Geometrical model for the retention of fullerenes in high-performance liquid chromatography

In high-performance liquid chromatography (HPLC) using a poly(octadecylsiloxane) as a stationary phase, methanol as a mobile phase, C(60) and C(70) fullerenes as solutes, and water as a mobile phase modifier, a study on the surface tension effect of water on fullerene retention was carried out by varying the water concentration [W] and the column temperature T. The thermodynamic parameters for fullerene transfer from the mobile to the stationary phase were determined from linear van't Hoff plots. An enthalpy-entropy compensation revealed that the types of interactions between fullerenes and the stationary phase were independent of the fullerene structure and the mobile phase composition. An analysis of the experimental variations of the retention factor and the selectivity values with [W] was performed using a novel geometrical model. It was shown that the increase in fullerene retention accompanying the water concentration was due to the increased effects of surface tension. This brought about an increase in the interactions between fullerene and the stationary phase, explaining the observed thermodynamic parameter trends over the water concentration range. The theoretical model provided an estimation of the radius of fullerene which was found for C(60) to be equal to 3.3 Å and an activation energy during the transfer equal to 9.8 kJ/mol.