Influence of intramolecular interactions on chromatographic behaviour of arylaliphatic acids : II. Enthalpy—entropy compensation in reversed-phase high-performance liquid chromatography

Abstract

The capacity factors of various arylaliphatic acids have been determined at various temperatures on a μBondapak C 18 reversed-phase system with buffered aqueous methanol as eluent (pH 3.0). Two groups of derivatives of arylaliphatic acids were evaluated, the lipophilicity of which is affected by intramolecular interactions. A series of arylacetic acids without intramolecular interactions was used for the sake of comparison. Analysis of the data obtained showed a linear correlation between the logarithms of the capacity factors and the corresponding changes in enthalpy. Enthalpy—entropy compensation is operating in this system, with differences in the slopes between the particular groups of acids. It may be concluded that the decrease in lipophilicity of some derivatized acids is probably due to the differences in the mechanism of the solute—stationary phase interactions. The linear relationship between log k′ and Δ H 0 values implies that a decrease in lipophilicity of the ortho-dialkoxy derivatives is connected with a decrease in changes in entropy on passing from a mobile to a stationary phase. Such a conclusion is in agreement with a hypothesis of steric hindrance of salvation.