Molecular dynamics simulation of hydration and free energy of ions in nanochannels of polyelectrolyte threaded metal organic framework and the impacts on selective ion transport

Abstract

Hierarchical porous materials constructed by threading polyelectrolyte in metal organic framework (MOF) display superior ion exchange behavior. To understand the ion exchange and ion transport mechanism, we conduct molecular dynamics simulations of ions inside the nanochannel of a MOF that is threaded with a polyelectrolyte (polyelectrolyte∼MOF), specifically the poly(sodium vinyl sulfonated-co-acrylic acid)∼MIL-53(Al). We compute the free energy change and hydration number for four representative ions, Na+, K+, Cl− and Mg2+ in the absence and presence of poly(sodium vinyl sulfonated-co-acrylic acid) in a channel of MIL-53(Al). The free energy barrier to ion entry, and the interaction between ion and polyelectrolyte help to understand the selectivity in ion transport. The negative charge of polyelectrolyte can exert a repulsion to anion and attract cation in the neighboring channel of MOF, as indicated by the lower energy valley for monovalent cations Na+ and K+, and higher energy barrier for Cl−. The second hydration shell explains size-selective prohibition of the transport of divalent Mg2+ in the nanochannel of MIL-53(Al). Our results demonstrate that the ion selectivity is attributed to the synergistic effect of charge and size-exclusion of the polyelectrolyte∼MOF structure.