Computational protocol: Robust water desalination membranes against degradation using high loads of carbon nanotubes

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[…] The activation energy barrier for chlorination of PA was studied by Nudged Elastic Band (NEB) calculations using three structural models: 1) plain PA, 2) SWCNT (5,5) + PA, and 3) graphene + PA, in order to evaluate the effect of sp2 hybridized carbon networks on the PA stability. The PA structure was built by one TMC and two MPD molecules, 44 atoms in total. The SWCNT (5,5) and graphene were both composed of 100 atoms. We performed structural relaxation for the three cases followed by the NEB20 simulations, which were performed by the Quantum Espresso package, considering van der Waals (vdW) interactions. During simulations, a plane wave basis set with 30 Ry cutoff energy and ultra-soft potential within PBE as exchange correlation interaction functional was considered, and the reaction path was set to ten images. For NEB simulation, we set the amide bond of PA + free chlorine in the initial state and the amide bond attached chlorine + free hydrogen atoms in final state. During NEB calculations, the initial and final states were also optimized by performing structural relaxation.Molecular dynamics on model PA and PA-CNT membranes were carried out as reported in the literature. Briefly, a PA matrix was simulated as interacting monomers (1,3-diaminobenzene or MPD, and 1,3,5-benzenetricarbonyl trichloride or TMC), using Lennard-Jones (LJ) and Coulomb type potentials for energy evaluations. Parametrized atom charges were previously determined with ANTECHAMBER 1.27 and AM1-BCC partial charges,. Interactions between carbon atoms from CNT and PA chains are determined by LJ only, as parametrized in the GAFF (force field). CNT interactions are described by the Tersoff potential and LJ potential among layers with cutoff radii set to 10.0 Å. For plain PA, a cell of 71.8 × 51.4 × 80.1 Å was filled with a PA molecule made with 360 and 282 MPD and TMC units respectively; isothermal-isobaric (NPT) ensemble relaxed for 2 ns, with a time step of 0.5 fs. For CNT-PA, the cell size was preserved, while reducing the number of PA molecules made with 291 and 228 MPD and TMC units, in order to maintain a constant density. The CNT was triple-walled [(15,15) > (10,10) > (5,5)] with a total of 2520 carbon atoms. Additionally, free volume (voids) determinations within the relaxed PA and CNT-PA cells were carried out with the Fpocket code, using probe spheres between 3.0 and 6.0 Å, 50 spheres as minimum clustering, and 4 neighboring spheres as interclustering criteria for free volume detection. Finally, 2D maps of potential for Cl− ions were calculated using:3Vi=∑j4εij[(σijrij)12−(σijrij)6]+∑jqiqj4πε0rijwhich defines the interaction potential Vi as a sum of vdW and Coulomb potentials. The first term is vdW interaction evaluated by LJ potential, and the second term is the Coulombic interaction. LJ parameter εij was 0.10 kcal mol−1 and σij was set to 4.40 Å for Cl. Charges of Cl, qi, are −1.0. The LJ interaction potentials for other atoms combinations were determined by Lorentz-Berthelot combination rules. The potential mapping surface for Na and Cl was calculated in the XY plane centered along the Z direction. […]

Pipeline specifications

Software tools Quantum ESPRESSO, fpocket
Application Mathematical modeling
Chemicals Carbon, Chlorine