Structure, property and transport mechanism of water and electrolytes in graphene nanopores
Advances in fabrication, measurement and characterization have led to intense research in the area of nanoporous membranes. Owing to its ultrathin thickness, graphene nanopores are promising candidates for various applications such as water desalination, single molecule sensing, etc. Understanding water and ion transport mechanisms and properties in membranes is essential to characterize and design membranes for various applications mentioned above. In nanopores, transport mechanisms can differ from the continuum theory and transport properties under confinement can be different from the bulk values. In such a situation, molecular dynamics simulation is a useful tool to observe molecular level details. In this thesis, structure, properties and transport mechanism of water and ions in graphene nanopores are investigated in detail to realize the full potential of graphene nanopores, using molecular dynamics simulations. Due to the unique water structure, confined in the radial direction and layered in the axial direction of the pore, water viscosity and slip length increase with a decrease in the pore radius, in contrast to water confined in a carbon nanotube. Due to the nanometer dimension of the pore, Reynolds number for pressure-driven water flow through graphene nanopores is very small and a linear relation between flow rate and applied pressure drop is observed. Hydrodynamic membrane length is introduced to effectively capture entrance and exit pressure losses. As the diameter of the pore increases, the water transport mechanism transitions from collective diffusion to frictional flow described by the modified Hagen–Poiseuille equation. Graphene membrane is shown to be ultra-efficient by comparing the permeation coefficient of graphene membrane to that of advanced membranes. Water transport through graphene is compared with water transport through thin carbon nanotube (CNT) membranes. For smaller diameter membranes, where single-file structure is observed, water flux is lower through the graphene membrane compared to that of the CNT membrane, primarily due to the frequent rupture of hydrogen bonding network and L/D defect-like water orientation in the graphene pore. For larger diameter pores, where the water structure is not single-file, graphene membranes provide higher water flux compared to CNT membranes. Furthermore, in thin CNT membranes, the water flux did not vary significantly with the thickness of the membrane. This result is explained by the pressure distribution and plug-like velocity distribution in the CNT. Finally, the static and dynamic properties of ions are investigated with and without an external electric field. Ion concentration in graphene nanopores sharply drops from the bulk concentration when the pore radius is smaller than 0.9 nm. Ion mobility in the pore is also smaller than bulk ion mobility due to the layered liquid structure in the pore-axial direction. The results show that a continuum analysis can be appropriate when pore radius is larger than 0.9 nm if pore conductivity is properly defined. Additionally, several orders of magnitude larger electro-osmotic water flow rate was observed in the subcontinuum regime (pore radius less than 0.9 nm) compared to that of porous alumina or carbon nanotube membranes. Since many applications of graphene nanopores, such as single molecule sensing and desalination, involve water and ion transport, the results presented here will be useful not only in understanding the behavior of water and ion transport but also in designing graphene nanopores for various applications.
Nanofluidics, Molecular dynamics simulation, Membrane transport
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign