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

Abstract

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.

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

“…The decrease in colloidal particle size on approaching pure acetonitrile (Figure ) is in line with Volmer’s known rules: smaller particles appear in the solvent where the solubility of the compound is lower. ,, For much more concentrated fullerene solutions, however, some other regularities are valid …”

“…The data are typified in Figure . Our studies on C 70 and C 60 fullerenes demonstrated that the critical composition of the binary solvent, which corresponds to the appearance of colloidal aggregates, depends on the fullerene working concentration. , Therefore, some discrepancies are observed between the results by different authors. ,, In Figure , such a critical composition corresponds to 38 vol % toluene.…”

“…As these authors have not used the DLS method in the cited work, we made the corresponding measurements. Indeed, even at the C 70 concentration of 1.2 × 10 –7 M, only species of about 1 nm were found in 70% acetonitrile …”

“…Formation of fullerene aggregates in the toluene–acetonitrile binary solvent is well documented. ,,− Recently, we conducted some new experiments, which allowed us to make additional observations for this and related systems. The data are typified in Figure .…”

Colloid Chemistry of Fullerene Solutions: Aggregation and Coagulation

“…The same can be expected for polar ("poor") solvents, where the solvophobic nature of fullerene colloids is undisputable [23,24,[98][99][100][101]. In the recently named systems, it leads to formation of colloidal aggregates owing to solvophobic interactions.…”