Anomalous solubility behaviour of C60

SOLUBILITIES of solids in liquids exhibit a temperature dependence that is correlated with the energetics of dissolution. For organic solutes in organic solvents the common experience is that solubility increases on warming (that is, dissolution is normally endothermic), forming the basis for purification by crystallization by slow cooling of hot solutions1. Interactions between inorganic compounds and water are often more energetic, and may lead to complicated temperature-dependent solubilities, often associated with the formation of hydrated solid phases2. We have investigated the temperature-dependent solubility of C60 in hexane, toluene and CS2. We observe a solubility maximum near room temperature (around 280 K) for all three solvents. Although the solubility of C60 in these three solvents differs by several orders of magnitude, the temperature dependence of the relative solubilities is much the same in each case. We conclude that dissolution is endothermic below room temperature and exothermic above. We interpret this change as being due to a phase change in solid C60 presumably the phase change observed previously in the absence of a solvent3–5, modified by solvent wetting. A solubility maximum (or minimum) for organic compounds in non-electrolytes is highly unusual, and may be unprecedented. The effect may have consequences for solvent-extraction techniques of fullerene purification.