters other than the ones studied here. The scaling factor for van der Waals interaction can also be used to model nHOF cluster interaction.