Some Enthalpies of Solution of C60 and C70. Thermodynamics of the Temperature Dependence of Fullerene Solubility

Abstract

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...