The peculiar behavior of fullerene C60 in mixtures of ‘good’ and polar solvents: Colloidal particles in the toluene–methanol mixtures and some other systems

https://doi.org/10.1016/j.colsurfa.2016.09.045Get rights and content

Highlights

  • Fullerene C60 readily forms colloidal particles in toluene–methanol mixtures.

  • The size of the species depends on the C60 concentration and solvent composition.

  • The species are negatively charged.

  • At low methanol content, single molecules coexist with aggregates.

Abstract

The state of C60 in toluene–methanol mixtures and some other solvents was studied within the range of fullerene concentrations from 4 × 10−7 to 4 × 10−5 M. The variations in the particle size from ca. 200–500 to ca. 1 nm, as followed up using the dynamic light scattering (DLS) method, were compared with the UV–vis absorption spectra. The threshold composition of the mixed solvent, which corresponds to the disappearance of single C60 molecules, depends on the fullerene concentration. In toluene–methanol systems, small amounts of large aggregates were detected via the DLS even under conditions of substantial predominance of single molecules. The examination of the fullerene in several other binary organic solvents reveals the specific features of the formation and behavior of the negatively charged colloidal particles.

Introduction

The numerous modern applications of fullerenes are based on the preparation of their solutions [1], [2], [3], [4]. Though the best (‘good’, or ‘strong’) solvents for the fullerenes are, first of all, non-polar aromatics and CS2 [5], [6], the polar (‘weak’, or ‘poor’) solvents or their mixtures with the good ones are preferable for some purposes [3], [6], [7], [8], [9], [10], for instance, owing to their higher electrical permittivity [3]. There are a number of procedures for preparation of C60 colloidal solutions in water, based on the dilution of previously prepared solutions in polar organic solvents; the corresponding papers have been regarded in detail in a review [6].

It was well documented already in the 1990th that, in polar organic solvents and polar–non-polar mixtures, fullerenes readily form colloidal aggregates with the size of hundreds of nanometers [11], [12], [13]. Further studies disclosed some regularities of fullerene aggregation in mixed solvents [14], [15] including the specific features of the toluene–N-methyl-pyrrolidine-2-one systems [16], [17], [18], [19].

Though such colloids may be obtained through sonication [8], [9] and sometimes after mechanical grinding of the solid fullerene before adding the solvent [20], the most popular procedure to prepare such systems is mixing of the (relatively concentrated) initial solution in a good, or strong solvent, such as toluene, benzene, or CS2 with larger volume of polar solvent [2], [7], [10], [11], [12], [14], [15], [16], [17], [18], [19], [21]. As polar solvents, acetonitrile [2], [10], [11], [12], [21], acetone [21], DMSO [10], dimethylformamide [22], and alcohols [4], [7], [8], [9], [10], [20], [21], [23] were used. Special case are some of the nitrogen-containing solvents [6], [16], [17], [18], [19], where even their covalent interactions with fullerenes are probable.

If the aforesaid mixtures contain commensurable fractions of good and polar solvents, the appearance of aggregates depends on the solvent composition [6], [10], [11], [12], [14], [15], [16], [17], [18], [19] and to some degree on the fullerene concentration [6], [11], [14], [15]. The threshold composition of the mixed solvent is reported to be rather sharp; some authors explain it in terms of critical relative permittivity value [14], [15]. For the buckminsterfullerene, C60, this value was found to be εr = 13, as result of examining the mixtures of benzonitrile with benzene, dichloromethane, and decalin, as well as benzene–acetonitrile and decalin–acetone mixed solvents [14].

The solutions of fullerenes in pure benzene, toluene, etc., are usually considered as molecular ones. Some earlier reports of aggregation in good solvents were ascribed rather to non-equilibrium methods of preparation, e.g., those using ultrasound or intensive stirring [18]. Some new papers again communicate either aggregation [24], [25] or rather unique organization of solutions [26] in entire good solvents, using a number of modern experimental methods.

The previous reports on mixed good + polar solvents are based mainly on the UV–vis spectra or light scattering data. A consideration of the publications until 2013 may be found in a review paper [6].

The present report is devoted mainly to a system, which was not studied in detail so far, namely, to C60 in the toluene–methanol mixed solvents. These and some other solutions were examined simultaneously by the dynamic light scattering (DLS) and UV–vis spectroscopy, in order to reveal the peculiarities of aggregate formation. The data confirming the formation of fullerene alcosols and suspensions in methanol [9], [10], ethanol [7], [8], [21], and other alcohols [20], [23] are available in the literature. Methanol is a typical poor solvent for fullerenes. According to the literature data [6], the solubility of C60 in this alcohol is (3.3–4.8) × 10−8 M (hereafter, 1 M = 1 mol dm−3).

Section snippets

Materials and methods

The C60 sample (Acros Organics, 99.9%) was used as received. Toluene, benzene, methyl alcohol, acetone, and acetonitrile were purified and dehydrated via standard procedures. The UV/vis absorption spectra were measured with the Hitachi U-2000 spectrophotometer against the solvent blanks. The particle size distribution was determined via DLS using Zetasizer Nano ZS Malvern Instruments, scattering angle 173°. Each measurement was repeated five times. In the figures given below, the averaged data

UV–vis molar absorptivities and the solubility of C60 in the aromatic solvents

The working solutions of C60 in toluene were prepared by dilution of either saturated solutions or solutions with concentrations ca. 3–10 times lower than the solubility limit. For determination of the molar absorptivities of C60 in toluene (Table 1), the solutions were carefully prepared by mixing weighed amounts of C60 and toluene, storing for three days and then measuring absorbance (A) against the solvent blank. For the UV and visible portions of the spectrum, different concentrations of

Conclusions

The fullerene C60 readily forms colloidal solutions in methyl alcohol with more or less content of toluene. The application of the dynamic light scattering method allows revealing that at some threshold composition of the mixed solvent, the single molecules disappear and large colloidal particles, up to ca. 500 nm, are formed. This critical composition depends on C60 concentration; so, for 4 × 10−7 to 4 × 10−6 M, it corresponds to 67.5 vol% methanol (xCH3OH = 0.84), whereas for 4 × 10−5 M C60, already at

Acknowledgements

The authors express their gratitude to Elena A. Korosteleva, Pharmstandart-Biolik, Kharkov, Ukraine, for the SLS measurements. The research was partly supported by the Ministry of Education and Science of Ukraine via the grant number 0116U000834.

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