The effect of molecular size and intermolecular interactions on the transport properties of the chloride and hydroxide ions in a PiperION anion exchange membrane is discerned using molecular dynamics simulations and a subsequent detailed comparison of the molecular features like radial distribution function (RDF), coordination number (CN), and mean square displacement (MSD). The pore characteristics of the membrane like pore limiting diameter (PLD), largest cavity diameter (LCD), and pore size distribution in the presence of the two anions are also analyzed to determine the effect of the molecular size on ionic movement through the membrane. Our results show that the anion–water RDF peak values for the hydroxide anions are more than two times that of chloride ions at all levels of hydration, implying much stronger interactions between hydroxide and water in comparison to those between chloride and water. The pore characteristics are also observed to be less in the hydroxide membrane compared with the chloride one at higher hydration levels. The stronger interactions and smaller pore sizes are observed to reduce the vehicular diffusivities of the hydroxide anion by a factor of 2.0 as compared to its chloride counterpart at high hydration levels (λ = 8.0, 12.0, 16.0, and 20.0) when the anions are fully hydrated and remain in the aqueous phase. At low hydration levels (λ = 4.0), however, when the pore sizes in the two samples are compatible and water channels segregated due to nonpercolation, the hydroxide ions diffuse faster on account of their smaller size compared to chloride, as is normally the case in anionic diffusion. Our simulation results are observed to match closely with experiments and other simulation data from the literature, thus verifying our findings.
Sharma et al. (Wed,) studied this question.