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.

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 C 60 and C 70 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 C 60 and C 70 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 C 70 and C 60 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.

In this paper, the formation of colloidal species of fullerene C 70 in organic solvents was studied. The examining of the UV-visible spectra was accompanied by particle size analysis using dynamic light scattering, DLS. Stock solutions of C 70 in non-polar toluene and n-hexane were diluted with polar solvents acetonitrile and methanol. The appearance of colloidal species with a size within the range of ≈ 50-500 nm is accompanied by alterations of the absorption spectra. In the toluene-acetonitrile and toluene-methanol binary mixed solvents at 25 o C, the absorption spectra of C 70 (5×10-6 M) tend to retain the features of the spectrum in neat aromatic solvent even if the C 70 molecules are gathered into colloidal aggregates. Earlier such phenomenon was observed for C 60 in benzeneacetonitrile and toluene-methanol solvent systems. This gives support to the idea of rather stable primary solvate shells formed by aromatic molecules around the fullerene molecules. The behavior of C 70 in toluene mixtures with methanol was compared with the earlier reported results from this laboratory for the C 60 fullerene in the same solvent system. The study of n-hexane-methanol mixtures was performed at elevated temperature because of limited miscibility of these solvents at 25 o C. Accordingly, the C 70-toluene-methanol system was also examined at 40 o C. A small but distinctly noticeable difference was revealed. Whereas in the case of the last-named system, the absorption spectrum typical for molecular form of C 70 is still observable when colloidal species are already present in the solution, the turning-point between molecules and colloids as determined by both UVvisible spectra and DLS coincides for the n-hexane-methanol binary mixed solvent. Hence, the solvation shells formed by the aliphatic solvent around C 70 are less stable as compared with those formed by toluene. Finally, the absorption spectra of C 70 in the mixed solvents toluene-n-hexane were analyzed. These data give some support to the assumption of preferable solvation of the C 70 molecules by the aromatic co-solvent.

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This article is devoted to the synthesis and characterization of the hydrosol of C70 of the son/nC70 type and to its coagulation by sodium chloride and cetyltrimethylammonium bromide (CTAB). At C70 concentration of 3.3×10–6 M, the electrokinetic potential is ζ= –40 ± 4 mV, the particle size expressed as Zeta-average is 97±3 nm; at higher C70 concentrations, 1.7×10–5 and 6.9×10–5 M, the size stays the same: 99 – 100 nm. The critical concentration of coagulation (CCC) values, were determined using the diameter increasing rate (DIR) on NaCl concentration. The CCCs are concentration-dependent: 250, 145, and 130 mM at C70 concentrations 3.3×10–6, 1.7×10–5, and 6.9×10–5 M, respectively. The CCC for the CTAB surfactant is much lower, about 5×10–3 mM. At 0.02 mM CTAB, however, the overcharging up to ζ = + 40 mV and stabilization of the colloidal particles take place. Interpretation of the hydrosol coagulation by NaCl using the Derjaguin–Landau–Verwey–Overbeek theory makes it possible to determine the Hamaker constant of the C70–C70 interaction in vacuum, if only electrostatic repulsion and molecular attraction are taking into account: AFF ≈ 7×10–20 J. On the other hand, if we use the value AFF = (16.0–16.6)×10–20 J, obtained earlier in the study of organosols, then the data for hydrosols can be explained only by the introduction of an additional type of interactions. Following the terms of Churaev and Derjaguin, one should take into account the structural contribution to the interaction energy, which stabilizes the hydrosol.