Sorption of enantiomers and alcohols into Nafion® and the role of air humidity in the experimental data evaluation

Abstract

Two enantiomers may exhibit completely different biological or pharmacological activities; therefore, the resolution of the enantiomers from a racemic mixture is an important task. The results of the total sorption measurements for the individual (+) and (−) liquid enantiomers from the enantiomeric pair revealed that there is a difference between extents of their sorption in Nafion®. The difference between liquid sorption of the (+) and (−) enantiomers of small molecules, such as the methyl lactates or 2-butanols, is higher than in the case of larger molecules, such as phenylethanols and methylbenzyl amines. This result could be explained by the hypothesis that the size of the chiral molecule must fit like a puzzle piece into the Nafion® helical polymer network. This hypothesis was supported by the 19F MAS NMR analysis, which confirmed the helical structure of Nafion®. Further, the branching of the molecules significantly influences the extent of sorption of the liquid in Nafion®, as shown for the butanols. The trend of this dependence is in agreement with the trend of heat of vaporisation and the normal boiling temperature data of the butanols. Because Nafion® easily absorbs water vapour from the air, the influence of the humidity on the sorption values was studied and discussed.