Molecular dynamics simulations were employed to study the transport of water and ions through pores created on the basal plane of one graphene sheet (GS). Graphene pore diameters ranged from 7.5 to 14.5 Å. Different pore functionalities were considered, obtained by tethering various functional groups to the terminal carbon atoms. The ease of ion and water translocation across the pores was monitored by calculating the potential of mean force along the direction perpendicular to the GS pore. The results indicate that effective ion exclusion can be achieved only using nonfunctionalized (pristine) pores of diameter ~7.5 Å, whereas the ions can easily penetrate pristine pores of diameters ~10.5 and 14.5 Å. Carboxyl functional groups can enhance ion exclusion for all pores considered, but the effect becomes less pronounced as both the ion concentration and the pore diameter increase. When compared to a carbon nanotube of similar pore diameter, our results suggest that GS pores functionalized with COO(-) groups are more effective in excluding Cl(-) ions from passing through the membrane. Our results suggest that narrow graphene pores functionalized with hydroxyl groups remain effective at excluding Cl(-) ions even at moderate solution ionic strength. The results presented could be useful for the design of water desalination membranes.

In this work, using molecular dynamics simulations, we demonstrate that it is possible to significantly reduce the Kapitza resistance [P. L. Kapitza, J. Phys. (USSR) 4, 181 (1941)] at the graphene sheet-liquid octane interface by appropriately functionalizing the graphene sheets. The key concept is that the functional groups, to be effective, must show vibrational modes compatible with those of the organic matrix. Because functionalizing graphene sheets at their edges should not compromise their exceptional intrinsic thermal-transport properties, our results suggest a practical recipe for manufacturing high-thermal-transport polymeric nanocomposites.

Molecular dynamics simulations have been employed to study the structural properties of aqueous electrolytes confined within graphene pores. The effects of pore size and graphene surface charge density were quantified by calculating ionic density profiles within the pores and pore-bulk partition coefficients. Carbon-slit pores of width 0.9, 1.2, and 1.6 nm were considered. The graphene surfaces were charged with densities ranging from 0 (neutral pore), 20, 30, and 40 μC/cm 2 , simulating various applied voltages. Aqueous solutions of NaCl at 1.5À1.6 M concentrations were considered at ambient conditions. When the graphene sheets are neutral, most electrolytes remain outside of the pores. The few sodium and chloride ions that are found within the pores remain preferentially at the center of pores, where they can be hydrated. As the graphene surface charge density increases, more Na + and Cl À enter the pores. At the maximum graphene surface charge density considered (40 μC/cm 2 ) the ionic concentration within the pores can be ∼10 times as high as that outside of the pores, with the maximum partition coefficient obtained when the pore width is 1.2 nm. In all pores, when the surface charge density is 40 μC/cm 2 the ions move toward the charged graphene surfaces because of counterion condensation effects, at the expense of losing part of their hydration shells. In some instances our results reveal the formation of multiple layers of adsorbed electrolytes near a charged graphene surface. These layers appear to form because of a number of effects including surfaceÀion electrostatic interactions, hydration phenomena, and also ionÀion correlations, especially at the maximum surface charge densities considered within the 1.2 nm wide pores. The results presented are useful for designing graphene-based electric double layer capacitors.

Graphene sheets, one-atom-thick layers of carbon atoms, are receiving enormous scientific attention because of extraordinary electronic and mechanical properties. These intrinsic properties will lead to innovative nanocomposite materials that could be used to produce novel transistors and thermally conductive polymeric materials. Such applications are currently hindered by the difficulty of producing large quantities of individual graphene sheets and by the propensity of these nanoparticles to agglomerate when dispersed in aqueous and/or organic matrixes. We report here molecular dynamics simulations for pristine and functionalized graphene nanosheets of 54 and 96 carbon atoms each dispersed in liquid organic linear alkanes (oils) at room conditions. For the first time, our results show that, although pristine graphene sheets agglomerate in the oils considered, graphene sheets functionalized at their edges with short branched alkanes yield stable dispersions. We characterized the simulated systems by computing radial distribution functions between the graphene sheets centers of mass, pair potentials of mean force between the graphene sheets in solution, and site-site radial distribution functions. The latter were used to determine the preferential orientation between approaching graphene sheets and the packing of the organic oils on the graphene sheets. Our results are useful not only for designing practical recipes for stabilizing graphene sheets in organic systems, but also for comparing the molecular mechanisms responsible for the graphene sheets aggregation to those that stabilize graphene sheets-containing dispersions, and for controlling the coupling between organic oils and graphene sheets used as fillers. In particular, we demonstrated that excluded-volume effects, generated by the branched architecture of the functional groups grafted on the graphene sheets, are responsible for the stabilization of small graphene sheets in the organic systems considered here.

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