Methane and carbon dioxide adsorption on edge-functionalized graphene: A comparative DFT study

With a view towards optimizing gas storage and separation in crystalline and disordered nanoporous carbon-based materials, we use ab initio density functional theory calculations to explore the effect of chemical functionalization on gas binding to exposed edges within model carbon nanostructures. We test the geometry, energetics, and charge distribution of in-plane and out-of-plane binding of CO2 and CH4 to model zigzag graphene nanoribbons edge-functionalized with COOH, OH, NH2, H2PO3, NO2, and CH3. Although different choices for the exchange-correlation functional lead to a spread of values for the binding energy, trends across the functional groups are largely preserved for each choice, as are the final orientations of the adsorbed gas molecules. We find binding of CO2 to exceed that of CH4 by roughly a factor of two. However, the two gases follow very similar trends with changes in the attached functional group, despite different molecular symmetries. Our results indicate that the presence of NH2, H2PO3, NO2, and COOH functional groups can significantly enhance gas binding with respect to a hydrogen-passivated edge, making the edges potentially viable binding sites in materials with high concentrations of edge carbons. To first order, in-plane binding strength correlates with the larger permanent and induced dipole moments on these groups. Implications for tailoring carbon structures for increased gas uptake and improved CO2/CH4 selectivity are discussed.Comment: 12 pages, 7 figure

Investigations on the melting and bending modulus of polymer grafted bilayers using dissipative particle dynamics

Understanding the influence of polymer grafted bilayers on the physicomechanical properties of lipid membranes is important while developing liposomal based drug delivery systems. The melting characteristics and bending moduli of polymer grafted bilayers are investigated using dissipative particle dynamics simulations as a function of the amount of grafted polymer and lipid tail length. Simulations are carried out using a modified Andersen barostat, whereby the membrane is maintained in a tensionless state. For lipids made up of four to six tail beads, the transition from the low temperature Lβ phase to the Lα phase is lowered only above a grafting fraction of Gf=0.12 for polymers made up of 20 beads. Below Gf=0.12 small changes are observed only for the HT4 bilayer. The bending modulus of the bilayers is obtained as a function of Gf from a Fourier analysis of the height fluctuations. Using the theory developed by Marsh et al. [Biochim. Biophys. Acta 1615, 33 (2003)] for polymer grafted membranes, the contributions to the bending modulus due to changes arising from the grafted polymer and bilayer thinning are partitioned. The contributions to the changes in κ from bilayer thinning were found to lie within 11% for the lipids with four to six tail beads, increasing to 15% for the lipids containing nine tail beads. The changes in the area stretch modulus were also assessed and were found to have a small influence on the overall contribution from membrane thinning. The increase in the area per head group of the lipids was found to be consistent with the scalings predicted by self-consistent mean field results

Enhancing the hydrophobic effect in confined water nanodrops

The distribution of hydrophobic solutes, such as methane, enclosed in a nanosized water droplet contained in a reverse micelle of diameter 2.82 nm is investigated using Monte Carlo simulations. The effect of the hydrophobic solute's atomic diameter on the solute-solute potential of mean force is also studied. The study reveals that confinement has a strong influence on the solute's tendency to associate. The potential of mean force exhibits only a single minimum, indicating that the contact pair is the only stable configuration between solutes. The solvent-separated pair that is universally observed for small solutes in bulk water is conspicuously absent. This enhanced hydrophobic effect is attributed to the lack of sufficient water to completely hydrate and stabilize the solvent-separated configurations. The study is expected to be important in understanding the role of hydrophobic forces during protein folding and nucleation under confinement

Mutual diffusion in a binary Ar-Kr mixture confined within zeolite NaY

Molecular dynamics investigations of the mutual diffusion coefficients in an Ar-Kr mixture confined in the zeolite NaY are reported. Velocity auto- and cross correlations were computed at two different temperatures (200 and 600 K). The importance of the appropriate choice of reference frame while evaluating the time correlation functions is illustrated for argon in the zeolite NaY. Mutual diffusivities in the mixture were obtained in the barycentric reference frame. Recently, Zhou and Miller showed that the distinct diffusivity Dd is zero for the Ar-Kr mixture in bulk. On confinement, it is seen that at 200 K the ratio R=D11/Ds=0.77, where D11 is the mutual diffusivity and Ds is the mixture self-diffusivity. However, at 600 K, R=0.97, implying that the contribution from distinct diffusion is only slightly negative. The large negative Dd at 200 K could be attributed to strong localization of Ar and Kr in the physisorption sites within the zeolite cages. Analysis of error bars and an efficient computational algorithm for evaluation of the velocity cross correlation function are also presented. The results have implications in biology, chemistry, and other situations where transport of confined mixtures is encountered