Evaluation of the Gibbs free energies of tris(β-diketonato) chromium (III),-cobalt(III), and bis(β-diketonato)palladium(II) in reversed-phase liquid chromatography on octadecyl-bonded silica gel with acetonitrile-water mobile phases

To clarify the mechanism of retention of several β-diketonates of chromium(III), cobalt(III), and palladium(II) in reversed-phase liquid chromatography (RPLC), their capacity factors (k) were measured on an octadecyl-bonded silica gel (ODS) stationary phase with acetonitrile-water binary mobile phases. The values obtained were compared with the liquid-liquid parition coefficients (P) of the metal β-diketonates in the dodecane/(acetonitrile-water mixture) system. This comparison showed that with regard to the distribution of the metal β-diketonates the nature of the stationary phase was similar to, but not identical with, that of dodecane. When the Gibbs free energies of the metal β-diketonates in the stationary phase were calculated, the values showed that the cavity-formation free energy for each metal β-diketonate in the stationary phase was close to that in dodecane, and the interaction free energy was almost equal to the sum of the energies of interaction with the alkyl moiety and with acetonitrile. This implies that acetonitrile molecules from the mobile phase penetrate the alkyl moiety of ODS.