EVALUATION OF STATIONARY PHASES FOR THE SEPARATION OF BUCKMINSTERFULLERENES BY HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY

The high-performance liquid chromatographic separation of the buckminsterfullerenes C60 and C70 was studied using different chromatographic systems. Normal-phase (silica), non-aqueous reversed-phase (C18-bonded silica and graphitized carbon black) and charge-transfer complexation with dinitrobenzoylpherrylglycine (DNBPG) and 2-(2,4,5,7-tetranitro-9-fluorenylideneaminooxy)propionic acid (TAPA) were considered as retention mechanisms. Mobile phases of different compositions, but all based on hydrocarbons (toluene, hexane, heptane) as the main solvent, were used. Although it was found that the adsorption of the fullerenes on all the other phase systems is exothermic, their interactions with the charge-transfer phases exhibit an unusual endothermic behavior. The retention times of C60 and C70) on these phases increase with increasing temperature. The adsorption enthalpy and entropy derived from the Van 't Hoff plots confirm this rare temperature dependence. The adsorption enthalpy is positive whereas the adsorption entropy is strongly positive.