Enthalpy-Entropy Compensation of Octylsilica Stationary Phase in Reversed-Phase HPLC

Differences between retentions of various classes of aromatic hydrocarbons in reversed-phase high-performance liquid chromatography. Implications of using retention data for characterizing hydrophobicity

Capacity factors of a series of alkylbenzenes (C1-C10), 12 chlorobenzenes, 9 chlorotoluenes, 17 chloronaphthalenes and 65 chlorobiphenyls have been measured on an octadecylsilica column. Aqueous methanol of four different compositions (80-95% methanol) was used as eluent. Logarithms of capacity factors of all eluites are linearly related to the amount of organic modifier in the eluent. In addition, linear relationships between the solvent strength and the logarithms of capacity factors extrapolated to zero methanol have been revealed. The proportionality factors are dependent on the structures of the eluites. Thermodynamic consideration of the retention processes shows that, within each type of eluite, enthalpy-entropy compensation is found. The compensation temperatures are not significantly different for the various types of eluite. Furthermore, it is shown that the compensation temperatures increase with increasing water content of the eluent. Since the intercepts of the delta G0-delta H0 plots are not equal for the various types of eluite, it was concluded that the distribution processes causing retention of benzene, naphthalene and biphenyl are different. When only the free energies of retention (i.e. the capacity factors) of different types of eluite are compared, no accurate information on the hydrophobicity of the eluites can be obtained if aqueous methanol is used as eluent. Therefore the possibilities for relating or predicting other physico-chemical parameters of the test compounds, such as octan-1-ol-water partition coefficients with isocratic retention data, will be limited.