The most recent and comprehensive set of solubility data of C 60 fullerene in various solvents at 298 and 303 K was treated by multivariate stepwise linear regression applied as the linear solvation energy approach. In a few cases where the fullerene formed crystalline solvates, the enthalpy of incongruent melting and the temperature of maximum solubility were used to determine the "hypothetical solubility" of the unsolvated C 60 , which was then used in the statistical procedure instead of the solubility of the solvate. The "ideal" solubility of the fullerene was estimated, and the "nonideal" part of the Gibbs energy of solution was then related to solute-solvent interactions. Increasing molar volume and solvent polarity (as measured by the Dimroth-Reichardt "general polarity" parameter, E T (30)) diminished the solubility of C 60 , whereas electron pair donation ability and polarizability enhanced solubility.

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To improve the understanding of the solution properties of C60 and C70 in aromatic solvents, binary systems of C60 and C70 with benzene, toluene, 1,2- and 1,3-dimethylbenzene, 1,2,4- and 1,3,5-trimethylbenzene, bromobenzene, and 1,2- and 1,3-dichlorobenzene were studied using differential scanning calorimetry, solution calorimetry, and thermogravimetry. Solid solvates with different compositions were identified in many of the systems. The solvates were characterized by composition and by the temperature and the enthalpy of the incongruent melting transition. Enthalpies of solution of C60 in toluene, 1,2-dimethylbenzene, 1,2- and 1,3-dichlorobenzene, and 1,2,4-trimethylbenzene and of C70 in 1,2-dimethylbenzene and in 1,2- and 1,3-dichlorobenzene were determined. The formation−incongruent melting of solid solvates causes maxima in the temperature−solubility curves of fullerenes in aromatic solvents. Trends in solubility behavior of fullerenes were discussed in terms of thermodynamics of solution and solvate formation.

The high-temperature structure of solvent-free C(70) has been determined with high-resolution x-ray powder difraction and electron microscopy. Samples crystallized from solution form hexagonal close-packed crystals that retain an appreciable amount of residual toluene, even after prolonged heating. Samples prepared by sublimation, which contain no detectable solvent, are primarily face-centered cubic with some admixture of a hexagonal phase. The relative volume of the hexagonal phase can be further reduced by annealing. The structures of both phases are described by a model of complete orientational disorder. The cubic phase contains an appreciable density of stacking faults along the [111] direction.

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The enthalpy of solution of C60 in toluene, carbon disulfide, decalin, and N,N-diethylaniline and of C70 in CS2 has been measured using an isoperibol solution calorimeter. Exothermic heats of solution are observed in all cases. The heat of solution of C60 in toluene is −8.6 ± 0.7 kJ mol-1; in carbon disulfide it is −20 ± 1 kJ mol-1. In decalin, slow dissolution prevents an accurate determination, but the heat of solution is less than −5 kJ mol-1. In N,N-diethylaniline, a fast exothermic step comparable to that observed in other systems is followed by a much larger heat evolution, implying that a chemical reaction is occurring. The heat of solution of C70 in CS2 is −9 ± 1 kJ mol-1. A thermodynamic theory of the temperature dependence of fullerene solubility is presented. The central idea is that the unusual temperature dependence of the solubilities of fullerenes is caused by the formation of solvated crystals. Conditions are derived under which a temperature maximum of solubility, caused by an incongruent melting point of a solvated crystal, will occur. The calorimetric results suggest that the negative temperature dependence of the solubility of fullerenes may be quite general if the solid phase in equilibrium with the saturated solution is unsolvated fullerene.

Differential scanning calorimetry, solution calorimetry, and room-temperature single-crystal X-ray diffraction were used to study the thermodynamic and structural properties of a solvated crystal C60·2C6H5Br. In the monoclinic solvate, two orientations of C60 were observed with fractional populations of 0.71 and 0.29. The enthalpy of solution of pure C60 in bromobenzene was determined to be Δsol H[C60(s)] = −11.5 ± 2.0 kJ/mol. The enthalpy of solution of the solvated crystal was Δsol H[C60·2C6H5Br(s)] = +28 ± 1 kJ/mol. The phase diagram of the system C60−C6H5Br for T < 423 K was constructed. It predicts the existence of a maximum in the temperature−solubility relationship for C60 in bromobenzene at 350 K. The activity of bromobenzene vapor over the solvated crystal is predicted to be reduced from its value over the pure liquid by a factor of 3.5.