Polarizable Molecular Simulations Reveal How Silicon-containing Functional Groups Govern the Desalination Mechanism in Nanoporous Graphene

TL;DR

This study uses polarizable molecular dynamics simulations to understand how silicon-based functional groups on nanoporous graphene influence desalination, revealing distinct ion rejection mechanisms and flux behaviors.

Contribution

Developed a detailed polarizable force field for simulating functionalized graphene nanopores, elucidating the molecular mechanisms governing ion rejection and water flux in desalination.

Findings

Si(OH)$_2$$ pores reject ions via electrostatic repulsion, especially Cl$^-$.

SiH$_2$$ pores exclude ions based on hydration layer size and flexibility.

Water flux correlates with accessible pore area but is lower than experimental estimates.

Abstract

We report a molecular dynamics (MD) simulation study of reverse osmosis desalination using nanoporous monolayer graphene passivated by SiH$_2$ and Si(OH)$_2$ functional groups. A highly accurate and detailed polarizable molecular mechanics force field model was developed for simulating graphene nanopores of various sizes and geometries. The simulated water fluxes and ion rejection percentages are explained using detailed atomistic mechanisms derived from analysis of the simulation trajectories. Our main findings are: (1) The Si(OH)$_2$ pores possess superior ion rejection rates due to selective electrostatic repulsion of Cl$^-$ ions, but Na$^+$ ions are attracted to the pore and block water transfer. (2) By contrast, the SiH$_2$ pores operate via a steric mechanism that excludes ions based on the size and flexibility of their hydration layers. (3) In absence of ions, water flux is directly proportional to the solvent accessible area within the pore; however, simulated fluxes are lower than those inferred from recent experimental work. We also provide some hypotheses that could resolve the differences between simulation and experiment.