Anomalous Self-Aggregation of Carbon Nanoparticles in Polar, Nonpolar, and Binary Solvents

Polydisperse preparations of multicarbon nanoparticles were observed to undergo anomalous time-dependent self-association leading to the formation of nanoclusters in carbon tetrachloride (nonpolar and hydrophobic solvent, high solubility), water (polar and nonhydrophobic solvent, low solubility), and ethanol-water binary solvents (polarity and hydrophobicity dependent on ethanol concentration, marginal solubility) at room temperature (20 °C). The association kinetics in CCl 4 , water, and binary solvents could be described through the Smoluchowski aggregation model where the average cluster diameter d av was found to scale with time, t as d av ∼ t β , where β = 0.58 ± 0.07, 0.45 ± 0.05, 0.57 ± 0.08, and 0.65 ± 0.09 for CCl 4 , water, and 45% (v/v) and 70% (v/v) ethanol binary solvents, respectively. The corresponding mass fractal dimensions, d f of these clusters were d f = 1/β = 1.72 for CCl 4 , 2.22-2.95 for water (depending on salt concentration), and 1.75 and 1.54 for 45% (v/v) and 70% (v/v) ethanol binary solvents, respectively. The intensity of scattered light, I s (t) did not exhibit such universal behavior: for the CCl 4 sample, Is(t) ∼ t α with a = 0.65, whereas, for all other solvents, a linear time-dependent increase was observed mimicking a Derjaguin-Landau-Verwey-Overbeek (DLVO) type growth mechanism. The β values obtained for clusters dispersed in CCl 4 and the binary solvents implied cluster-cluster aggregation while the same in water behaved like diffusion limited aggregation (DLA) clusters. The physical morphology of these clusters was observed through high-resolution transmission electron microscopy (HRTEM) studies. UV spectroscopy data showed characteristic solute-solvent interactions. The electrophoretic mobility studies in the binary solvents indicated interesting polyampholyte-type behavior of these clusters. The results taken together indicate the specific role played by hydrophobicity in solubilization of carbonaceous nanoparticles.