Interactions of Nanosized Aggregates of Fullerene C60 with Electrolytes in Methanol: Coagulation and Overcharging of Particles.

Contrary to numerous studies on the stability of fullerene aqueous colloidal solutions in the presence of electrolytes, the corresponding issue for the organosols was until recently almost unexplored. In this article, the state of C60 in methyl alcohol and the regularities of the coagulation of colloidal solution in this solvent were examined in the presence of electrolytes. Alcosols with a fullerene concentration of 4 × 10-6 M were prepared by the dilution of the C60 saturated solution in toluene by methanol. The ca. 300 nm-sized aggregates possess a negative electrokinetic potential value, ζ = -37 ± 8 mV. To determine the critical coagulation concentrations, CCC, the size increase of the species was followed up using the dynamic light scattering method. The analysis of the coagulation in terms of the Fuchs function, W, was accompanied by zeta potential monitoring. The consideration of the data for 1:1 electrolytes NaClO4 and N(n-C4H9)4ClO4 allows a rough estimate of the Hamaker constant of fullerene-fullerene attraction. Whereas in the case of these two electrolytes the colloidal species are negatively charged at the CCC, expressed overcharging of up to ζ = +36 mV by H+, Ca2+, Ba2+, and La3+ ions was observed. The action of HClO4 should be attributed to the interfacial acid-base reaction, whereas the excessive attraction of metal cations is caused by poor solvation in methanol; the negative charge is restored when the metal cations are shielded by a macrocyclic ligand.

Towards better understanding of C60 organosols.

It is of common knowledge that fullerenes form colloids in polar solvents. However, the coagulation via electrolytes and the origin of the negative charge of species are still unexplored. Using a 'radical scavenger' and electrospray ionization spectroscopy (ESI), we proved the formation of ion-radical C60˙(-) and its (probable) transformation into C60(2-) or (C60)2(2-). The coagulation of C60 organosols by NaClO4 and other perchlorates and nitrates in acetonitrile and its mixture with benzene obeys the Schulze-Hardy rule. At higher Ca(ClO4)2 and La(ClO4)3 concentrations, instead of coagulation, stable re-charged colloidal particles appeared, up to a zeta-potential of +(20-42) mV, as compared with -(33-35) mV of the initial organosols. The influence of both HClO4 and CF3SO3H was similar. This phenomenon is attributed to poor solvation of inorganic cations in cationo- and protophobic acetonitrile, which was proven using [2.2.2] cryptand. Further increasing the concentration of Ca(ClO4)2 led again to coagulation, thus demonstrating a novel type of 'coagulation zones'.