Temperature dependence of the Kováts retention index: The entropy index

According to a novel equation, the temperature dependence of the Kováts retention index, dI/dT is proportional to the difference of the Kováts retention index, I, and the new entropy index, I(S), defined similarly as the retention index, but based on solvation entropy instead of the free energy of solvations. The new relationship was tested with the experimental retention and thermodynamic data published by Kováts and coworkers for 32 compounds on 6 different stationary phases. Very good correlations (r > 0.99) were observed for dI/dT versus (I − I(S)) and dI/dT versus δΔCp, the molar heat capacity difference of the solute and the hypothetical n-alkane, which has the same retention index as the solute. Deviations in the dI/dT versus δΔCp relationship were observed only for alcohols, suggesting a different solvation mechanism for alcohols as compared with other compounds.