Relating colloidal stability of fullerene (C60) nanoparticles to nanoparticle charge and electrokinetic properties.

The stability and aggregation kinetics of two different suspensions of fullerene (C60) nanoparticles and their relation to nanoparticle charge (electrokinetic) properties were investigated. The two synthesis methods employed--a solvent exchange method involving sonication of fullerene initially dissolved in toluene and prolonged stirring of bulk fullerene in water--produce negatively charged fullerene nanoparticles. With an increase in electrolyte (KCl) concentration, the electrophoretic mobilities of both fullerene nanoparticles became less negative, while the corresponding aggregation rates increased until maximum rates were reached at their respective critical coagulation concentrations. This behavior is consistent with the classic Derjaguin-Landau-Verwey-Overbeek (DLVO) theory for the stability of charged colloidal particles. The nanoparticles prepared by prolonged stirring of bulk fullerene in water were much more stable than those prepared by sonication in toluene, as evident from their significantly higher critical coagulation concentration (166 and 40 mM KCl, respectively). A comparison of the aggregation kinetics with predictions based on DLVO theory yielded the same Hamaker constant (8.5 x 10(-21) J) for both fullerene nanoparticles, indicating that they have the same material composition. Further investigation shows that both fullerene nanoparticles are more negatively charged and stable at higher pH conditions, suggesting that dissociation of surface functional groups contributes to surface charge for both nanoparticles. This hypothesis is further supported by oxidation which occurs on the surface of bulk fullerene that has been exposed to water over a prolonged period of time, as detected through X-ray photoelectron spectroscopy (XPS). However, since both nanoparticles remain negatively charged at pH 2, it is likely that there are other contributing factors to the surface charge of fullerene nanoparticles.