Modelling the Uptake, Transport and Separation of Multiply Charged Ions Through MoS₂ Membranes

This talk will present recent studies of the Schatz group in collaboration with the Chong Liu and her group concerning the uptake and transport of multiply charged ions (Cu²⁺, Pb²⁺, lanthanides) through membranes that consist of stacked MoS₂ layers. Two levels of theory have been considered: electronic structure calculations using VASP, and molecular dynamics using empirical potentials. The electronic structure calculations involve studies of ion hydration both in the bulk and inside the membrane with models that contain 10-60 explicit water molecules, along with implicit water. These calculations are able to capture important features of the experiments concerning ion-solvent coordination number, membrane interlayer separation, relative binding energy of ions in the membrane, and how hydration varies with location of the ion and whether the MoS₂ is or isn’t functionalized with acetate. This leads to a realistic picture of the uptake experiments. Our MD studies of ion transport and separation are able to provide qualitative understanding of the dependence of ion dehydration that takes place as the ion enters the membrane, and dependence of diffusion on strength of interaction of the ion with the membrane, but the empirical potentials are apparently not accurate enough to capture subtle effects that reflect chemical interactions of the ions with the membrane, especially concerning variation of the interlayer separation while the ion diffuses, and the transport of mixtures of different ions.