(487h) Finite-Size Effects in Molecular Dynamics Simulations of Ion Transport through Nanoporous Membranes

The ability of porous membranes to impede the transport of certain ions is fundamental to procedures such as reverse osmosis desalination as well as numerous biological and chemical separation processes. The passage of undesirable ions through such membranes can, however, occur over timescales that are too long to be accessible to conventional molecular simulation techniques. In recent years, nonequilibrium molecular dynamics (MD) simulations augmented with path sampling techniques such as forward flux sampling (FFS) [1] have emerged as key tools for exploring these processes due to their ability to access arbitrarily long transport timescales while providing detailed molecular-level mechanistic information about the transport process [2]. One of the under-explored aspects of such simulations is the extent by which they are impacted by the size of the simulation box. Using nonequilibrium MD alongside FFS, we simulate ion transport through several graphitic membranes with a single sub-nm cylindrical pore but with different membrane cross-sectional surface areas and find the ionic passage times to be extremely sensitive to systems size. We attribute ­­­­these finite-size effects to ion-ion interactions and variations in charge density within the feed compartment during ion transport and present a simple scaling relationship which allows for the prediction of passage times in the thermodynamic limit, i.e., for systems with infinitely large membrane cross-sectional areas. These results are of interest, because limits on computational resources generally necessitate utilizing periodic boundary conditions and simulating systems that are considerably smaller than their experimental counterparts. Our finidngs also warrant caution in interpreting salt rejection rates reported in computational studies of desalination and membrane separation.

[1] Hussain, Haji-Akbari, J. Chem. Phys., 152: 060901 (2020).

[2] Malmir, Epsztein, Elimelech, Haji-Akbari, Matter, 2: 735-750 (2020).