Pressure-controlled high-performance liquid chromatographic study on the influence of rim chemistry on partial molar volume differences between free and complexed cyclodextrins

Abstract

Pragmatic comparison of pressure dependent retention for differing cyclodextrin rim chemistries is assessed using controlled-pressure HPLC. For pressure differences of <300 bar, systematic shifts in solute capacity factor are observed for both native and methylated β-cyclodextrin stationary phases. In addition to the importance of this observation for the practice of liquid chromatography, this technique can also be implemented in the fundamental determination of the influence of rim chemistry on the cyclodextrin partial molar volume both with and without solute inclusion. That is, pressure-controlled measurements provide a direct comparison between the partial molar volumes for native cyclodextrin (CD) and methylated cyclodextrin (MCD) in the presence and absence of the complexing solute (comp). Surprisingly, direct comparison of the measured partial molar volumes for the two rim chemistries indicates that the presence of neutral solutes does not contribute significantly to the volumetric component of complexation, V comp,CD− V comp,MCD≈ V CD− V MCD. In contrast, their ionized counterparts are shown to exhibit marked rim chemistry differences in the partial molar volume of cyclodextrins with and without anion inclusion, V comp(−),CD− V comp(−),MCD < V CD− V MCD. Not previously demonstrated by direct chromatographic measurement, these results have interesting implications for advancing the fundamental understanding of host–guest solvation properties.