Synthesis, Characterization, and Application of Chiral Ionic Liquids and Their Polymers in Micellar Electrokinetic Chromatography

Abstract

Two amino acid-derived (leucinol and N-methylpyrrolidinol) chiral ionic liquids are synthesized and characterized in both monomeric and polymeric forms. Leucinol-based chiral cationic surfactant is a room-temperature ionic liquid, and pyrrolidinol-based chiral cationic surfactant melts at 30−35 °C to form an ionic liquid (IL). The monomeric and polymeric ILs are thoroughly characterized to determine critical micelle concentration, aggregation number, polarity, optical rotation, and partial specific volume. Herein, we present the first enantioseparation using chiral IL as a pseudostationary phase in capillary electrophoresis. Chiral separation of two acidic analytes, (±)-α-bromophenylacetic acid and (±)-2-(2-chlorophenoxy)propanoic acid (±)-(2-PPA) can be achieved with both monomers and polymers of undecenoxycarbonyl-l-pryrrolidinol bromide (l-UCPB) and undecenoxycarbonyl-l-leucinol bromide (l-UCLB) at 25 mM surfactant concentration using phosphate buffer at pH 7.50. The chiral recognition seems to be facilitated by the extent of interaction of the acidic analytes with the cationic headgroup of chiral selectors. Polysodium N-undecenoxycarbonyl-l-leucine sulfate (poly-l-SUCLS) and polysodium N-undecenoxycarbonyl-l-leucinate (poly-l-SUCL) were compared at high and low pH for the enantioseparation of (±)-(2-PPA). At pH 7.5, poly-l-SUCLS, poly-l-SUCL, and (±)-(2-PPA) are negatively charged resulting in no enantioseparation. However, chiral separation was observed for (±)-(2-PPA) using poly-l-SUCLS at low pH (pH 2.00) at which the analyte is neutral. The comparison of chiral separation of anionic and cationic surfactants demonstrates that the electrostatic interaction between the acidic analyte and cationic micelle plays a profound role in enantioseparation.