Abstract
Osmosis is the key process in establishing versatile functions of cellular systems and enabling clean-water harvesting technologies. Membranes with single-atom thickness not only hold great promises in approaching the ultimate limit of these functions, but also offer an ideal test-bed to explore the underlying physical mechanisms. In this work, we explore diffusive and osmotic transport of water and ions through carbon nanotube and porous graphene based membranes by performing molecular dynamics simulations. Our comparative study shows that the cylindrical confinement in carbon nanotubes offers much higher salt rejection at similar permeability in osmosis compared to porous graphene. Moreover, chemical functionalization of the pores modulates the membrane performance by its steric and electrostatic nature, especially at small-size pores due to the fact that the optimal transport is achieved by ordered water transport near pore edges. These findings lay the ground for the ultimate design of forward osmosis membranes with optimized performance trade-off, given the capability of nano-engineering nanostructures by their geometry and chemistry.
Highlights
Osmosis is the key process in establishing versatile functions of cellular systems and enabling cleanwater harvesting technologies
Carbon nanostructures, including carbon nanotubes (CNTs) that naturally possess tubular pores[2,10], graphene where holes can be implanted through chemical or irradiation treatments[4,11,12], and graphyne webs[13,14,15,16] are attractive candidates for both forward and reverse osmosis (FO and RO) applications
Considering the relatively low osmotic pressure of a few megapascals in FO applications, two-dimensional (2D) materials such as graphene with nanosized pores is much favored due to its monatomic thickness and could be considered as the ultimate material in design[19]. In addition to their promising figures of merits for energy and environmental applications, carbon nanostructures with well-defined porous structures offer ideal platforms to explore the atomistic details of osmotic water transport as well, which is the basis for understanding biological functions of cells
Summary
Osmosis is the key process in establishing versatile functions of cellular systems and enabling cleanwater harvesting technologies. Considering the relatively low osmotic pressure of a few megapascals in FO applications, two-dimensional (2D) materials such as graphene with nanosized pores is much favored due to its monatomic thickness and could be considered as the ultimate material in design[19] In addition to their promising figures of merits for energy and environmental applications, carbon nanostructures with well-defined porous structures offer ideal platforms to explore the atomistic details of osmotic water transport as well, which is the basis for understanding biological functions of cells. Due to the facts we introduce above, porous carbon nanostructures are the promising candidates for this question and it would be interesting to explore their optimized performance by engineering their structures and chemistry To this end, we perform molecular dynamics (MD) simulations for a comparative study of diffusive and osmotic transport through membranes composed of porous graphene and CNTs (Fig. 1). The performance of FO applications based on these membranes is discussed in this work, which is demonstrated to outperform most of conventional membranes reported in the literature
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