Molecular Dynamics Insights into the Structural and Water Transport Properties of a Forward Osmosis Polyamide Thin-Film Nanocomposite Membrane Modified with Graphene Quantum Dots

An
approach combining molecular dynamics (MD) simulations and laboratory
experiments was applied to provide new theoretical insights into the
chemical structure of polyamide (PA) thin-film composite (TFC) membranes
modified with graphene quantum dots (GQDs). Interaction energies,
fractional free volumes, mean-square displacements, densities, and
water diffusion coefficients were computed for PA and four likely
chemical structures of the GQD-embedded PA membranes. These theoretical
results aided with experimentally measured water fluxes allowed for
determining the most likely structure of the GQD–PA membrane.
The compatibility of the GQDs and PA chains was found to be due to
the formation of hydrogen and covalent bonds to m-phenylenediamine units. The modified membrane has a higher water
diffusivity but a lower overall free volume, compared to the pristine
PA membrane. MD simulations in concert with laboratory experiments
were found to provide a good understanding of the relationship between
the microscopic characteristics and macroscopic transport properties
of TFC membranes.