Henri Szwarc

Toxicity Studies of Carbon Nanotubes

As for fullerenes, the potential and the growing use of CNT and their mass production have raised several questions about their safety and environmental impact. Research on the toxicity of carbon nanotubes has just begun and the data are still fragmentary and subject to criticisms. Preliminary results highlight the difficulties in evaluating the toxicity of this new and heterogeneous carbon nanoparticle family. A number of parameters including structure, size distribution and surface area, surface chemistry and surface charge, and agglomeration state as well as purity of the samples, have considerable impact on the reactivity of carbon nanotubes. However, available data clearly show that, under some conditions, nanotubes can cross the membrane barriers and suggests that if raw materials reach the organs they can induce harmful effects as inflammatory and fibrotic reactions. Therefore, many further studies on well-characterized materials are necessary to determine the safety of carbon nanotubes as well as their environmental impact.

The prolongation of the lifespan of rats by repeated oral administration of [60]fullerene.

Countless studies showed that [60]fullerene (C(60)) and derivatives could have many potential biomedical applications. However, while several independent research groups showed that C(60) has no acute or sub-acute toxicity in various experimental models, more than 25 years after its discovery the in vivo fate and the chronic effects of this fullerene remain unknown. If the potential of C(60) and derivatives in the biomedical field have to be fulfilled these issues must be addressed. Here we show that oral administration of C(60) dissolved in olive oil (0.8 mg/ml) at reiterated doses (1.7 mg/kg of body weight) to rats not only does not entail chronic toxicity but it almost doubles their lifespan. The effects of C(60)-olive oil solutions in an experimental model of CCl(4) intoxication in rat strongly suggest that the effect on lifespan is mainly due to the attenuation of age-associated increases in oxidative stress. Pharmacokinetic studies show that dissolved C(60) is absorbed by the gastro-intestinal tract and eliminated in a few tens of hours. These results of importance in the fields of medicine and toxicology should open the way for the many possible -and waited for- biomedical applications of C(60) including cancer therapy, neurodegenerative disorders, and ageing.

'Low-pressure' orthorhombic phase formed from pressure-treated C60

Abstract X-ray and electron diffraction and Raman spectroscopy have shown that the solid formed by a pressure-temperature treatment of C 60 at 1.5 GPa-723 K has an orthorhombic structure O′. It may be considered that phases O′ and O, the previously known orthorhombic phase formed at 8 GPa-573 K, are the same so that our pressure-temperature treatment provides “the optimal way of obtaining the linear-chain orthorhombic fullerene phase”. However, differences in the respective spectra of these two phases led us to re-examine the structure of the previously described phase O: it is found that a rhombohedral structure fits the experimental X-ray data at least as well as an orthorhombic one does. In any case, phase O′ may be an intermediary for the formation of the tetragonal high-pressure modification.

A new hexagonal phase of fullerene C60

Abstract A new phase of fullerene C 60 with a simple hexagonal unit cell with 6/mmm symmetry was grown by slowly evaporating solutions of C 60 in dichloromethane. X-ray measurements reveal that the c / a ratio is 1.616 at 298 K and increases as temperature decreases. At low temperature, it seems to extrapolate to 1.633, the ideal ratio for a close-packed hexagonal lattice. The unit cell volume which is higher than that of the usual cubic C 60 phases at high temperature decreases near 90 K. A structural model is proposed according to which a 3-fold molecular axis is parallel to the 6-fold crystallographic z -axis. This suggests that the molecules could reorient around this axis at high temperature.

Fullerene C60, 2CCl4 solvate. A solid-state study

By slowly evaporating solutions of fullerene C60 in CCl4 at room temperature, a stable solvate (C60, 2CCl4) crystallizes in a simple hexagonal system (Laue class 6/mmm) with a=10.10(5) A and c = 10.75(5) A. This solvate undergoes a phase transition at 210–220 K and decomposes at 397 K. Atomic force microscopy of the (001) face shows a C60 molecular packing analogous to that which exists in the {111} planes of face-centered cubic C60. It seems likely that C60 and CCl4 molecules are orientationally disordered at room temperature.

Decagonal C60 crystals grown from n-hexane solutions: solid-state and aging studies

Abstract Decagonal C60 crystals grown from n-hexane solutions correspond to an orthorhombic 1:1 solvate (a=10.249 A, b=31.308 A, c=10.164 A). It forms with negative excess volume ( −55.5 A 3 per formula unit) and transforms on heating into fcc C60 (desolvation enthalpy of +50.6 kJ per solvate mole, close to the sublimation enthalpy for pure n-hexane) while n-hexane desorption from fcc C60 is accompanied by an enthalpy of +48.6 kJ per solvent mole. Thus solvate formation is preferred to solvent adsorption. Orthorhombic C 60 ·1 n -hexane undergoes no degradation when stored in air for 9 years at room temperature in the dark.

Thermal studies of C60 transformed by temperature and pressure treatments

Raman spectroscopy, X-ray and electron diffractions have been applied to study samples of fullerene C60 after they have undergone pressure and temperature treatments up to 8 GPa and 1073 K. It is confirmed that mixtures of rhombohedral and tetragonal structures are formed in the 2–4 GPa and 673–1073 K pressure-temperature domain. Traces of a hexagonal phase are also observed in the same range. Only the rhombohedral one is obtained in the 4–8 GPa range in the same temperature interval. The material obtained at temperatures between 473 and 673 K in the whole pressure range which has previously been described in terms of a cubic structure can be understood in terms of an orthorhombic one. It may also contain mixtures of different phases including a cubic one and even a cubic superstructure. The widened poorly resolved X-ray profiles indicate the existence of disorder within the materials formed and the increasing complexity of Raman spectra as temperature increases at fixed pressures (from 2 to 8 GPa) suggests that this disorder is related to phase mixtures in almost all samples. A rhombohedral sample formed at 6 GPa-873 K reverted to a mixture of rhombohedral and tetragonal phases at 2.5 GPa-873 K. Thus thermodynamic equilibrium between these two kinds of systems could exist.

Molecular packing of fullerene C60 at room temperature

A single crystal of C60 grown from a toluene solution has been studied by X-ray diffraction at room temperature. The lattice is face-centered cubic, with space group Fm3m and Z = 4, which agrees with previous powder diffraction measurements. It is shown that, contrary to what is obtained in other plastic crystals, the Pauling-Fowler model (the so-called free rotation one), which implies an isotropic molecular disorder, gives a better description of the molecular packing than the site model does. It is concluded that the molecules undergo a rotational diffusion as previous molecular dynamics simulations have described.

Solid-state studies on C60 solvates grown from n-heptane

Crystallographic and thermodynamic experiments show that crystalline C60 obtained by slow evaporation of solutions in n-heptane is a C60, n-heptane 1:1 solvate. Its lattice is hexagonal, Laue class 6/mmm, with a = 10.00(4) A and c = 10.16(1) A. No transition is observed at temperatures higher than 100 K, and desolvation into fcc C60 occurs at about 360 K with ΔH = +43.5 J g−1, close to the sublimation enthalpy for pure n-heptane. Another binary compound, presumably a polymorph of the former, is sometimes obtained by rapid evaporation of toluene + n-heptane mixtures. Its lattice is orthorhombic, Immm, with a = 10.07 A, b = 10.22 A and c = 48.9 A.

Crystalline thiophene—I: Phase diagram and structures of two orientationally disordered crystalline phases. crystallographic evidence for a metastable low temperature phase

Abstract The first order phase diagram of thiophene has been determined from 217.5 to 298 K up to 400 MPa. It shows that a high pressure phase which melts at room temperature is in fact Waddington et al.s atmospheric pressure phase II and discloses a ∼ R ln2 entropy increment at the II→I phase transition. The structures of phases I and II have accordingly been investigated taking this relationship into account. For phase I, the best result is obtained for space group Cmca with 20 equiprobable molecular orientations. This leads us to assume that phase II is better described by space group Pnma with 10 molecular orientations. Finally, a metastable phase I′ can easily be obtained by cooling phase I at atmospheric pressure. Its unit cell is derived from that of I by multiplying parameter b by 2 and parameter c by 20: this can be considered as an a posteriori justification of the multiple-of-five number of molecular orientations in phases I and II.