Yudan Zhu

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.

Carbon titania mesoporous composite whisker as stable supercapacitor electrode material

Titania carbon composites were prepared via in situ carbonization on mesoporous titania whiskers. Their microstructures were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), showing that the composites, after carbonization, still retain the original morphology of the whiskers and the crystalline structure of titania. Based on N2 sorption isotherms, the average pore sizes of the as-prepared composites were found to depend on the amount of filled carbon. The electrochemical capacitance performance of the as-prepared composites was investigated by cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and galvanostatic charge–discharge cycles. Although the specific surface area of the composite TiO2/0.252C is moderate at 156 m2 g−1, its specific volumetric capacitance of 25 F cm−3 was much higher than the value of 10 F cm−3 for Vulcan XC-72, which has a specific surface area of 236 m2 g−1. This enhanced capacitance may come from the composite mesopores derived from porous titania whiskers. They provide readily accessible diffusion pathways for electrolyte ions. There is better conductivity with carbon in the composite. After 2000 cycles, we observed a change of −2.8%, −2.6% and −1.9% decrease in the specific volumetric capacitance compared to the values at the 100th cycle of the composites TiO2/0.252C, TiO2/0.143C and TiO2/0.08C, respectively. This decrease is small and significantly less than the 10% decrease of capacitance in Vulcan XC-72 in the same period. The more consistent capacitance in the composite suggests a more stable interface between titania, carbon filling and electrolyte compared to that of Vulcan XC-72 without titania.

Simple physical approach to reducing frictional and adhesive forces on a TiO2 surface via creating heterogeneous nanopores.

A simple physical strategy to reduce the frictional and adhesive forces on TiO(2) films was proposed by constructing mesoporous TiO(2) films with heterogeneously distributed nanopores on the film surfaces. In comparison, TiO(2) films with densely packed nanoparticles were also prepared. The crystal structure and morphology of the films were characterized with Raman spectroscopy, field emission scanning electron microscopy (FESEM), and atomic force microscopy (AFM). It was found that the TiO(2)(B) phase exists in the mesoporuos TiO(2) films but not in the densely packed films. The existence of TiO(2)(B) plays a significant role in creating and maintaining the nanopores in the mesoporous TiO(2) films. The frictional and adhesive forces were measured on both films using AFM. The mesoporous films exhibit two typical adhesion forces of around 3 and 12 nN in the force distribution profile whereas the densely packed films show only one around 12 nN. The frictional coefficients were 2.6 × 10(-3) and 6.7 × 10(-2) for the mesoporous and densely packed TiO(2) films, respectively. A model based on the atomic structures of a thin film of water molecules adsorbed on TiO(2) surfaces leading to hydrophobic effects was proposed to understand the lower frictional and adhesive forces observed on the mesoporous TiO(2) films. This simple physical approach to reducing the frictional and adhesive forces on TiO(2) films could have broad applications to a variety of surface coatings.

Theoretical Study of Hydration Effects on the Selectivity of 18-Crown-6 Between K+ and Na+

Abstract A combination of molecular dynamics (MD) and density functional theory (DFT) calculations were used to study the hydration structures of K + and Na + ions under the confinement of 18-crown-6 in order to identify the role of water in the selectivity of 18-crown-6 towards K + . The radial distribution functions, coordination numbers, orientation structures and interaction energies were analyzed to investigate the hydration of K + and Na + in 18-crown-6/cation complexes. All calculations of K + and Na + in bulk water were also conducted for comparison. The simulation results show that the orientation distributions of the water molecules in the first coordination shell of K + are more sensitive to the confinement of 18-crown-6 than those of Na + . It is more favorable to confine a K + in 18-crown-6 than a Na + in terms of interaction energy. Good agreement is obtained between MD results and DFT results.

Wetting behavior of ionic liquid on mesoporous titanium dioxide surface by atomic force microscopy.

Ionic liquids based on 1-butyl-3-methylimidazolium hexafluoro-phosphate (ILs [Bmim][PF6]) has been employed to wet the mesoporous and dense titanium dioxide (TiO2) films. It has been found from atomic force microscopy (AFM) analysis that ILs [Bmim][PF6] can form a wetting phase on mesoporous TiO2 films, but nonwetting and sphere shaped droplets on dense films. AFM topography, phase images, and adhesion measurements suggest a remarkable dependence of wetting ILs [Bmim][PF6] films on the TiO2 porous geometry. On mesoporous TiO2 films, the adhesive force of ILs [Bmim][PF6] reaches at 40 nN, but only 4 nN on dense TiO2 films. The weak interacting ILs [Bmim][PF6] on dense TiO2 films forms rounded liquid spheres (contact angle as 40°), which helps to reduce friction locally but not on the whole surface. The stronger adhesive force on mesoporous TiO2 films makes ILs [Bmim][PF6] adhere to the surface tightly (contact angle as 5°), and this feature remains after five months. The stable spreading ILs [Bmim][PF6] films provide low friction coefficient (0.0025), large wetting areas, and short CO2 diffusion distance on the whole mesoporous TiO2 surface, avoiding the significant decelerating effect through equilibrium limitations to enable CO2 capture rate up to 1.6 and 10 times faster than that on dense TiO2 and pure ILs, respectively. And importantly, ILs wetted on mesoporous TiO2 shorten the time reaching to the maximum adsorption rate (2.8 min), faster than that on mesoporous TiO2 (6.1 min), and dense TiO2 (11.2 min). This work provides an important guidance for the improvement of the efficiency of CO2 capture, gas separation, and the lubrication of micro/nanoelectromechanical systems (M/NEMs).

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.

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.

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.

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.