Enthalpy-entropy compensation in reversed-phase high-performance liquid chromatography of series of aryloxoalkanoic and arylhydroxyalkanoic acids

Abstract

The capacity factors of various arylacetic (I), aryloxoalkanoic (II), arylhydroxyalkanoic (III) and substituted benzyloxyphenylacetic (IV) acids have been determined at various temperatures on a μBondapak C 18 reversed-phase system with buffered aqueous methanol, acetonitrile or tetrahydrofuran (pH 3.0) as the mobile phases. Three groups of acids were evaluated, the lipophilicity and retention behaviour of which were affected by intramolecular interactions. A series of acids I, used for comparison, showed a linear correlation between the logarithms of capacity factors, log k′, and the corresponding changes in enthalpy, -Δ H 0. Interactions of two hydrophilic groups (so-called H/H interactions) affect the lipophilicity and retention properties of the acids II, III in a similar way. Thus it is possible to calculate log P values from the linear dependence log P vs. log k′, regardless of the modifier used. These interactions did not show in the enthalpy-entropy compensation of the systems containing methanol or tetrahydrofuran as a modifier. Thus, the linear relationships between log k′ and ΔH 0 are valid for the whole group of acids I-III. An effect of intramolecular hydrophobic interactions upon the retention properties of acids IV in 50% methanol was accompanied by a deviation from the isokinetic relationship derived for the acids I-III. This type of interaction did not occur in 40% tetrahydrofuran and the entropy-enthalpy compensation operated over the whole group of acids I-IV. The enthalpy changes were minimal in the system with 40% acetonitrile as the mobile phase and thus the isokinetic relationship was affected by a large error.