Abstract
Solvation and ion valency effects on selectivity of metal oxyanions at redox–polymer interfaces are explored through in situ spatial-temporally resolved neutron reflectometry combined with large scale ab initio molecular dynamics. The selectivity of ReO4– vs MoO42– for two redox-metallopolymers, poly(vinyl ferrocene) (PVFc) and poly(3-ferrocenylpropyl methacrylamide) (PFPMAm) is evaluated. PVFc has a higher Re/Mo separation factor compared to PFPMAm at 0.6 V vs Ag/AgCl. In situ techniques show that both PVFc and PFPMAm swell in the presence of ReO4– (having higher solvation with PFPMAm), but do not swell in contact with MoO42–. Ab initio molecular simulations suggest that MoO42– maintains a well-defined double solvation shell compared to ReO4–. The more loosely solvated anion (ReO4–) is preferably adsorbed by the more hydrophobic redox polymer (PVFc), and electrostatic cross-linking driven by divalent anionic interactions could impair film swelling. Thus, the in-depth understanding of selectivity mechanisms can accelerate the design of ion-selective redox-mediated separation systems for transition metal recovery and recycling.
