•Functionalized nanoporous graphene membranes could realize the removal of heavy metal ions.•Water permeation is correlated with the interfacial pore chemistry and confined structure.•Interfacial electrostatic interaction and ion dehydration effect cooperatively determine ion rejection.Currently, elimination of heavy metal ions from contaminated water resource is an important issue in environmental protection. In this study, we simulated the separation performance of heavy metal ions using nanoporous graphene surfaces as reverse osmosis membranes with functionalized groups (boron, nitrogen and hydroxyl groups). We show these nanoporous graphenes could realize high water permeation and ion rejection for various conditions. The simulated water permeability is 2–5 orders of magnitude greater than that of currently commercial membranes. The interfacial water structures and flow velocity of water molecules within the nanopores were characterized. The calculations of the potential of mean force reveal water molecules generally face lower free energy barrier than ions when passing through graphene pores. The free energy barriers for ions can be explained as the combining contributions from the ion dehydration effect and the surface electrostatic interaction. Overall, the functionalized nanoporous graphene membranes exhibit potential application in the removal of heavy metal ions, and meanwhile our simulation results provide new insights into the ion rejection mechanism.Download high-res image (108KB)Download full-size image

Multilayer graphene oxide (GO) is an attractive candidate for new applications in nanoelectromechanical materials and structural reinforcement nanocomposites due to its strong mechanical properties. In this study, the mechanical properties and failure mechanism of multilayer GO nanosheets were studied by non-equilibrium molecular dynamics simulation. The simulated Young's modulus, fracture stresses, and fracture strains were found to be consistent with the experimentally measured values. The effects of the surface oxidation content of GO and the stacking layer number on these mechanical properties were investigated. The oxidation content has a larger influence on the mechanical properties compared with the layer number. The failure of multilayer GO nanosheets undergoes a relatively slow cracking process due to the existence of functional groups and the stacking layers. There appears to be different two-dimensional stress distributions on multilayer GO sheets from the outer layer to inner layer. The Young's modulus and the fracture strength of the middle layer are generally larger than those in the outer layer. The fracture of the outside GO sheet begins first, and then the failure of the inner GO sheet occurs with a delayering process. The simulation result is expected to improve understanding of the mechanical behavior of multilayer GO nanosheets.

A frontal chromatographic technique was used to measure the adsorption isotherms of benzaldehyde and benzyl alcohol onto a polymeric resin in supercritical CO2 (scCO2). The effect of temperature and pressure (density) on adsorption behavior was investigated. It was observed that benzyl alcohol has stronger adsorption than benzaldehyde. The desorption of benzaldehyde using scCO2 from the polymeric resin adsorbent was studied under the same temperature and pressure ranges. The local equilibrium theory was found to reasonably predict the desorption profiles. A phenomenological statistical thermodynamic model combined with classic Peng–Robinson equations of state was used to correlate the adsorption equilibrium isotherms of the solutes from scCO2. This theoretical model with three parameters is able to describe the adsorption behavior over wide temperature and pressure ranges.

•Kinetics model was developed for adsorption/desorption process of benzaldehyde in scCO2.•The theoretical model can satisfactorily describe the experimental data with one fitting parameter.•Mass transfer mechanism within pore medium was clarified from the parameter analysis.•Molecular pore diffusion is the rate-controlling step in the scCO2-based process.In this work, a kinetics model was developed for the adsorption and desorption processes of benzaldehyde and benzyl alcohol on the polymeric resin adsorbent in supercritical carbon dioxide (scCO2). This kinetics model takes into account adsorption equilibrium, internal diffusion in porous medium, mass transfer between fluid-sold interfaces, and axial dispersion in supercritical fluid. The breakthrough curves measured in a fixed-bed have been fitted by this model under various operational conditions. This model uses the intraparticle diffusion coefficient as the fitting parameter and other transfer parameters can be calculated through current correlation equations. A satisfactory agreement between the modeling results and the experimental data has been obtained with just one fitting parameter. The effect of temperature, pressure, and concentration on the intraparticle diffusion coefficient has been investigated and the molecular pore diffusion within the solid adsorbent is found to be the rate-controlling step for the overall adsorption process. Moreover, the theoretical model has also been demonstrated to reasonably predict the desorption behavior for the two solutes in scCO2. Our work presents an effective and concise theoretical framework for adsorption/desorption kinetics in scCO2 fluid.Download high-res image (164KB)Download full-size image

Graphyne has been proposed as a distinctive molecular sieving membrane due to its intrinsic nanoscale pores and single-atom thickness. However, this novel application requires a precise quantification and understanding of the molecular interaction at graphyne interfaces, which can modulate molecular transport across graphyne. Herein, interfacial adsorption and permeation of ethanol–water mixtures on graphynes are studied by a multiscale simulation strategy, in which dispersion-corrected density functional theory (DFT-D) and classical molecular dynamics (MD) are combined. Our results show that graphyne possesses differential surface affinities with ethanol and water, provoking a preferential adsorption layer of ethanol. The adsorption on the graphyne surface is dominated by attractive dispersion force, even for polar water molecules. As a joint function of ethanol-rich segregation adsorption on graphyne and preferred pore occupation of ethanol, polyporous graphyne with a suitable pore size is envisioned to act as an alcohol-permselective membrane. Our simulation results present new insights into interfacial interaction and have an impact on the promising application of two-dimensional graphyne membranes.

The interfacial friction of fluid within nanoscale pores is important to nanofluidic devices and processes. Herein, molecular dynamics simulations have been used to study the interfacial flow resistance of ethanol–water mixtures confined within graphene-based nanochannels. The friction coefficients of the mixtures were investigated by considering the effects of slit pore width and mixture composition. The simulated results show that the flow friction coefficient is sensitive to the graphene slit pore size for ethanol-containing solution systems. In particular, the mixture composition has a significant impact on the friction coefficients for the mixture in 7–10 Å nanoslits, while the composition dependence of friction coefficients becomes weak at larger pore widths. In addition, qualitative theoretical analysis has been carried out to reveal the molecular origin of mixture friction behavior. The ethanol–wall interaction accounts for the major role on the mixture friction coefficients. The changing behavior of mixture friction coefficient is caused by the joint effects from the interfacial ethanol density and the potential energy barrier felt by ethanol molecules.

Layer-by-layer assembled graphene oxide (GO) has been considered as a high-efficiency novel membrane material. However, its performance of water permeation and ion rejection remains largely unresolved. Herein we constructed a model of a GO membrane using laminate nanochannels with aligned flexible multilayered GO sheets, on which functional groups were randomly distributed based on the Lerf–Klinowski model. The water permeation and ion rejection in the flexible GO membranes with various pore widths and surface oxidization degrees were simulated. Our results indicate water flow rate in the GO nanochannels is significantly slowed, which is quantitatively equivalent with the prediction using the no-slip Poiseuille equation. The simulated results suggest the capillary channels within GO stacked laminated membranes might not always work as the major flow route for water to permeate. It is observed that confined water structure becomes more disordered and loose within the corrugated GO nanochannels. The interfacial friction provides huge corrugation of surface energy landscape for moving water and largely suppresses the water flow. The microscopic mechanism of ion rejection has been ascribed to the size exclusion of ion hydration and the surface interaction from functional groups. Overall, our results provide new physical pictures for capillary channels in GO-related stacked membranes.

In this work, we simulate the hydraulic permeation of liquid ethanol–water mixtures through a series of nanoporous graphene membranes. Ethanol was found to have larger permeability as compared to water in the mixture. For the first time, we present direct computational evidence that nanoporous graphenes exhibit promising potential as ethanol-permselective sieving membranes for the separation of ethanol–water mixtures, with ethanol permeability several orders of magnitude higher than current pervaporation membranes. The underlying sieving mechanism is not the process of pore-size sieving, but has been distinctively revealed as the sieving mode based on interfacial affinity. The enhanced hydrophobic surface adsorption and preferential pore trapping function in nanoporous graphene structure lead to the selective penetration of ethanol. Our results provide new insight into the molecular penetration across atomically thick nanoporous graphenes, and it further represents a proof of concept design of highly efficient nanoporous graphenes in membrane separation and nanofluidic devices for liquid-phase mixtures.

The chemical vapor deposition (CVD) synthesis using the solid/liquid carbon sources provides important alternative to economical and large-scale production of graphene-like materials. Herein, we applied the reactive molecular dynamics simulation to study the formation and growth of graphene on nickel surfaces using naphthalene/fluorene as carbon sources. The kinetic CVD process has been demonstrated. A series of fundamental mechanism steps were revealed and identified, where surface-assisted dehydrogenation reaction occurs at first stage, followed by coalescence reaction of active molecular species, which includes complicated multi-step processes. This unique behavior is different from the nucleation and growth mechanisms in the conventional graphene CVD process. The effect of annealing temperature, precursor concentration, and surface types was systematically investigated. Our result suggests that there exist optimal temperature and concentration in the CVD process. The moderate surface interaction on Ni (1 1 1) promotes the formation and growth of large and continuous graphene-like carbon network structure. Finally, we evaluate the self-healing function of surface graphene structures by extending the annealing time. Our simulation provides a new insight into the graphene surface growth and will be valuable to further develop the CVD process.

Molecular dynamics simulation was conducted to study ethanol–water mixtures and the corresponding pure species, confined within slit-shaped graphene nanopores. Extensive structural and dynamical properties of the confined fluids, including hydrogen-bonding behavior, were investigated. The effects of pore width and mixture composition on the confined behavior were illustrated. It is observed that a layered structure is formed within the confined spaces and the ethanol–water mixtures show segregation at larger pores, with ethanol molecules preferentially adsorbing on graphene surfaces. This microphase demixing behavior stems from the competitive effect of the solid–fluid and fluid–fluid interactions. Moreover, miscellaneous diffusion mechanisms have been revealed for the hydrogen-bonding mixtures within the graphene pores. In the mixtures, water and ethanol generally display analogous diffusion mechanism due to ethanol–water association, converting from short-time subdiffusion to long-time Fickian diffusion in the larger nanopores. In the smaller pore (7 Å), both ethanol and water show a suppressed single-file diffusion behavior at the initial time and then display subdiffusion or single-file diffusion behavior. The complex diffusion behavior of ethanol–water mixtures can be described by the collaborating effects of pore confinement and enhanced interaction in the hydrogen-bonding mixtures.

A molecular dynamics simulation was conducted to study the structure and morphology of sodium dodecyl sulfate (SDS) surfactants adsorbed on a nanoscale graphene nanostructure in the presence of an electrolyte. The self-assembly structure can be reorganized by the electrolyte-induced effect. An increase in the ionic strength of the added electrolyte can enhance the stretching of adsorbed surfactants toward the bulk aqueous phase and make headgroups assemble densely, leading to a more compact structure of the SDS/graphene composite. The change in the self-assembly structure is attributed to the accumulation/condensation of electrolyte cations near the surfactant aggregate, consequently screening the electrostatic repulsion between charged headgroups. The role of the electrolyte revealed here provides direct microscopic evidence or an explanation of the reported experiments in the electrolyte tuning of the interfacial structure of a surfactant aggregate on the surface of carbon nanoparticles. Additionally, the buoyant density of the SDS/graphene assembly has been computed. With an increase in the ionic strength of the electrolyte, the buoyant density of the SDS/graphene composite rises. The interfacial accumulation of electrolytes provides an important contribution to the density enhancement. The study will be valuable for the dispersion and application of graphene nanomaterials.Keywords: electrolyte; graphene; molecular simulation; SDS; self-assembly; surfactant;

A combined density functional theory and molecular dynamics study has been used to study reactions relevant to the crystallization of a model cluster based upon the metastable phase NH2-MOF-235(Al), which has been previously shown to be an important intermediate in the synthesis of NH2-MIL-101(Al). The clusters studied were of the form Al3O(BDC)6(DMF)n(H2O)m+, where BDC– = NH2-benzenedicarboxylate and DMF = dimethylformamide (n = 1–3; m = {n – 3}). The ionic bonding interaction of the Al3O7+ core with BDC– is much stronger than that with a coordinated solvent and is independent of the bulk solvent medium (water or DMF). The exchange reactions of a coordinated solvent are predicted to be facile, and the dynamic solvent organization indicates that they are kinetically allowed because of the ability of the solvent to migrate into the cleft created by the BDC–Al3O–BDC coordination angle. As BDC– binds to the Al3O7+ core, the solvation free energy (Gsolv) of the cluster becomes less favorable, presumably because of the overall hydrophobicity of the cluster. These data indicate that as the crystal grows there is a balance between the energy gained by BDC– coordination and an increasingly unfavorable Gsolv. Ultimately, unfavorable solvation energies will inhibit the formation of quantifiable metal–organic framework (MOF) crystals unless solution-phase conditions can be used to maintain thermodynamically favorable solute–solvent interactions. Toward this end, the addition of a cosolvent is found to alter solvation of Al3O(BDC)6(DMF)3+ because more hydrophobic solvents (DMF, methanol, acetonitrile, and isopropyl alcohol) preferentially solvate the MOF cluster and exclude water from the immediate solvation shells. The preferential solvation is maintained even at temperatures relevant to the hydrothermal synthesis of MOFs. While all cosolvents exhibit this preferential solvation, trends do exist. Ranking the cosolvents based upon their observed ability to exclude water from the MOF cluster yields acetonitrile < DMF ∼ methanol < isopropyl alcohol. These observations are anticipated to impact the intermediate and final phases observed in MOF synthesis by creating favorable solvation environments for specific MOF topologies. This adds further insight into recent reports wherein DMF has been implicated in the reactive transformation of NH2-MOF-235(Al) to NH2-MOF-101(Al), suggesting that that DMF additionally plays a vital role in stabilizing the metastable NH2-MOF-235(Al) phase early in the synthesis.

The flow behavior of pressure-driven water infiltration through graphene-based slit nanopores has been studied by molecular simulation. The simulated flow rate is close to the experimental values, which demonstrates the reasonability of simulation results. Water molecules can spontaneously infiltrate into the nanopores, but an external driving force is generally required to pass through the whole pores. The exit of nanopore has a large obstruction on the water effusion. The flow velocity within the graphene nanochannels does not display monotonous dependence upon the pore width, indicating that the flow is related to the microscopic structures of water confined in the nanopores. Extensive structures of confined water are characterized in order to understand the flow behavior. This simulation improves the understanding of graphene-based nanofluidics, which helps in developing a new type of membrane separation technique.Flow behavior of pressure-driven water through graphene-based subnanometer nanochannels is simulated, along with the microscopic structures of water confined in the graphene nanopores. The flow behavior can be related to the microscopic structures of confined water.Download full-size image