Pressure effects on complexation equilibria as elucidated by liquid chromatography.

Although its effects are not as familiar to chemists as temperature, pressure has also been shown to influence broad classes of chemical interactions. In these investigations, pressure-controlled liquid chromatography is utilized to assess the impact of typical chromatographic pressures (10 to 350 bar) on complexation equilibria in liquid solution. Using stationary phases which contain immobilized-host molecules, the retention of solutes is directly related to the equilibrium constant for solute-host complexation. Therefore, the change in capacity factor with pressure may be used to measure the change in partial molar volume upon solute complexation with the host species. This important measurement assesses changes in solvation as the solute and host form a complex, which play an important role in molecular recognition. Using several positional isomers and chiral drugs as probes, significant changes in retention are observed for stationary phases containing immobilized cyclodextrins and protein. Pressure-induced shifts in capacity factor are quite significant in many cases, and range from +24 to -36% with a 300 bar increase in pressure. In addition, many solute pairs indicate significant changes in selectivity with pressure. The change in capacity factor with pressure is shown to be sensitive to changes in solute ionization state and solute size, as well as the organic-solvent content of the mobile phase. By contrast, the rim chemistry of the cyclodextrin affects the change in capacity factor with pressure only under those conditions in which the solute is significantly present in the ionized form. Clearly, these pressure effects have considerable impact for the practical implementation and quality control of liquid chromatographic separations using these stationary phases. Moreover, the measurement of the change in partial molar volume upon complexation using this sensitive method will advance the fundamental understanding of complexation equilibria in liquid chromatographic separations and other technological applications.PhDAnalytical chemistryPhysical chemistryPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/131984/2/9938520.pd