Linghong Lu

Anomalous Hydration Shell Order of Na + and K + inside Carbon Nanotubes

We performed molecular dynamics simulations of the hydration of Na+ and K+ in infinitely long single-walled armchair carbon nanotubes (CNTs) at 298 K. Simulation results indicate that the preferential orientation of water molecules in coordination shells of these two cations presents an anomalous change in the CNTs and causes a diameter-dependent variation for the interaction energy between the cation and water molecules in its coordination shell. In the five CNTs of this work, it is energetically favorable for confining a hydrated K+ inside the two narrow CNTs with diameters of 0.60 and 0.73 nm, whereas the situation is reverse inside the wide CNTs with diameters of 0.87, 1.0, and 1.28 nm. This finding is important for CNT applications in ionic systems that control the selectivity and the ionic flow.

Helicity and temperature effects on static properties of water molecules confined in modified carbon nanotubes

Carbon nanotubes show exceptional properties that render them promising candidates as building blocks for nanostructured materials. Many ambitious applications, ranging from molecular detection to membrane separation, require the delivery of fluids, in particular aqueous solutions, through the interior of carbon nanotubes (CNT). To foster such applications, an understanding of the properties of water molecules confined in carbon nanotubes at the molecular level is needed. In this work we report a study of temperature and helicity effects on static properties of water molecules confined in modified CNT by molecular dynamics simulations. It was found that the temperature has little effect on the confined water molecules in carbon nanotubes. But on the other hand, the simulation results showed that because of the difference in helicity between (6, 6) and (10, 0) CNTs, the modification by hydrophilic carboxyl acid functional groups (-COOH) results in a different response to the CNTs, which in turn have control over the flow direction of water molecules in these CNTs.

Changes in CNT-confined water structural properties induced by the variation in water molecule orientation

Using a (8, 8) carbon nanotube (CNT) as a model for nanochannel, we studied the effects of carbonyl (–CO) groups and external electric field on the properties of confined water molecules by molecular dynamics simulation. We analysed the density profiles, the distribution of orientation and the hydrogen bonds of water molecules in the axial and radial directions of the CNT. The results show that –CO groups are more powerful than electric fields on controlling water properties, and property changes induced by –CO groups are less affected by electric fields. Meanwhile, the particular behaviour of water molecules induced by the –CO groups is not limited near the –CO groups, but also expands to the whole CNT.

Nanomaterial-oriented molecular simulations of ion behaviour in aqueous solution under nanoconfinement

Abstract Various nanotube- and graphene-based materials have been used in a wide range of applications associated with nanoconfined ions and these nanomaterial designers aim to learn more about the underlying mechanisms of ion behaviour at the nanoscale through molecular simulation. In the present work, we summarised recent progress of molecular simulation studies on the nanoconfined ionic transport behaviour in aqueous solutions. With regard to nanomaterial design, the significance of molecular simulation studies and the derived important design principles were selectively highlighted in three major applications, including separation based on nanoporous membranes, electrochemical-related energy storage and DNA nanopore sequencing. Molecular simulation evaluations of influencing factors were also reviewed based on ionic hydration and ion pairing under nanoconfinement, which primarily contribute to ionic transport. Moreover, we also discussed useful analysis methods based on the microscopic molecular dynamics trajectory information for improved evaluation of the microscopic properties of the nanoporous material towards different applications.

Diffusion of CO2/CH4 confined in narrow carbon nanotube bundles

ABSTRACT The diffusion of a CO2/CH4 mixture in carbon nanotube (CNT) bundles was studied using molecular simulations. The effect of diameter and temperature on the diffusion of the mixture was investigated. Our results show that the single-file diffusion occurs when CO2 and CH4 are confined in CNTs of diameter less than 1.0 nm. In CNTs of diameter larger than 1.0 nm, both molecules diffuse in the Fickian style. The transition from single-file to Fickian diffusion was demonstrated for both CO2 and CH4 molecules. A dual diffusion mechanism was observed in the studied (20, 0) CNT bundle, single-file diffusion of CO2 in the interstitial sites of (20, 0) CNT bundle and Fickian diffusion of CO2 and CH4 in the pores. For CO2, the interaction energies (CO2–CO2 and CO2–CNT) are larger than that of CH4 in all cases. But only a very small difference in the diffusion coefficient was observed between CO2 and CH4. Temperature has a negligible effect on the difference between diffusion coefficients of CO2 and CH4 in the studied CNT bundles. The adsorption, diffusion and permeation selectivities are discussed and compared, and the adsorption is demonstrated to be the rate limiting step for the separation of CO2/CH4 in CNT bundle membranes.

Adsorption of N-Butane/I-Butane in Zeolites: Simulation and Theory Study

Zeolite adsorption is one of the best available technologies for gas separation. We use molecular simulation to produce the data of adsorption of n-butane and i-butane and their mixture at different temperatures and pressures in different zeolites. In BEA, MOR, CFI, ISV, and BOG the amount of adsorption of i-butane is higher than n-butane, but the case is just the contrary in MFI, MEL, TER, and TON. The heats of adsorption decrease with temperature increasing for all investigated cases except for i-butane in MFI and MEL. The IAST (ideal adsorbed solution theory) is used to predict the n-butane/i-butane mixture adsorption. For the systems of BEA, MOR, CFI, and ISV, IAST theory provides predictions that are in good agreement with simulation. We propose SACIAST (surface area corrected IAST) by introducing a correcting parameter Cr into IAST to predict the adsorption of mixture in MFI, MEL, TER, TON, and BOG since the original IAST failed to predict the mixture data in these zeolites because of the pore structure of these zeolites. The SACIAST we proposed is a new modified theory based on IAST which has never been reported in the literature before, and it predicts the mixture adsorption very well.

Wetting control through topography and surface hydrophilic/hydrophobic property changes by coarse grained simulation

Abstract The changes of wetting state of water droplet on the solid surface featuring pillared structures are quantitatively studied by Coarse Grained simulation. Our results demonstrate that wetting state changes with the different topography (surface roughness), and it depends on the intrinsic hydrophilic/hydrophobic property of surface as well. Only if the contact angle of water droplet on the smooth surface is larger than 93.13°, the wetting state translates from the Wenzel state to the Cassie state on the rough surface with certain pillar height and width, and the contact angle climb up to the highest point and then remain almost unchanged with the increasing of pillar height and the same pillar distance. However, the wetting state does not change if the contact angle on the smooth surface is 85.1° or less, no matter what pillar structure the surface has. Additionally, the contact angles will remain almost unchanged if the pillar height is higher than a certain value. Our simulation results provide a quantitative understanding about the wetting state of water droplet on solid rough surfaces, and the results show the wetting state can be controlled by combining rough structure design and hydrophilic/hydrophobic property change of surfaces.

Effect of Adsorbed Alcohol Layers on the Behavior of Water Molecules Confined in a Graphene Nanoslit: A Molecular Dynamics Study

With the rapid development of a two-dimensional (2D) nanomaterial, the confined liquid binary mixture has attracted increasing attention, which has significant potential in membrane separation. Alcohol/water is one of the most common systems in liquid-liquid separation. As one of the most focused systems, recent studies have found that ethanol molecules were preferentially adsorbed on the inner surface of the pore wall and formed an adsorbed ethanol layer under 2D nanoconfinement. To evaluate the effect of the alcohol adsorption layer on the mobility of water molecules, molecular simulations were performed to investigate four types of alcohol/water binary mixtures confined under a 20 Å graphene slit. Residence times of the water molecules covering the alcohol layer were in the order of methanol/water < ethanol/water < 1-propanol/water < 1-butanol/water. Detailed microstructural analysis of the hydrogen bonding (H-bond) network elucidated the underlying mechanism on the molecular scale in which a small average number of H-bonds between the preferentially adsorbed alcohol molecules and the surrounding water molecules could induce a small degree of damage to the H-bond network of the water molecules covering the alcohol layer, resulting in the long residence time of the water molecules.

Molecular Interactions of Protein with TiO2 by the AFM-Measured Adhesion Force

Understanding the interactions between porous materials and biosystems is of great important in biomedical and environmental sciences. Upon atomic force microscopy (AFM) adhesion measurement, a new experimental approach was presented here to determine the molecular interaction force between proteins and mesoporous TiO2 of various surface roughnesses. The interaction force between each protein molecule and the pure anatase TiO2 surface was characterized by fitting the adhesion and adsorption capacity per unit contact area, and it was found that the adhesion forces were approximately 0.86, 2.63, and 4.41 nN for lysozyme, myoglobin, and BSA, respectively. Moreover, we reported that the molecular interaction force was independent of the surface topography of the material but the protein type is a factor of the interaction. These experimental results on the molecular level provide helpful insights for stimulating model calculation and molecular simulation studies of protein interaction with surfaces.

Mg2+-Channel-Inspired Nanopores for Mg2+/Li+ Separation: The Effect of Coordination on the Ionic Hydration Microstructures

The separation behaviors of Mg2+ and Li+ were investigated using molecular dynamics. Two functionalized graphene nanopore models (i.e., co_5 and coo_5) inspired by the characteristic structural features of Mg2+ channels were used. Both nanopores exhibited a higher preference to Mg2+ than to Li+, and the selectivity ratios were higher for coo_5 than for co_5 under all the studied transmembrane voltages. An evaluation of the effect of coordination on the ionic hydration microstructures for both nanopores showed that the positioning of the modified groups could better fit a hydrated Mg2+ than a hydrated Li+, as if Mg2+ was not dehydrated according to hydrogen bond analysis of the ionic hydration shells. This condition led to a lower resistance for Mg2+ than for Li+ when traveling through the nanopores. Moreover, a distinct increase in hydrogen bonds occurred with coo_5 compared with co_5 for hydrated Li+, which made it more difficult for Li+ to pass through coo_5. Thus, a higher Mg2+/Li+ selectivity was found in for coo_5 than for co_5. These findings provide some design principles for developing artificial Mg2+ channels, which have potential applications as Mg2+ sensors and novel devices for Mg2+/Li+ separation.