Enantioselective Molecular Transport in Multilayer Graphene Nanopores

Multilayer superstructures based on stacked layered nanomaterials offer the possibility to design three-dimensional (3D) nanopores with highly specific properties analogous to protein channels. In a layer-by-layer design and stacking, analogous to molecular printing, superstructures with lock-and-key molecular nesting and transport characteristics could be prepared. To examine this possibility, we use molecular dynamics simulations to study electric field-driven transport of ions through stacked porous graphene flakes. First, highly selective, tunable, and correlated passage rates of monovalent atomic ions through these superstructures are observed in dependence on the ion type, nanopore type, and relative position and dynamics of neighboring porous flakes. Next, enantioselective molecular transport of ionized l- and d-leucine is observed in graphene stacks with helical nanopores. The outlined approach provides a general scheme for synthesis of functional 3D superstructures

Gated Water Transport through Graphene Nanochannels: From Ionic Coulomb Blockade to Electroosmotic Pump

Understanding and controlling water or ion transport in nanochannels plays an important role in further unravelling the transport mechanism of biological membrane channels and designing functional nanofluidic devices. Molecular dynamics simulations were conducted to investigate water and ion transport in graphene nanochannels. Similar to electron coulomb blockade phenomenon observed in quantum dots, we discovered an ionic coulomb blockade phenomenon in our graphene nanochannels, and another two ion transport modes were also proposed to rationalize the observed phenomena under different electric-field intensities. Furthermore, on the basis of this blockade phenomenon we found that the Open and Closed states of the graphene nanochannels for water transport could be switched according to external electric-field intensities, and electroosmotic flow could further enhance the water transport. These findings might have potential applications in designing and fabricating controllable valves in lab-on-chip nanodevices

Correlated Rectification Transport in Ultranarrow Charged Nanocones

Using molecular dynamics simulations, we reveal ion rectification in charged nanocones with exit diameters of 1–2 nm. The simulations exhibit an opposite rectification current direction than experiments performed in conical channels with exit diameters larger than 5 nm. This can be understood by the fact that in ultranarrow charged cones screening ions are trapped close to the cone tip at both field directions, which necessitates them to be released from the cone in a correlated multi-ion fashion. Electroosmosis induced by a unidirectional ion flow is also observed

Combined Molecular Dynamics and Quantum Mechanics Study of Oil Droplet Adsorption on Different Self-Assembly Monolayers in Aqueous Solution

In this paper, the dodecane is selected as the oil phase, and the adsorption behavior of an oil droplet on six self-assembled monolayer (SAM) surfaces in aqueous solution is investigated by a combined molecular dynamics and quantum mechanics method. First, the adsorption configuration of an oil droplet on these six surfaces is investigated, which indicates that the oil droplets spread on SAMs of −CH₃, −OCH₃, and −COOCH₃ and detach from SAMs of −NH₂, −OH, and −COOH. After that, the interactions of oil–SAM and water–SAM are calculated to rationalize the driving force controlling the conformational change of the oil droplet on various SAMs. Researched results suggest that the conformational change of the oil droplet is mainly driven by the interaction between water and SAMs. Finally, the mechanism of oil spreading or detachment behavior is discussed from the aspect of water property near the SAM, and calculated results show that the adsorption capability and dynamic property of interfacial water have a profound effect on the adsorption behavior of the oil droplet. The microscopic adsorption behavior of the oil droplet on SAMs discussed here is helpful to understand many phenomena in scientific and industrial processes better

Molecular Insight into the Methane Occurrence inside a Shale Nanochannel with Formation Water

The occurrence status of shale gas inside a nanochannel of a shale reservoir is crucial for shale gas reserve assessment and exploitation. In this work, adopting molecular dynamics simulations, the methane occurrence inside a shale nanochannel was investigated under different formation water saturations. Moreover, employing three common inorganic minerals including of quartz, calcite, and kaolinite, the influence of mineral hydrophilicity on methane occurrence was also examined. The simulations indicate that there are three occurrence statuses of methane inside nanochannels including a free status located in the internal nanochannel, an adsorption status on mineral and water film surfaces, and a dissolution status inside the water phase. Among them, free-status methane is the dominant contribution to the recoverable reserve assessment and could be feasibly exploited. Without formation water, methane gives two occurrence statuses including an adsorption status on mineral surfaces and a free status. Once the formation water emerges, it will preferentially adsorb onto the mineral surface to form a water film. With the increase of formation water saturation, the proportions of free-status and dissolution-status methanes increase, while the proportion of adsorption-status methane decreases. In three mineral nanochannels with hydrophilicity orders of quartz > calcite > kaolinite, the proportion of adsorption-status methane follows the order of kaolinite > calcite > quartz, and free-status methane gives the order of kaolinite ≈ quartz > calcite. The underlying mechanisms of these occurrence features were discussed at length from the view of microscopic interactions among mineral surfaces, water, and methane. Our work presents the methane occurrence structure inside a shale nanochannel, and the discussion of the three methane occurrence statuses could provide fundamental data and theoretical guidance for shale gas reserve assessment and exploitation

Study on the Asphaltene Precipitation in CO₂ Flooding: A Perspective from Molecular Dynamics Simulation

Asphaltene precipitation is a common phenomenon in the exploitation of crude oil and closely correlates with oil recovery, especially in CO₂ flooding. In this work, employing molecular dynamics simulations, the asphaltene precipitation process in CO₂ was investigated. The simulation results reveal that the CO₂ could stepwise extract nonpolar and light polar components from asphaltene micelle, and a two-step asphaltene precipitation process was illustrated. In our eight molecule asphaltene system, first four asphaltene dimers formed. Two dimers get together into one aggregation in bulk; the other two dimers get together and adsorbed onto the silica surface. After that, the surface aggregation further induces the adsorption of bulk aggregation onto it to complete asphaltene precipitation. In addition, we also studied the pressure effect on asphaltene precipitation. Our work provided a molecular-level understanding of asphaltene precipitation phenomenon in CO₂ flooding, and the results have significant promise for oil exploitation

Molecular Dynamics Simulations of CO₂/N₂ Separation through Two-Dimensional Graphene Oxide Membranes

Graphene oxide (GO), as an ultrathin, high-flux, and energy-efficient separation membrane, has shown great potential for CO₂ capture. In this study, using molecular dynamics simulations, the separation of CO₂ and N₂ through the interlayer gallery of GO membranes was studied. The preferential adsorption of CO₂ in the GO channel derived from their strong interaction is responsible for the selectivity of CO₂ over N₂. Furthermore, the influences of interlayer spacing, oxidization degree, and channel length on the separation of CO₂/N₂ were investigated. Our studies unveil the underlying mechanism of CO₂/N₂ separation in the interlayer GO channel, and the results may be helpful in guiding rational design of GO membranes for gas separation

Tuning and Designing the Self-Assembly of Surfactants: The Magic of Carbon Nanotube Arrays

Controlling the self-assembly and polymorphic transition of surfactants is a challenging but meaningful topic for chemists and materials scientists, which is significant for the preparation of advanced nanomaterials. We presented coarse-grained (CG) molecular dynamics (MD) simulations on the self-assembly of surfactants confined within the carbon nanotube (CNT) arrays. Under the effect of confinement, an intriguing “rod-double helix-hexagon-worm” polymorphic transition was observed with varying the size of the confining space. The simulations also showed that the confinement of CNT arrays does not break the characteristic of surfactant assemblies at certain concentration. Based on these results, a plausible strategy for designing complex assemblies was presented. And then, “nano-drill” and “dartboard” assemblies were created with the strategy. This work demonstrated that the confinement of CNT arrays may be an efficient method to tune and design the self-assembly of surfactants, shedding light on the development of nanotechnology and advanced materials