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

In this paper, the formation of colloidal species of fullerene C70 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 C70 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 oC, the absorption spectra of C70 (5×10–6 M) tend to retain the features of the spectrum in neat aromatic solvent even if the C70 molecules are gathered into colloidal aggregates. Earlier such phenomenon was observed for C60 in benzene–acetonitrile 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 C70 in toluene mixtures with methanol was compared with the earlier reported results from this laboratory for the C60 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 oC. Accordingly, the C70–toluene–methanol system was also examined at 40 oC. 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 C70 is still observable when colloidal species are already present in the solution, the turning-point between molecules and colloids as determined by both UV-visible spectra and DLS coincides for the n-hexane–methanol binary mixed solvent. Hence, the solvation shells formed by the aliphatic solvent around C70 are less stable as compared with those formed by toluene. Finally, the absorption spectra of C70 in the mixed solvents toluene–n-hexane were analyzed. These data give some support to the assumption of preferable solvation of the C70 molecules by the aromatic co-solvent.