Solubility of fullerenes

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.

C
₆₀ in solution

C
₆₀ in extra virgin olive oil showing the characteristic purple color of pristine C
₆₀ solutions

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]}

Solvent	C 60	C 70
1-chloronaphthalene	51	ND
1-methylnaphthalene	33	ND
1,2-dichlorobenzene	24	36.2
1,2,4-trimethylbenzene	18	ND
tetrahydronaphthalene	16	ND
carbon disulfide	8	9.875
1,2,3-tribromopropane	8	ND
chlorobenzene	7	ND
p-xylene	5	3.985
bromoform	5	ND
cumene	4	ND
toluene	3	1.406
benzene	1.5	1.3
carbon tetrachloride	0.447	0.121
chloroform	0.25	ND
n-hexane	0.046	0.013
cyclohexane	0.035	0.08
tetrahydrofuran	0.006	ND
acetonitrile	0.004	ND
methanol	4.0×10−5	ND
water	1.3×10−11	ND
pentane	0.004	0.002
heptane	ND	0.047
octane	0.025	0.042
isooctane	0.026	ND
decane	0.070	0.053
dodecane	0.091	0.098
tetradecane	0.126	ND
acetone	ND	0.0019
isopropanol	ND	0.0021
dioxane	0.0041	ND
mesitylene	0.997	1.472
dichloromethane	0.254	0.080
ND = not determined

See also

• Fullerene chemistry