Separation of racemic mixture by ultrafiltration of enantioselective micelles. 1. Effect of pH on separation and regeneration

Many enantiomer separation systems are studied to meet the increasing deman d for enantiopure compounds. One way to obtain pure enantiomers is to apply enantioselective micelles in ultrafiltration systems. We have studied the separation of phenylalanine (Phe) enantiomers by the ultrafiltration of non selective nonionic micelles containing selector molecules, cholesteryl-L-gl utamate:Cu-II (CLG:Cu-II). Because the net charges of enantiomer and CLG ar e pH-dependent, it is foreseen that pH will be an important factor in the d esign of a cascaded separation process that yields enantiopure products. Ex periments at pH 7, 9, and 11 showed that the complexation can be described by multicomponent Langmuir isotherms. The CLG enantioselectivity for D-Phe increases with decreasing pH, being 1.4, 1.7, and 1.9 for pH 11, 9, and 7, respectively. Accordingly, the saturation concentration and the affinity co nstants decrease with decreasing pH, finally resulting in no complexation a t pH 6. To design an economically attractive separation process, the regene ration of D-Phe-saturated micelles leaving the multistage system is inevita ble. Regeneration, i.e., recovery of enantioselective micelles for reuse, i s possible at pH less than or equal to 4. To keep salt production to a mini mum, the shift in pH between the separation and regeneration processes must be minimized. Therefore, a separation process at pH 7 seems most attractiv e.