Langmuir isotherms for enantioselective complexation of ( d/ l)-phenylalanine by cholesteryl- l-glutamate in nonionic micelles

• Published in: Colloids and surfaces. A, Physicochemical and engineering aspects Physicochemical and engineering aspects, 2000-04, Vol.163 (2), p.209-224
• Main Authors: Overdevest, Pieter E.M, van der Padt, Albert, Keurentjes, Jos T.F, van ’t Riet, Klaas
• Format: Article
• Language: English
• Subjects: Chiral selector; Enantiomer separation; Langmuir isotherm; Micellar-enhanced ultrafiltration; Sectie Proceskunde; Selectivity; Sub-department of Food and Bioprocess Engineering; VLAG
• ISSN: 0927-7757; 1873-4359
• DOI: 10.1016/S0927-7757(99)00308-8

An increased demand for enantiopure compounds has induced a significant effort in the development of enantiomer separation technologies. The conventional path to obtain homochiral products at a preparative scale is diastereoisomer crystallization. Disadvantages of this separation process are costly scale-up and have a high energy requirement. An alternative can be ultrafiltration (UF) of enantioselective micelles, which is an easily scalable process with a low energy requirement. The micelles preferentially form a complex with one of the enantiomers. Only unbound enantiomers can pass the membrane during the UF process. The work described in this paper aims at the description of the complexation of phenylalanine enantiomers by cholesteryl- l-glutamate anchored in nonionic micelles of nonyl-phenyl polyoxyethylene [E10] ether (NNP10). The description of this model system is used to develop a separation process capable of complete enantiomer resolution from their racemic mixtures. The influence of membrane rejection and of non-selective complexation on the apparent selectivity is investigated. Both statistical analyses of complexation models and UF experiments in the absence of a chiral selector show that membrane rejection and non-selective complexation are not significant compared to enantioselective complexation. It is concluded that the complexation can be described by straightforward competitive multicomponent Langmuir isotherms. The apparent enantioselectivity appears to be constant over a wide concentration range and equals 1.4. Only at extremely low total enantiomer concentrations the selectivity increases to a value of 4.5. A multi-stage separation process is required in order to separate a racemate for 99.9%. Preliminary calculations using the Langmuir model have shown that 60 stages are sufficient to reach a 99.9% separation.