RP-HPLC study of hydrophilic C60 fullerene derivatives

The chromatographic retention (expressed by the retention factor k') on RP-HPLC stationary phases is proportional to the lipophylic character of the solutes. The retention also depends on the composition of the mobile phase, i.e. the nature and the volume fraction of the organic solvent. We have measured k’ at different percentages (v/v) of organic solvent, phi, of the mobile phase for three C60 fullerene derivatives (FD1-3 ) characterised by a different side chain length. Plots of Log k’ vs eluent composition gave straight lines in all cases. Extrapolation to phi = 0 gave the Log Kw values, namely the logarithmic retention that the solute would have using pure water as the eluent. The experimental k’ data, obtained using both a C4 and a C18 stationary phase, highlight the dependence of the Log Kw values on the intrinsic properties of the studied molecules and their independence on the length of the alkyl chains constituting the stationary phase. The obtained results point out to a decreasing lipophylic character of the substrates in the order FD1>FD2>FD3 on both columns. The effect of the temperature (25-65°C) on the chromatographic behaviour of FD1-3 have also been investigated. A progressive decrease of the retention time upon increasing the temperature has been observed and straight lines in the Van't Hoff plots (ln k' vs 1/T) have been obtained. These results allow to determine H and S of the association process between FD and the stationary phase. We have also measured the Log P octanol/water, or more precisely the Log P m/a, of the same derivatives using the shake-flask technique. In fact for these substrates the partition is between the aggregate form of FD in water and the monomer of the same FD in octanol. The existence of a correlation between Log Kw and Log P had already been evidenced . We have found for FD1-3 a corresponding correlation between Log Kw and Log Pm/a.