Chiral separations in microemulsion electrokinetic chromatography: Use of micelle polymers and microemulsion polymers

In this study, microemulsions of the chiral surfactant polysodium N-undecenoyl-d-valinate (poly-d-SUV) was utilized for enantiomeric separation by investigating two approaches using polymeric chiral surfactant in microemulsion electrokinetic chromatography (MEEKC). In the first approach, poly-d-SUV was used as an emulsifier surfactant along with 1-butanol and n-heptane. Enantioseparation of anionic or partially anionic binaphthyl derivatives, anionic barbiturates, and cationic paveroline derivatives were achieved by varying the mass fraction of 1-butanol, n-heptane and poly-d-SUV. For anionic or partially anionic analytes, relatively lower mass fractions of n-heptane, and poly-d-SUV were found to give optimum chiral separations as compared to that for cationic solutes. In the second approach, the chiral microemulsion polymer was prepared by polymerizing mixtures of 3.50% (w/w) of sodium N-undecenoyl-d-valinate (d-SUV) and 0.82% (w/w) of n-heptane (core phase) at varying concentration of 1-butanol. After polymerization, the n-heptane and 1-butanol were removed to yield solvent free microemulsion polymers (MPs) which were then utilized for the separation of anionic binaphthyl derivatives and anionic barbiturates. When MPs of D-SUV were utilized for chiral separation, 1.00% (w/w) 1-butanol and 3.50% (w/w) 1-butanol was optimum for enantioseparation of (±)-BNP and (±)-BOH, respectively. On the other hand, for anionic (±)-barbiturates very low concentration of butanol (0.25%, w/w) provided optimum resolution. Compared with micellar electrokinetic chromatography (MEKC), the use of micelle polymers or microemulsion polymers in MEEKC showed dramatic enhancement for resolution of (±)-BNP, while this enhancement was less dramatic for other binaphthyls [(±)-BOH, (±)-BNA] as well as for (±)-barbiturates and (±)-paveroline derivatives. However, higher separation efficiency of the enantiomers was always observed with MEEKC than in MEKC.