Aggregation Kinetics of Higher-Order Fullerene Clusters in Aquatic Systems

The aggregation kinetics of <i>n</i>C₆₀ and higher-order fullerene (HOF) clusters, i.e., <i>n</i>C₇₀, <i>n</i>C₇₆, and <i>n</i>C₈₄, 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 <i>n</i>HOF aggregation. An increase in electrolyte concentration resulted in electrical double-layer compression of the negatively charged fullerene clusters, and the <i>n</i>C₆₀s and <i>n</i>HOFs 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 <i>n</i>C₆₀, <i>n</i>C₇₀, <i>n</i>C₇₆, and <i>n</i>C₈₄, 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 <i>n</i>HOF clusters other than the ones studied here. The scaling factor for van der Waals interaction can also be used to model <i>n</i>HOF cluster interaction