TY - JOUR

T1 - Negative and Positive Confinement Effects in Chiral Separation Chromatography Monitored with Molecular-Scale Precision by In-Situ Electron Paramagnetic Resonance Techniques

AU - Wessig, Martin

AU - Spitzbarth, Martin

AU - Klaiber, Alexander

AU - Drescher, Malte

AU - Polarz, Sebastian

N1 - Funding Information: We acknowledge the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) for funding within the SPP 1570 (PO 780/14-1). Funding Information: *E-mail: sebastian.polarz@uni-konstanz.de. ORCID Malte Drescher: 0000-0002-3571-3452 Sebastian Polarz: 0000-0003-1651-4906 Author Contributions M.W. prepared all samples, recorded the EPR data, and evaluated them. M.S. and M.D. performed the imaging EPR studies. A.K. measured SAXS data. S.P. supervised the research and wrote the paper. Funding German Research Foundation (Deutsche Forschungsgemein-schaft, DFG) Notes The authors declare no competing financial interest. Publisher Copyright: © 2017 American Chemical Society. Copyright: Copyright 2017 Elsevier B.V., All rights reserved.

PY - 2017/10/31

Y1 - 2017/10/31

N2 - Separation of compounds using liquid chromatography is a process of enormous technological importance. This is true in particular for chiral substances, when one enantiomer has the desired set of properties and the other one may be harmful. The degree of development in liquid chromatography is extremely high, but still there is a lack in understanding based on experimental data how selectivity works on a molecular level directly at the surfaces of a porous host material. We have prepared amino-acid containing organosilica as such host materials. Watching the rotational dynamics of chiral spin probes using electron paramagnetic resonance spectroscopy allows us to differentiate between surface adsorbed and free guest species. Diastereotopic selectivity factors were determined, and the influence of chiral surface group density, chemical character of the surface groups, pore-size, and temperature was investigated. We found higher selectivity values in macroporous solids with a rather rigid organosilica network and at lower temperature, indicating the significant effect of confinement effects. In mesoporous materials features are opposed with regards to the T-dependent behavior. From EPR imaging techniques and the resulting (macroscopic) diffusion coefficients, we could confirm that the correlations found on the microscopic level transform also to the macroscopic behavior. Thus, our study is of value for the development of future chromatography materials by design.

AB - Separation of compounds using liquid chromatography is a process of enormous technological importance. This is true in particular for chiral substances, when one enantiomer has the desired set of properties and the other one may be harmful. The degree of development in liquid chromatography is extremely high, but still there is a lack in understanding based on experimental data how selectivity works on a molecular level directly at the surfaces of a porous host material. We have prepared amino-acid containing organosilica as such host materials. Watching the rotational dynamics of chiral spin probes using electron paramagnetic resonance spectroscopy allows us to differentiate between surface adsorbed and free guest species. Diastereotopic selectivity factors were determined, and the influence of chiral surface group density, chemical character of the surface groups, pore-size, and temperature was investigated. We found higher selectivity values in macroporous solids with a rather rigid organosilica network and at lower temperature, indicating the significant effect of confinement effects. In mesoporous materials features are opposed with regards to the T-dependent behavior. From EPR imaging techniques and the resulting (macroscopic) diffusion coefficients, we could confirm that the correlations found on the microscopic level transform also to the macroscopic behavior. Thus, our study is of value for the development of future chromatography materials by design.

UR - http://www.scopus.com/inward/record.url?scp=85032665544&partnerID=8YFLogxK

U2 - 10.1021/acs.langmuir.7b02713

DO - 10.1021/acs.langmuir.7b02713

M3 - Article

VL - 33

SP - 11968

EP - 11976

JO - LANGMUIR

JF - LANGMUIR

SN - 0743-7463

IS - 43

ER -