Fig. 4

MD simulations of ion transport in UiO-66 channels. a The simulation cartoon shows UiO-66 cavities filled with water molecules (sky blue spheres), and they are connected via narrow windows. For clarity, UiO-66 is shown as a green wireframe. K⁺ and F⁻ ions are represented by red and dark blue spheres, respectively. b The mobility of Cl⁻ and F⁻ ions in UiO-66. F⁻ mobility is presented as a function of the strength of F–Zr LJ potential at 0.0457 M and 3.748 M, respectively. At 3.748 M, F⁻ mobility is enhanced by around 10 times compared with that at 0.0457 M. Error bars represent the standard deviation of calculations of 5 samples. c, e, g Radial distribution function of water molecules around anions sitting at cavity center and window center. Cl⁻ ions have a weaker second hydration shell compared with F⁻ ions at 0.0457 M when sitting at cavity center (c, e). However, at 3.748 M, F⁻ ions have a relative smaller hydration shell as part of water molecules within second shell were shared with neighbored F⁻ ions (g). d, f, h The simulation cartoon shows the arrangement of water molecules around anions sitting at cavity center (left) and window center (right), corresponding to c, e and g, respectively. Water molecules are red (O) and white (H), and ions (F⁻ or Cl⁻) are dark blue. F⁻ ions have two hydration layers at cavity center, and the second hydration shell should peel off at window center at 0.0457 M (d). Cl⁻ ions do not have a distinct second hydration shell (f), thus, smaller dehydration energy would be required for transport through windows compared with F⁻ ions at 0.0457 M. At 3.748 M, water molecules of F⁻ second hydration shell are shared with neighbored F⁻ ions (h)