The solubility of fullerenes is generally low. Carbon disulfide dissolves 8g/L of C60, and the best solvent (1-chloronaphthalene) dissolves 53 g/L. up Still, fullerenes are the only known allotrope of carbon that can be dissolved in common solvents at room temperature. Besides those two, good solvents for fullerenes include 1,2-dichlorobenzene, toluene, p-xylene, and 1,2,3-tribromopropane. Fullerenes are highly insoluble in water, and practically insoluble in methanol.

Solutions of pure C₆₀ (buckminsterfullerene) have a deep purple color. Solutions of C₇₀ are reddish brown. Larger fullerenes C
₇₆ to C
₈₄ have a variety of colors. C
₇₆ has two optical forms, while other larger fullerenes have several structural isomers.

General considerations

Some fullerene structures are not soluble because they have a small band gap between the ground and excited states. These include the small fullerenes C
₂₈,^[1] C
₃₆ and C
₅₀. The C
₇₂ structure is also in this class, but the endohedral version with a trapped lanthanide-group atom is soluble due to the interaction of the metal atom and the electronic states of the fullerene. Researchers had originally been puzzled by C
₇₂ being absent in fullerene plasma-generated soot extract, but found in endohedral samples. Small band gap fullerenes are highly reactive and bind to other fullerenes or to soot particles.

Solubility of C
₆₀ in some solvents shows unusual behaviour due to existence of solvate phases (analogues of crystallohydrates). For example, solubility of C
₆₀ in benzene solution shows maximum at about 313 K. Crystallization from benzene solution at temperatures below maximum results in formation of triclinic solid solvate with four benzene molecules C
₆₀·4C
₆H₆ which is rather unstable in air. Out of solution, this structure decomposes into usual face-centered cubic (fcc) C
₆₀ in few minutes' time. At temperatures above solubility maximum the solvate is not stable even when immersed in saturated solution and melts with formation of fcc C
₆₀. Crystallization at temperatures above the solubility maximum results in formation of pure fcc C
₆₀. Millimeter-sized crystals of C
₆₀ and C
₇₀ can be grown from solution both for solvates and for pure fullerenes.^[2][3]

Solubility table

The following are some solubility values for C
₆₀ and C
₇₀ from the literature, in grams per liter.^{[4][5][6][7][8]}

See also

• Fullerene chemistry

References