Formation and ageing of the fullerene C 60 colloids in polar organic solvents

Unexpected Colloidal Stability of Fullerenes in Dimethyl Sulfoxide and Related Systems

It is known that fullerenes are poorly soluble in polar solvents, but readily form colloidal solutions in such media. These solutions are typically solvophobic (hydrophobic when prepared in water), that is, thermodynamically unstable colloidal systems with negatively charged particles. To understand the stability factors of a colloidal system, the thresholds for coagulation of a sol or suspension by electrolytes are of key importance. While hydrosols and aqueous suspensions coagulate at concentrations of 1:1 inorganic electrolytes about 0.1-0.2 M, in acetonitrile and methanol, the corresponding critical concentrations of coagulation are ca. 3 orders of magnitude lower. Given the wide variety of properties of organic solvents, it seemed important to complete the picture to study solvents with more basic properties. This is all the more reasonable since electrophilic fullerenes are in fact Lewis acids. Our choice was dimethyl sulfoxide, DMSO, and related solvent systems. The colloidal solutions of fullerenes C₆₀ and C₇₀ in DMSO and N,N-dimethyl formamide, DMF, are unexpectedly easy to prepare by mechanical methods, and addition of water leads to formation of relatively stable organo-hydrosols. UV-visible spectra and dynamic light scattering were used to characterize the solutions of C₆₀ and C₇₀ in DMSO, benzene-DMSO, acetonitrile-DMSO, and benzene-acetonitrile-DMSO systems, as well as in DMF. Our present study demonstrated that, in contrast to organosols in methanol and acetonitrile, colloids of C₇₀ and C₆₀ fullerenes in DMSO and DMF are surprisingly as stable with respect to electrolytes as the corresponding hydrosols are. Such high stability is caused by the non-DLVO interactions, or, in terms proposed by Churaev and Derjaguin, by the so-called structural effect. These results shed light on the nature of the solvation of colloidal fullerene particles in solvents of various chemical natures.

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.