Graphene can be considered as an ideal membrane since its thickness is only one carbon diameter. In this study, using molecular dynamics simulations, we investigate water transport through a porous graphene membrane and compare the results with water transport through thin (less than 10 nm in thickness/ length) carbon nanotube (CNT) membranes. For smaller diameter pores, where a single-file water structure is obtained, CNT membranes provide higher water flux compared to graphene membranes. 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. We explain the results through a detailed analysis considering pressure distribution, velocity profiles, and potential of mean force. This work opens up opportunities for graphene-based membranes in molecular sieving, water filtration, fuel cells, and so forth. SECTION Surfactants, Membranes

An ultrathin graphene membrane is a promising candidate for various applications such as gas separation, water purification, biosensors, etc. In this study, we investigate water transport mechanisms and hydrodynamic properties such as water flux, pressure variation, velocity, viscosity, slip length, etc. 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. 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 a carbon nanotube and an ultrathin silicon membrane. We envision that the study presented here will help to understand and design various membrane separation processes using graphene membrane.

We investigate reverse osmosis through commonly used polymeric and advanced inorganic nanotube based semipermeable membranes by performing nonequilibrium molecular dynamics simulations. Simulations indicate that there is a significantly higher water flux through boron-nitride nanotube ͑BNNT͒ and carbon nanotube ͑CNT͒ compared to a polymethyl methacrylate ͑PMMA͒ pore, and a slightly higher water flux through BNNT as compared to CNT. The calculated permeation coefficient is in reasonable agreement with the theoretical single-file "hopping" model. Potential of mean force analysis indicates that the irregular nature of PMMA pore surface can cause significant localized energy barriers inside the pore, thereby reducing the water flux.

Graphene nanopore is a promising device for single molecule sensing, including DNA bases, as its single atom thickness provides high spatial resolution. To attain high sensitivity, the size of the molecule should be comparable to the pore diameter. However, when the pore diameter approaches the size of the molecule, ion properties and dynamics may deviate from the bulk values and continuum analysis may not be accurate. In this paper, we investigate the static and dynamic properties of ions with and without an external voltage drop in sub-5-nm graphene nanopores using molecular dynamics simulations. 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. Our results show that a continuum analysis can be appropriate when the pore radius is larger than 0.9 nm if pore conductivity is properly defined. Since many applications of graphene nanopores, such as DNA and protein sensing, involve ion transport, the results presented here will be useful not only in understanding the behavior of ion transport but also in designing bio-molecular sensors.

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