Permeability and diffusion coefficients of single methyl lactate enantiomers in Nafion® and cellophane membranes measured in diffusion cell

A two-compartment horizontal diffusion cell of own construction was used to measure the permeability and diffusion coefficients for the test system toluene in low density polyethylene (LDPE) and for chiral D- and L-methyl lactates in Nafion (R) and cellophane. The solvent was chosen such that it was not absorbed in the membrane. The evaluation process was developed to include the accumulation of the solute in the membrane or the change in the membrane thickness. There is a significant difference between the transport and sorption of methyl lactates in Nafion (R) and cellophane, respectively. The methyl lactate enantiomers show a comparable relative difference (50% approximately) in the case of the mass sorption and permeability coefficient values in Nafion (R) and therefore very similar values of diffusion coefficients, but a tenfold higher permeability coefficient of one enantiomer in comparison with the second one in cellophane while sorption coefficient is, again, different by 50% approximately in this case resulting in fifteenfold higher diffusion coefficient of one enantiomer. Furthermore, there is an opposite difference between the mass sorption and swelling of the membrane in the case of methyl lactate enantiomers in Nafion (R) : D-methyl lactate shows a higher mass sorption but lower areal swelling compared with the results of L-methyl lactate in Nafion (R) in the case of dilute solutions. A possible explanation for this fact is that D-methyl lactate fits like a puzzle piece into Nafion's secondary or primary structure and that L-methyl lactate molecules cause more extensive stretching of polymer backbones due to specific force actions. Despite of the observed chiral membrane properties, chiral resolution of racemic mixture of methyl lactates was not reached. (C) 2015 Elsevier B.V. All rights reserved.