Behavior of fullerene C70 in binary organic solvent mixtures as studied using UV-Vis spectra and dynamic light scattering

Formation, Stability, and Coagulation of Fullerene Organosols: C70 in Acetonitrile-Toluene Solutions and Related Systems

This paper is aimed at better understanding the nature of C₇₀ aggregates in organic solvents. As liquid media, acetonitrile-toluene mixed solvents were chosen. At a high content of CH₃CN, e.g., 90 vol %, colloidal particles with a size of ca. 225 ± 10 nm are formed with a negative ζ-potential of -(55 ± 5) mV and are stable over time. The interaction with electrolytes containing single-, double-, and triple-charged cations was examined using dynamic light scattering and UV-visible spectra. Additional experiments were carried out with methanol and benzene instead of acetonitrile and toluene, respectively. For comparison, data were obtained with C₆₀ organosols. It was found that coagulation obeys the classical Schulze-Hardy rule. The specificity of the coagulating power of various single-charged cations was explained by their different abilities to adsorb on negatively charged C₇₀ aggregates. The overcharging effect is expressed not only for Ca²⁺ and La³⁺ ions but even for Li⁺ and is caused by poor solvation of such cations in a cationophobic solvent, acetonitrile. After introduction of the cryptand [2.2.2], a substantial increase in the critical concentrations of coagulation for Na⁺, Li⁺, and Ca²⁺ was observed owing to conversion of "bare" metal cations into their cryptates. The application of the Derjaguin-Landau-Verwey-Overbeek theory allowed for evaluation of the Hamaker constant of the C₇₀-C₇₀ interaction in vacuum, A_FF, which lies in the interval of 5.8-16.6 × 10^-20 J. Such an estimate, close to that made previously for C₆₀ organosols, was received after withdrawing electrolytic systems where the hetero- and mutual coagulation were highly likely. However, it is impossible to completely exclude the interfering influence of the latter phenomena. Based on the obtained A_FF values, two approaches to understanding the behavior of fullerenes in water were proposed.