18-crown-6-aminophenol isomer complexes studied by RPLC

The reserved-phase liquid chromatography retention of aminophenol isomer wa s investigated over a concentration range of 18-crown-6 (c: 0 - 0.15 M) usi ng a C18 as the stationary phase and a phosphate buffer pH 2 as the mobile phase. A theoretical treatment was developed to investigate the effect of t he polyether on the equilibrium between the solutes with the C18 phase and the aqueous medium respectively. This was found to be adequate to accuratel y describe the isomer retention behavior when c varied. it was expected tha t the addition of 18-crown-6 was responsible for an increase in the solute affinity for the C18 stationary phase due to complex formation and blockage of the polar amino group of the solute. The thermodynamic parameter variat ions were calculated using van't Hoff plots. The enthalphy-entropy compensa tion was analyzed in relation to this retention model to confirm the influe nce of the modifier on the solute affinity towards the C18 phase. From the experimental data, the complexation and transfer constants for the differen t species were determined for the three isomers. This indicated that the hy droxyl group of the solute governed both the complexation and retention pro cesses via hydrogen bonding and steric effects.