The application of enantioselective micelles in ultrafiltration systems can be an alternative route to meet the increasing demand for enantiopure products. We have studied the separation of D,L-phenylalanine (Phe) by cholesteryl-L-glutamate:CuII (CLG:CuII) anchored in nonionic micelles (intrinsic enantioselectivity D/L,int = 1.9). A cascaded system is needed to complete the separation, as a single stage is insufficient to obtain >99% optically pure products. It is shown that the complexation and decomplexation processes are not instantaneous; hence, elucidation of the complexation kinetics is essential to the design of a multistage system. Linear driving force (LDF) models describe both the complexation and decomplexation rates of enantiomers. It can be concluded that the complexation rates of D- and L-Phe, (32 ± 11) × 10-5 s-1 and (28 ± 14) × 10-5 s-1, respectively, are not limited by enantiomer diffusion in the hydrophilic shell of the micelles. Consequently, the formation and rearrangement of the chelate complexes must be rate-limiting. In addition, decomplexation of both enantiomers is even slower, on the order of 10-6 s-1. Fortunately, ultrafiltration experiments indicate that a rapid exchange rate of bound L-Phe by unbound D-Phe improves the decomplexation of L-Phe to (360 ± 250) × 10-5 mM-1 s-1. This exchange process can be described by a second-order LDF model.
Overdevest, P. E. M., Schutyser, M. A. I., Bruin, de, T. J. M., Riet, van 't, K., Keurentjes, J. T. F., & Padt, van der, A. (2001). Separation of racemic mixture by ultrafiltration of enantioselective micelles 2. (De)complexation kinetics. Industrial and Engineering Chemistry Research, 40(25), 5998-6003. https://doi.org/10.1021/ie010014t