Computational Investigation of Structure-Selectivity Relationship in Membranes Using Non-Equilibrium Molecular Dynamics Simulations and Advanced Path Sampling Techniques

The ability of semipermeable membranes to selectively impede the transport of undesirable ions and molecules is key to many applications, from desalination and gas separation, to biological membrane transport. For instance, membranes that are only permeable to water and reject most other ions and molecules are used in water desalination. Designing more selective membranes requires characterizing the kinetics and mechanism of the transport of unwanted entities. The timescales associated with such processes, however, can be too long to be accessible to conventional molecular dynamics (MD) simulations. Moreover, the separation of timescales between the transport of desirable entities (such as water) and undesirable entities (such as salt) will result in considerable changes in concentration throughout an MD trajectory, which can skew the kinetics and mechanism of transport in ways that are difficult to quantify and correct. Recently, we utilized jumpy forward flux sampling (jFFS) [1] and non-equilibrium MD to study pressure-driven ion transport through graphitic nanoporous membranes [2]. This approach not only allows us to accurately estimate arbitrarily long mean passage times, but also to compute fluxes and permeabilities within a pseudo-equilibrium ensemble in which both reservoirs are at different— but almost constant— chemical potentials. In this presentation, we will discuss the technical aspects of this approach in the context of a few more examples, with the goal of addressing interesting questions about ion transport and selectivity. We will also provide a framework for correcting for finite size artifacts in studies of ion transport through ultra-selective membranes [3].<br/>[1] Haji-Akbari, J. Chem. Phys., 149: 072303 (2018).<br/>[2] Malmir, et al., Matter, 2: 735 (2020).<br/>[3] Shoemaker, et al., Under preparation.