Extending Reverse Non-Equilibrium Molecular Dynamics for Mass and Charge Transport in Fluids

With a pressing water shortage and prevalent, long-lasting droughts, desalination of seawater by reverse osmosis provides an attractive technology to counteract these worsening conditions. Reverse osmosis uses an applied hydraulic pressure to force a salt solution through a semi-permeable membrane, separating the salt ions from the water. Because of the nature of ion transport in aqueous solutions, molecular dynamics (MD) positions itself well for the study of these systems. A useful MD simulation technique for studying transport in bulk and interfacial systems, reverse nonequilibrium molecular dynamics (RNEMD), could be applied to the study of water desalination if an algorithm were developed which imposes an unphysical flux across an interface, while the system responds by creating a concentration difference across the same membrane. In this work, we present a novel algorithm that allows particle positions to be swapped between non-adjacent regions of a simulation box, resulting in an applied particle flux. We show that the scaled particle flux (SPF) method accurately calculates diffusion coefficients while maintaining energy and linear momentum constraints without the need for an external thermostat. As a result, we can map the temperature dependence of diffusion when used in tandem with a thermal flux. The development of the SPF-RNEMD algorithm, and the consequences of this development procedure are explored for our open-source molecular dynamics engine, OpenMD. Here we show how the method can be applied to charge separation techniques in ionic solutions. We also applied the method to the interfacial systems of nanoporous graphene in molecular and aqueous salt solutions. In the molecular case, permeabilities are computed by imposing a molecular flux between regions on opposite sides of the membrane and measuring both the hydraulic and osmotic pressures that develop as a result of this flux. For aqueous salt solutions, the effect of pore size on salt permeabilities is studied; however, because of the concentration dependence of the method, our findings suggest that the use of a solvent flux in a desalination study may yield better results.