Liquid membranes for chiral separations. Application of cinchonidine as a chiral carrier

Enantiomeric enrichment of mandelic acid and phenylglycine racemics is achieved by using a liquid membrane containing cinchonidine as a chiral carrier (α = 1.5 and 0.9, respectively). Transport of the respective enantiomers through a liquid membrane was characterized by using mandelic acid as target analyte. To this end, the influence on this transport of different parameters was determined, for instance the amount of decanol added as modifier to the cinchonidine in dodecane solution (constituting the liquid membrane), the initial carrier:analyte ratio in the feed phase, the pH of the aqueous solutions (both feed and receiving), and the effect of varying the membrane solvent from dodecane-decanol to the more solvating chloroform. Additionally, and for the sake of comparison with the mandelic acid results, the transport of phenylglycine and phenylalanine was studied. The membrane showed a slight selectivity only for the phenylglycine transport, as phenylalanine was transported as a 50% racemic mixture under all the conditions examined. The observed results may perhaps be accounted for by hydrogen bonding interactions between carrier and analytes.