The effect of modest hydrostatic pressure (<350 bar) on condensed-phase equilibrium processes has been largely overlooked, due in large part to the small compressibility of these phases relative to gases or supercritical fluids. Although the bulk properties of condensed phases are not significantly modified by pressure in this modest regime, the solvation processes driving inclusion complexation may be appreciably affected. In this paper, we examine this hypothesis using steady-state fluorescence spectroscopy to determine the pressure dependence of association constants. The widely used host molecule, β-cyclodextrin, provides an incompressible hydrophobic cavity into which structurally analogous fluorescent probes are encapsulated. By comparing the unique pressure dependencies of these equilibria, the importance of local site solvation and rim interactions in influencing the pressure dependence is demonstrated. The structurally analogous complexes chosen for these studies are expected to have similar pressure-dependent behavior based on comparable solvation structures. However, pressure-induced changes in the association constant for these two analogs are quite distinct, with differences in K(c) ranging from clearly pressure dependent (-14%) to pressure independent over 338 bar. Additional solvation perturbations are observed in the pressure dependence of the quantum efficiency for both complexes (-7.3% and -9.4%). Thus, pressure-induced perturbation in the fluorescence properties of the complex need not be accompanied by simultaneous changes in the complexation equilibrium. Finally, these pressure-induced changes in complexation selectivity are important for all measurements conducted under variable pressure conditions, including liquid chromatography and process monitoring.
On the basis of equilibrium thermodynamics, pressure can cause a shift in equilibrium for any interaction that exhibits a change in partial molar volume. This shift in equilibrium can be observed in liquid chromatography as a pressure-dependent shift in solute retention. In this paper, the impact of pressure on liquid chromatographic separations with mobile-phase additives is examined from both theoretical and experimental perspectives. The theoretical development for coupled-equilibria separations shown here is general and can be applied to any separation using mobile-phase additives. Predictions indicate that the coupled nature of these equilibria leads to pressure-induced perturbations in partitioning and complexation that can either compete with or complement one another. Using positional isomers and enantiomers as model solutes, experimental retention observations are fully consistent with these predictions, showing the diminution of individual pressure effects for competing cases and enhanced pressure effects for complementary cases. When pressure-induced changes in capacity or retention factor differ between individual solutes, changes in solute selectivity are predicted and observed. Using a C18 stationary phase with beta-cyclodextrin as the mobile-phase additive, solutes studied here exhibit changes in selectivity ranging from - 7 to + 10% for a change in average pressure of approximately 215 bar. Perhaps the most dramatic change in selectivity is observed for the separation of positional isomers where pressure-induced changes in selectivity actually reverse solute elution order.
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