Comparative solubility study of C60 and C60-Piperazine and applications on quartz crystal microbalance / heat conduction calorimeter

Abstract

Although C60, Buckminsterfullerene, was discovered in 1985, it was not until KrA¤tschmer and coworkers discovered in 1990 that C60 was soluble in benzene that the scientific literature exploded with experimental studies of these remarkable molecules. A large number of derivatives have been synthesized. However, some fundamental physical and chemical properties remain uncertain. Fullerene derivatives have some similarities with pure fullerene but there also exist a wide range of differences. Studying the thermodynamics of fullerene solubility led to the discovery that at room temperature, the thermodynamically stable phase of a saturated solution of fullerenes is its presence as solvates, stoichiometric binary compounds of fullerene and solvent. The incongruent melting of these solvates accounts for a highly unusual maximum in the temperature-solubility curves for these systems. This behavior has also been found in fullerene derivatives particularly in C60-piperazine which we have synthesized in gram quantity. The solubility of C60-piperazine has been measured in 12 aromatic solvents and the values have been compared with the solubility of C60. Hypothetical solubility determinations by various thermodynamic relationships have been worked. Also, a Theoretical Linear Solvation Energy Relationship (TLSER) has been applied to describe a relationship between the solvents and C60 / 60-piperazine. Besides organic solvents, the water solubility of 60-piperazine has been studied and an interesting, pH dependent, color change phenomenon has been studied. We have developed a new, highly sensitive method for measuring simultaneously the mass change (to ± 2 ng) and the resulting heat flow (to ± 100 nW) in a thin film when exposed to a gas mixture of changing composition. We call the device a quartz crystal microbalance/heat conduction calorimeter (QCM/HCC). Under isothermal conditions, a quartz crystal microbalance (QCM) is in intimate thermal contact with a heat flow sensor and thence to a heat sink. A sample QCM is coated with a thin film (0.1 – 20 µ). By introducing a slow flow of a nitrogen carrier gas with different known concentrations of water or organic solvent vapor to the QCM surface, we have been able to measure simultaneously the change in mass and the resulting heat flows from the film to the heat sink when the film sample takes up or releases solvent vapor. Exposing to water and organic solvents, films of C60 and C60-piperazine has been studied in the QCM/HCC. This sort of molecular crystal was a new application on QCM/HCC. Results from QCM/HCC were compared with other thermal analysis methods. Commercial specialized and generic coating materials for pharmaceutical products were studied in QCN/HCC. Film samples were exposed to high humidity and high temperature. The absorption, desorption, and corresponding enthalpy and mechanical property changes were also measured.%%%%Ph.D., Chemistry – Drexel University, 2002