Salt parameterization can drastically affect the results from classical atomistic simulations of water desalination by MoS₂ nanopores

Abstract

Water scarcity is a reality in our world, and scenarios predicted by leading scientists in this area indicate that it will worsen in the next decades. However, new technologies based in low-cost seawater desalination can prevent the worst scenarios, providing fresh water for humanity. With this goal, membranes based in nanoporous materials have been suggested in recent years. One of the materials suggested is MoS 2 , and classical Molecular Dynamics (MD) simulation is one of the most powerful tools to explore these nanomaterials. However, distinct Force Fields employed in MD simulations are parameterized based on distinct experimental quantities. In this paper, we compare two models of salt that were build based on distinct properties of water-salt mixtures. One model fits the hydration free energy and lattice properties, the second fits the crystal density and the density and the dielectric constant of water and salt mixtures. To compare the models, MD simulations for salty water flow through two nanopores sizes were used -one pore big enough to accommodate hydrated ions, and one smaller in which the ion has to dehydrate to enter, and two rigid water models from the TIP4P family -the TIP4P/2005 and TIP4P/ε. Our results indicate that the water permeability and salt rejection by the membrane are more influenced by the salt model than by the water model, especially for the narrow pore. In fact, completely distinct mechanisms were observed, and they are related to the characteristics employed in the ion model parameterization. The results show that not only the water model can influence the outcomes, but the ion model plays a crucial role.

Confinement-Controlled Aqueous Chemistry within Nanometric Slit Pores

“…We close by stressing that nagging problems may appear when using force fields to describe aqueous ion solutions in slit pores based on molecular dynamics simulations. For MoS 2 /graphene nanopore setups mimicking desalination devices, it has been found that both water permeability and salt rejection by the membrane are most sensitive to the parametrization of the ion model, especially for narrow pores, up to the point that qualitatively distinct mechanisms have been observed depending on the salt model …”

“…For MoS 2 /graphene nanopore setups mimicking desalination devices, it has been found that both water permeability and salt rejection by the membrane are most sensitive to the parametrization of the ion model, especially for narrow pores, up to the point that qualitatively distinct mechanisms have been observed depending on the salt model. 161…”

Molecular fluid flow in MoS2 nanoporous membranes and hydrodynamics interactions

“…MoS 2 has nanopores with 0.97 nm diameter (defined as the center to center distance of atoms), which is the minimum size that does not show the ion blockage effect. 18 The simulations were performed using the Large-scale Atomic/ Molecular Massively Parallel Simulator (LAMMPS). 37 The particles interact with each other via Lennard-Jones (LJ) and Coulomb potentials.…”

“…One suited simulation branch to better understand the desalination process is to mimic the reverse osmosis desalination system at the nanoscale. [8][9][10][11][12][13][14][15][16][17][18][19][20] This technique enters in the scope of Non-Equilibrium Molecular Dynamics (NEMD). In addition, its procedure has been used to get insights into designing new membrane materials for desalination.…”

How Grain Boundaries and Interfacial Electrostatic Interactions Modulate Water Desalination via Nanoporous Hexagonal Boron Nitride

“…To calculate the interactions between unlike atoms in the system, geometric mean combining rules, σ ij = (σ i σ j ) 1/2 and ε ij = (ε i ε j ) 1/2 , were used, where σ represents the distance at which the interatomic LJ potential is zero and ε represents the LJ well-depth parameter. In Section S4, we present a more detailed discussion on the choice of force fields for hBN, water molecules, and salt ions (see, e.g., refs and ).…”