Understanding Nonlinear Composition Dependency of Enantioselectivity in Chiral Separation Using Mixed Micelle

Hypothesis: Mixtures of chiral and achiral building blocks of supramolecules exhibit interesting cooperative phenomena, indicat-ed by the nonlinear composition dependence of the chiral properties. However, the nonlinear composition depend-ence of the enantioselectivity of mixed micelles is not well understood. It was hypothesized that in-depth understand-ing can be achieved by carefully investigating the composition dependence of the properties. Experiments: In this work, the nonlinear composition dependence of the enantioselectivity was found for the mixed micelle of achi-ral and chiral N-acyl amino acids by micellar electrokinetic chromatography (MEKC). Capillary electrophoresis, circu-lar dichroism (CD) spectroscopy, surface tension measurement, and fluorescence spectroscopy were used to investi-gate the mechanisms. Findings: Four mechanisms that could be causing the nonlinearity were investigated: (i) synergistic and antagonistic interac-tions of the surfactants; (ii) the chiral transfer from chiral to achiral surfactant; (iii) differences in the retention factor; and (iv) cooperative chiral recognition of the chiral and achiral surfactant. The investigation of the composition de-pendency of critical micelle concentration (CMC) and molar circular dichroism revealed that the effect of (i) and (ii) was negligibly small. The newly derived equations for (iii) and (iv) revealed that (iii) and (iv) have a major or medi-um effect on the nonlinear enantioselectivity.