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In this work the effects of the wall-fluid interaction on the critical point shift are studied by using a discrete and attractive wall-fluid interaction and density functional theory. In contrast to the previous assumption, it is found that the dependence of critical temperature shift on the wall-fluid interaction does not simply show a monotonic manner, but increases with the strength of the interaction for weak surfaces, then decreases for strong surfaces. The similar trend holds for the systems with different fluid-fluid interactions and different confined spaces. Unlike the capillary critical temperature, the critical density of square-well fluids in a confined space increases monotonically as the wall-fluid interaction becomes more attractive.

A novel united-atom (UA) force field is proposed from our previously developed all-atom (AA) force field for the imidazolium-based ionic liquids by the introduction of a coarse-grained method. The Lennard-Jones parameters for CH(2) and CH(3) in alkyls are fitted to match the AA force field, and the partial atomic charges are re-fitted by the one conformation two-step RESP method. The force field is verified by molecular dynamics simulations of pure ionic liquids and the mixture of [bmim][BF(4)] and acetonitrile. The densities, self-diffusion coefficients, vaporization enthalpies, cohesive energy densities, and microscopic structures of both the pure components and mixtures are simulated. The simulated results from the UA force field agree well with those from the AA force field. In addition, the predictive capability of the UA force field for the liquid densities of [C(n)mim][PF(6)] is tested. The UA force field proposed in this work provides a useful tool with good accuracy and much less computational intensity for future molecular design of ionic liquids.

A new method for synthesizing the labeled L-tryptophan is described in this work. L-tryptophan, labeled with 98% (15)N at position 1 was synthesized from the labeled anthranilic acid using Candida utilis mutants. The conversion ratio of (15)N of 50% was achieved. The labeled anthranilic acid was synthesized by [(15)N] phthalimide that was prepared by 99.34% [(15)N] urea and phthalic anhydride in ortho-xylene medium at 140 degrees C and under atmospheric pressure.

A hybrid cylindrical model for characterization of MCM-41 by density functional theory (DFT) is proposed in this work, where the surface heterogeneity of MCM-41 is taken into account by using a hybrid potential model to represent the interactions between a pore wall and molecules inside the pore. This model consists of two parts: (1) the potential energies from the oxygen atoms inside the wall, represented by the potential model proposed by our group; (2) the potential energies from the silanol coverage and/or other unknown factors in the surface of the channel of MCM-41, represented by the cylindrical surface potential function of Tjatjopoulos et al. (G. J. Tjatjopoulos, D. L. Feke and J. A. Mann, J. Phys. Chem., 1988, 92, 4006–4007). To test the new model, the DFT method was used to calculate the adsorption isotherm of nitrogen in MCM-41 at 77 K. The isotherm calculated is compared with the experimental data as well as the calculated results of Maddox et al., who divided the surface of MCM-41 into eight sectors and adopted different parameters for each sector to consider the heterogeneity of the surface. Compared with the work of Maddox et al. (M. W. Maddox, J. P. Olivier and K. E. Gubbins, Langmuir, 1997, 13, 1737–1745), our model gives a much better fit to the experimental isotherm of nitrogen at 77 K in the pressure range of P/P0 = 0.2–0.5 with much less parameter and computation effort, where phase transition and capillary condensation occur. Furthermore, the relationship between the reduced pressure, at which capillary condensation takes place, and the pore diameter by the hybrid model is in good agreement with that obtained by Maddox et al. In addition, adsorption and phase behavior of methane and ethane are studied by the model, and the calculated results also coincide well with the experimental isotherms of methane and ethane at 264 K–373 K. Therefore, the hybrid potential model incorporating into the DFT method provides a useful tool for characterization of MCM-41.

We report a theoretical study of nanobubble stabilization on a substrate by contact line pinning. In particular, we predict the magnitude of the pinning force required to stabilize a nanobubble and the threshold values of the pinning force that the substrate can provide. We show that the substrate chemistry and the local structures of substrate heterogeneity together determine whether or not surface nanobubbles are stable. We find that for stable nanobubbles, the contact angles are independent of substrate chemistry as its effects are cancelled out by the pinning effect. This prediction is in agreement with available experimental data.

In this work we perform large scale dissipative particle dynamics (DPD) simulations to study the interaction between nanoparticles (NPs) and vesicles, and discuss their potential implications for NP–cell interactions. First, we determine the different pathways of NP internalization and their dependence on NP size, NP concentration, vesicle excess area, and NP–vesicle attraction. In particular, we identify three pathways for spontaneous NP penetration, which are here called the cooperative chain-like penetration, direct penetration and inverted micelle-like penetration, and investigate how different factors affect the penetration processes. Then, we demonstrate that adsorption of NPs often induces NP clustering on the outer and/or inner surfaces of vesicles, leading to different vesicle responses, including the change of vesicle morphology, formation of protuberance, and vesicle rupture.

Selective epitaxial growth (SEG) of silicon-germanium (SiGe) films on patterned-oxide silicon substrates, using a tubular hot-wall low pressure chemical vapor deposition (LPCVD) system, is demonstrated in this study. This conventional system is proposed as a low cost alternative for SiGe epitaxial growth. Three process improvements needed to achieve quality growth are discussed. First, the hydrogen bake process is modified to eliminate Ge-outgassing. Secondly, a Si SEG buffer layer is deposited prior to SiGe SEG. Finally, a small flow of dichlorosilane is introduced during the temperature ramp-down period prior to SiGe SEG. The growth results are discussed in terms of growth selectivity, thickness uniformity, growth rate, defect density, SiGe film composition, and electrical properties.

The chemical doping method is often adopted to obtain metal-free conducting materials. To date, it is still a great challenge to controllably prepare metal-free semiconducting and conducting materials by tuning the inherent structure of a material. In this work, a class of novel one-dimensional (1D) covalent-organic polymer (COP) has been designed, whose cross-sections are triangular, tetragonal, pentagonal and hexagonal, and their electronic properties are explored. The tetragonal 1D COP exhibits unique phenomena in electronic properties, i.e. the tetragonal COPs with mono- or trilayer defects (odd defects) show semiconducting properties, while they become conductors for the two cases of non- or bilayer defects (even defects). This observation indicates that they comply with the characteristics of semiconducting and conducting switches induced by the odd-even defects. Therefore, we infer that for the tetragonal configuration, the odd-even defects could potentially manipulate the electrical behavior of the COP material. The discovery provides a new direction for the targeted synthesis of semiconducting and conducting materials by tuning the inherent structure of materials, which is entirely different from the chemical doping method yielding metal-free conducting materials.

Replacement of CH4 in hydrate form with CO2 is a candidate for recovering CH4 gas from its hydrates and storing CO2. In this work, microsecond molecular dynamics simulations were performed to study the replacement mechanism of CH4 hydrate by CO2 molecules. The replacement process is found to be controlled cooperatively by the chemical potentials of guest molecules, “memory effect”, and mass transfer. The replacement pathway includes the melting of CH4 hydrate near the hydrate surface and the subsequent formation of an amorphous CO2 hydrate layer. A large number of hydrate residual rings left after the melting of CH4 hydrate facilitate the nucleation of CO2 hydrate and enhance the dynamic process, indicating the existence of so-called “memory effect”. In the dynamic aspect, the replacement process takes place near the surface of CH4 hydrate rather easily. However, as the replacement process proceeds, the formation of the amorphous layer of the CO2 hydrate provides a significant barrier to the mass transfer of the guest CH4 and CO2 molecules, which prevents the CH4 hydrate from further dissociation and slows down the replacement rate.

Capillary bridge is a common phenomenon in nature and can significantly contribute to the adhesion of biological and artificial micro- and nanoscale objects. Especially, it plays a crucial role in the operation of atomic force microscopy (AFM) and influences in the measured force. In the present work, we study the rupture kinetics and transition pathways of liquid bridges connecting an AFM tip and a flat substrate during a process of pulling the tip off. Depending on thermodynamic conditions and the tip velocity, two regimes corresponding to different transition pathways are identified. In the single-bridge regime, the initial equilibrium bridge persists as a single one during the pulling process until the liquid bridge breaks. While, in the multibridge regime the stretched liquid bridge transforms into an intermediate state with a collection of slender liquid bridges, which then break gradually during the pulling process. Moreover, the critical rupture distance at which the bridges break changes with the tip velocity and thermodynamic conditions, and its maximum value occurs near the boundary between the single-bridge regime and the multibridge regime, where the longest range capillary force is produced. In this work, the effects of tip velocity, tip size, tip-fluid interaction, and humidity on rupture kinetics and transition pathways are also systematically studied.

The mechanism of biomembrane curvature generation has been studied for decades because of its role in many cellular functions. In this article, N-varied dissipative particle dynamics was used to investigate the relationship between membrane curvature generation and self-assembly of anchored proteins, and a protein aggregation mechanism of curvature generation was proposed. According to the mechanism, the curvature production is enhanced by the self-assembly of proteins, and the enhancement depends on the protein hydrophobic length. Contrary to the theoretic predictions that shallow insertion depth of proteins is more effective in producing positive membrane curvature, our simulations show the opposite trend if the self-assembly of proteins is taken into account. Furthermore, for the membrane proteins with deep insertion, simulations indicate that the self-assembly of proteins may induce membrane vesiculation at negative membrane tensions. In addition, the protein aggregates can sense the membrane curvature, although the way they respond to the local curvature again depends on the protein hydrophobic length. Especially, the self-assembly of shallow inserting proteins is significantly affected by the local membrane curvature.

In this work, we study the fission process of large micelles by using dissipative particle dynamics. In general, there exist four different stages during a fission process. For the first stage, the morphological transition to the dumbbell intermediate structures occurs due to a perturbation, in this case, a change in head-head interaction. Then, in the next stage the dumbbell-like intermediate structure fluctuates for a longer or less time until it reaches mechanical instability. Simulation results indicate that the fluctuation of the intermediate structure in this stage can be regarded as a nucleation process. In the third stage, the neck breaks as a result of continuous narrowing. In the last stage the two or more freshly formed, small micelles retract and equilibrate to their final shapes. Simulation results demonstrate that the first, second and third stages are characterized by distinct dynamic features, depending on the surfactant architecture. It is found that surfactant architecture controls the micelle fission process, especially for the second stage, similar to the way it controls the size and shape of the aggregates at equilibrium. Since for most cases it is the second stage that dominates the lifetime of a micelle, we suggest that the molecule packing parameter is not only a predictor of the shape and size of aggregates, but also a predictor of kinetic properties for micelle fission.

Computational modelling is a powerful tool for the study of gas–solid interactions, and can be used both to complement experiment and design new materials. For the modelling of gas adsorption by nanoporous media, a multiscale approach can be used, in which the molecular force fields required for Grand Canonical Monte Carlo (GCMC) simulations are derived from first-principles calculations. This can result in significantly enhanced accuracy, in comparison with conventional empirical force field-based GCMC methods. In this article, we review the application of this multiscale approach to the simulation of the adsorption of hydrogen, methane and carbon dioxide in Porous Coordination Frameworks (PCFs) for the purpose of gas storage for energy transportation and Carbon Capture and Storage (CCS) technology. We also define a scheme for the design of new materials with improved adsorption performance for the storage of these gases through the combination of multiscale simulation and experimental work, and discuss some of the issues regarding gas adsorption measurement accuracy in the context of the validation of simulation results using experimental data.

Nano-strengthening by employing nanoparticles is necessary for high-efficiency strengthening of elastomers, which has already been validated by numerous researches and industrial applications, but the underlying mechanism is still an open challenge. In this work, we mainly focus our attention on studying the variation of the tensile strength of nanofilled elastomers by gradually increasing the filler content, within a low loading range. Interestingly, the percolation phenomenon is observed in the relationship between the tensile strength and the filler loading, which shares some similarities with the percolation phenomenon occurring in rubber toughened plastics. That is, as the loading of nanofillers (carbon black, zinc oxide) increases, the tensile strength of rubber nanocomposites (SBR, EPDM) increases slowly at first, then increases abruptly and finally levels off. Meanwhile, the bigger the particle size, the higher the filler content at the percolation point, and the lower the corresponding tensile strength of rubber nanocomposites. The concept of a critical particle-particle distance (CPD) is proposed to explain the observed percolation phenomenon. It is suggested that rubber strengthening through nanoparticles is attributed to the formation of stretched straight polymer chains between neighbor particles, induced by the slippage of adsorbed polymer chains on the filler surface during tension. Meanwhile, the factors to govern this CPD and the critical minimum particle size (CMPS) figured out in this work are both discussed and analyzed in detail. Within the framework of this percolation phenomenon, this paper also clearly answers two important and intriguing issues: (1) why is it necessary and essential to strengthen elastomers through nanofillers; (2) why does it need enough loading of nanofillers to effectively strengthen elastomers. Moreover, on the basis of the percolation phenomenon, we give out some guidance for reinforcement design of rubbery materials: the interfacial interactions between rubber and fillers cannot be complete chemical bonding, and partial physical absorption of macromolecular chains on the filler surface is necessary, otherwise the formation of stretched straight chains would be seriously hindered. There should exist such an optimum crosslinking density for a certain filler reinforced rubber system, and as well an optimum filler loading for rubber strengthening. Additionally, the different percolation behaviors of Young's modulus, the tensile strength and the electrical conductivity are compared and analyzed in our work. Lastly, molecular simulation indicates that it is not possible to strengthen glassy or hard polymer matrices by incorporating spherical nanoparticles. In general, by providing substantial experimental data and detailed analyses, this work is believed to promote the fundamental understanding of rubber reinforcement, as well provide better guidance for the design of high-performance and multi-functional rubber nanocomposites.

Atomic force microscopy (AFM) can work in a variety of environment with different humidities. When the tip of AFM approaches a sample, the measured adhesion force would be significantly affected by the presence of nanometer-sized liquid bridge. The formation and rupture of liquid bridges can occur either through equilibrium or nonequilibrium process. In this work, the liquid bridges are assumed to be in thermodynamic equilibrium with the surrounding vapor medium. To study theoretically the stability of liquid bridge, a constraint is added into the lattice density functional theory to stabilize a series of bridges with different radii at a given tip-substrate distance. With the help of the constraint, we can identify not only stable and metastable states but also transition states for the formation and rupture of liquid bridges. Using this constrained method we calculate the energy barriers involved in the formation and rupture of the liquid bridges, respectively, and then discuss their stability as well as the origin of the hysteresis behavior observed with atomic force microscope measurements. On the whole, the calculated force-distance curves are found to be qualitatively in agreement with experimental observations. The energy barriers for the formation and rupture of liquid bridges are also analyzed as a function of tip-sample distance, humidity, and tip-fluid interaction.

A density functional theory (DFT) is used to investigate molecular orientation of rod fluids in selective slits. The DFT approach combines a modified fundamental measure theory (MFMT) for excluded-volume effect, the first-order thermodynamics perturbation theory for chain connectivity and the mean-field approximation for van der Waals (vdW) attraction. To study the molecular orientation, the intramolecular bonding orientation function is introduced into the DFT. First, we investigate the orientation of the surfactant-like rod molecule of AB6 (i.e. ABBBBBB) in a nanoslit of H = 20σ, where the walls selectively adsorb segment 'A'. It is observed that, with the increase of the surface energy of the wall to head segment (i.e. 'A' segment) of the rod molecule, the rod molecules adsorbed on the wall present the perpendicular orientation gradually, and assemble into a smectic-A-like monolayer finally. In addition, we also explore the molecular orientation of the rods with both end segments preferring to the wall, i.e. AB8A and AB7A, in a nanoslit of H = 10σ. Interestingly, the AB8A rod monolayer is compatible with either a smectic-A-like or a smectic-C-like organization, but AB7A rod molecules exhibit the smectic-A-like organization. The orientation factor of the AB7A rod molecule reaches 1, suggesting that AB7A rod molecules self-assemble into an ordered structure with perfectly perpendicular orientation to the wall.

We present hypergeometric function expressions for the molecule-micropore Lennard--Jones potential in cylindrical pores, in which the cylindrical wall can be a surface, or have thickness or have infinite thickness. These expressions are useful in theoretical study and computer simulations of fluids in micropore of circular cross sections.

Understanding the composition effect on the melting processes of bimetallic clusters is important for their applications. Here, we report the relationship between the melting point and the metal composition for the 55-atom icosahedral Ag-Pd bimetallic clusters by canonical Monte Carlo simulations, using the second-moment approximation of the tight-binding potentials (TB-SMA) for the metal-metal interactions. Abnormal melting phenomena for the systems of interest are found. Our simulation results reveal that the dependence of the melting point on the composition is not a monotonic change, but experiences three different stages. The melting temperatures of the Ag-Pd bimetallic clusters increase monotonically with the concentration of the Ag atoms first. Then, they reach a plateau presenting almost a constant value. Finally, they decrease sharply at a specific composition. The main reason for this change can be explained in terms of the relative stability of the Ag-Pd bimetallic clusters at different compositions. The results suggest that the more stable the cluster, the higher the melting point for the 55-atom icosahedral Ag-Pd bimetallic clusters at different compositions.

Segregation phenomena of Ag–Cu–Au trimetallic clusters with icosahedral structure are investigated by using a Monte Carlo method based on the second-moment approximation of the tight-binding (TB-SMA) potentials. We predict that the Ag atoms segregate to the surface of the Ag–Cu–Au trimetallic icosahedral clusters. The Ag concentrations in the surface layer of the clusters are about 11–29 at.% higher than the overall Ag concentration in all the cases studied. The simulation results also indicate that the Au atoms are mainly distributed in the middle shell and the Cu atoms are located in the center for the 147-, 309-, 561- and 923-atom clusters at 300 K. The segregation phenomena of the Ag, Au and Cu atoms in the Ag–Cu–Au trimetallic clusters are mainly due to the different surface energies of the Ag, Au and Cu atoms. It is found that the size and composition have little effect on the segregation phenomena of Ag, Au and Cu atoms in the Ag–Cu–Au trimetallic cluster.

We report the structures of Ag–Cu and Ag–Ni bimetallic clusters and nanowires (NWs), which are well known as effective Ag-based catalysts, by using an effective semi-grand-canonical ensemble Monte Carlo method. The metal–metal interactions are modeled by the second-moment approximation of the tight-binding potentials. The simulation results show that the Ag–Cu and Ag–Ni bimetallic nanomaterials, including clusters and NWs, possess core–shell structures at different compositions, in which the Ag atoms lie on the surface, while the Cu or Ni atoms occupy the cores of the clusters and NWs. It is found that the pentagonal multi-shell-type structure can be transformed into cylindrical multi-shell-type structures for Ag–Cu and Ag–Ni bimetallic NWs at 100, 300, and 500 K. On the other hand, with the increase of Ag mole fraction in the Ag–Cu and Ag–Ni bimetallic clusters, the Ag atoms occupy the surface shell first, then the interior shell, and finally the central sites of the clusters. It is also found that the initial shape, composition, and temperature have little effect on the core–shell structures of the bimetallic clusters and NWs. The formation of core–shell Ag–Cu and Ag–Ni bimetallic clusters and NWs is due to the fact that a single Ag impurity is favorable to be situated in the core of the Cu or Ni clusters and NWs.

1-Butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) is one of the promising room-temperature ionic liquids. To test the refined force field for [bmim][BF4] proposed in our previously work (J. Phys. Chem. B, 2004, 108, 12978–12989), thermodynamic properties of mixtures of [bmim][BF4] (1) + acetonitrile (2) are presented by using molecular dynamics over the whole concentration range. The calculated densities are in good agreement with the experimental data with deviations less than 2%, indicating the force field is applicable to the mixtures. In addition, the diffusion constants, viscosities, heats of vaporization, cohesive energy densities and excess properties of the mixtures are reported. The microscopic structures are discussed in detail, corresponding to the thermodynamic properties.

Wilson's local concentration model of solution non-ideality is extended to include an enthalpic contribution in addition to the original exclusively entropic contribution. The new complete model improves the Wilson equation and makes it possible to describe liquid—liquid immiscibility phenomena. We report the representation of vapor—liquid equilibrium data of a number of mixture systems with the new complete equation. Examples are shown of application to liquid—liquid equilibrium.

The coordination numbers of rigid spheres were simulated on a computer to obtain an improved empirical equation when surrounding spheres are of uniform size. In the case of two different species of surrounding spheres, the coordination numbers vary as a function of local composition and radius ratios. By fitting simulation results, correlations were proposed, which are expected to be of practical use in thermodynamic of real solutions.