Enhanced chiral recognition by β-cyclodextrin at liquid/liquid interfaces as revealed by chromatographic and interfacial tension measurements

The chiral selectivity of β-cyclodextrin (β-CD) in the water/hexane (3.0% THF) two-phase system is studied with chromatography and interfacial tension measurements. Chromatography using silica gel impregnated with aqueous β-CD as the stationary phase reveals that the chiral selectivity of this system is higher than that predicted from the β-CD complexation that occurs in bulk water. The retention of the solutes and chiral selectivity can be explained by the adsorption of β-CD at the interface between the aqueous phase (AP) and hexane (3.0% THF) phase. The interfacial tension measurements suggest that β-CD molecules forms a monolayer at this interface. The interfacial complexation constants are larger than the corresponding bulk water constants by at least one order of magnitude. The β-CD molecules adsorbed at the interface are preferably oriented for the formation of inclusion complexes, and thereby solute molecules are directly accommodated in the CD cavity at the interface without partitioning into the AP. The present chromatography-based method probes the interfacial phenomena that are not accessible by other methods. Hence, the liquid/liquid interface is a new field for molecular recognition that does not occur in bulk solution phases.