Molecular dynamics simulations of nafion and sulfonated poly ether sulfone membranes II. Dynamic properties of water and hydronium.

We measured the self-diffusion coefficients of water in a Nafion membrane and two sulfonated polyethersulfone (SPES) membranes with varying ion-exchange capacities (IEC) in terms of relative humidity using the pulse field gradient NMR (PFG-NMR) technique. The self-diffusion coefficients were plotted against the number of water molecules per sulfonic acid group, λ, and compare these values with the results of molecular dynamics (MD) simulations. Classical MD simulations for all membranes were carried out using a consistent force field at λ = 3, 6, 9, 12, and 15. The dynamic properties of water (H(2)O) and hydronium (H(3)O(+)) on a molecular level were estimated as self-diffusion coefficients and residence times around a sulfonate group ([Formula: see text]). The diffusion coefficients of H(2)O and H(3)O(+) followed the order, Nafion > SPES with IEC = 1.4 > SPES with IEC = 1.0 > SPES with IEC = 0.75, which agreed with the experimental data. The residence time distribution of H(2)O around [Formula: see text] in Nafion was in the range of 1-6 ps, whereas H(2)O in the SPES exhibited a residence time of greater than 20 ps.