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 degrees C). The association kinetics in CCl4, 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) similar to t(beta), where beta = 0.58 +/- 0.07, 0.45 +/- 0.05, 0.57 +/- 0.08, and 0.65 +/- 0.09 for CCl4, 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/beta = 1.72 for CCl4, 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, 4,(t) did not exhibit such universal behavior: for the CCl4 sample, I-s(t) similar to t(alpha) with alpha = 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 beta values obtained for clusters dispersed in CCl4 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.