Correlation between macroscopic sugar transfer and nanoscale interactions in cation exchange membranes
Résumé
Previous experimental work has shown that the transfer of organic solutes through ion-exchange membranes depends on the membrane counter-ion and that this dependence is probably linked to the interactions taking place at the nanoscale inside the membrane matrix. In this paper, a computational approach is carried out, combining quantum mechanics and molecular mechanics to determine the interactions occurring at the nanoscale, taking a cation exchange membrane as example. Building blocks are first accurately studied at high level of quantum theory, before being merged in macromolecular models. The computed interactions are then compared to the experimental values of the solute flux in order to point out the nanoscale mechanisms governing the solute transfer. The computed glucose-polymer fragment interactions, related to the sugar solubility inside the membrane, are found to be almost independent from the membrane counter ion. On the contrary, significant variations of the chain-chain interaction, i.e. the interaction energies per trapped water molecule or hydrogen bonding wire connecting the polymer fragments, were observed according to the cation. Moreover, a correlation was pointed out with the experimental sugar fluxes obtained with 3 different sugars. Increasing chain-chain interactions inside the membrane was found to give decreasing sugar flux. Then this work shows that the cohesion energy between the polymer fragments fixes the dependence of the sugar flux versus the membrane counter-ion. The crucial role of the water molecules coordinating the cations is also highlighted.
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