The effect of temperature on water desalination through two-dimensional nanopores.

Two-dimensional (2D) materials such as graphene, molybdenum sulfide, and hexagonal boron nitride are widely studied for separation applications such as water desalination. Desalination across such 2D nanoporous membranes is largely influenced by the bulk transport properties of water, which are, in turn, sensitive to the operating temperature. However, there have been no studies on the effect of temperature on desalination through 2D nanopores. We investigated water desalination through hydrogen functionalized graphene nanopores of varying pore areas at temperatures 275.0 K, 300.0 K, 325.0 K, and 350.0 K. The water flux showed a direct relation with the diffusion coefficient and an inverse relation with the hydrogen-bond lifetime. As a direct consequence, the water flux was found to be related to the temperature as per the Arrhenius equation, similar to an activated process. The results from the present study improve the understanding on water and ion permeation across nanoporous 2D materials at different temperatures. Furthermore, the present investigation suggests a kinetic model, which can predict the water and ion permeation based on the characteristics of the nanopore.

Water flow in carbon nanotubes: the role of tube chirality.

We investigated the effects of the chirality of carbon nanotubes (CNTs) on water transport using molecular dynamics simulations. For the study, we considered CNTs with similar diameter and varying chiralities, obtained by altering the chiral indices (n,m) of the nanotubes. The tubes with an armchair (n = m) structure show the maximum streaming velocity, flux, flow rate enhancement and slip length, whereas the corresponding values are lower for chiral (n≠m) tubes, and are the lowest in zigzag (m = 0) CNTs. The difference in flow rates with varying tube structures can be primarily attributed to the alteration in potential energy landscape experienced by the water molecules, leading to changes in the friction coefficient at the fluid-solid interface. The water molecules experienced the least resistance to flow in an armchair tube, while the force exerted by the CNT surface on the water molecules increased monotonically with the change in the CNT type to chiral and then to zigzag. The chirality effects on water transport are, however, found to decrease with an increase in tube diameter. Furthermore, an analysis of the influence of the CNT type on ion (Na+ or Cl-) transport in water-filled CNTs showed the interaction energy of ions with water to be much higher than that with the CNT surface, demonstrating minimal dependence of ion transport on the chiral structure. Hence, the tube chirality should be considered an ineludible factor in controlling the water transport through CNTs and in the designing of novel devices in nanotechnology.

Water desalination using graphene nanopores: influence of the water models used in simulations.

Molecular dynamics simulations are widely employed to analyze water and ion permeation through nanoporous membranes for reverse osmosis applications. In such simulations, water models play an important role in accurately reproducing the properties of water. We investigated the water and ion transport across a hydroxyl (OH) functionalized graphene nanopore using six water models: SPC, SPC/E, SPC/Fw, TIP3P, TIP4P, and TIP4P/2005. The water flux thus obtained varied up to 84% between the models. The water and ion flux showed a correlation with the bulk transport properties of the models such as the diffusion coefficient and shear viscosity. We found that the hydrogen-bond lifetime, resulting from the partial charges of the model, influenced the flux. Our results are useful in the selection of a water model for computer simulations of desalination using nanomembranes. Our findings also suggest that lowering the hydrogen-bond lifetime and enhancing the rate of diffusion of water would lead to enhanced water/ion flux.