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Electrocatalytic conversion of nitrate (NO3-) into ammonia can not only eliminate harmful pollutant but also provide a green method for a low-temperature ammonia synthesis. The electrochemical NO3- reduction reactions (NO3RRs) of a series of transition-metal-doped hexagonal boron phosphide (h-BP) monolayers were comprehensively evaluated using density functional theory. The V-doped h-BP monolayer was found to stand near the top of the volcano plot with the limiting potential of -0.22 V versus a reversible hydrogen electrode, exhibiting the lowest overpotential among the investigated systems in this work. Besides, the competing hydrogen evolution reaction is significantly suppressed due to the weak adsorption of the H atom. Importantly, the structure of the V-doped h-BP monolayer can be retained very well until 900 K, illustrating the initial indication of high thermal stability and great promise for synthesis. This study not only offers an eligible NO3RR electrocatalyst but also provides an atomic understanding of the behind mechanisms of the NO3RR process.

Due to wide applications of lithium-ion batteries (LIBs), scientists have never stopped pursuing larger-capacity and higher-stability lithium storage materials. To obtain better LIBs, three distinc...

Two-dimensional transition metal oxides have been widely explored as a catalyst in the high capacity non-aqueous lithium-oxygen batteries due to their excellent electrochemical performance in the o...

Group-III phosphide (MP, M = B, Al, Ga and In) monolayers have promising applications in photocatalysis, nanoelectronics and optoelectronics. Their stabilities in oxygen and water environments are crucial to their practical applications. In order to investigate the stabilities of the MP monolayers, density functional theory calculations combined with ab initio molecular dynamics (AIMD) simulations were performed to illustrate their interactions with oxygen and water molecules. It was found that an oxygen molecule spontaneously dissociates on the AlP, GaP and InP monolayers. In the meantime, the corresponding monolayer structures are severely distorted after annealing. In contrast, BP monolayer is stable in the oxygen environment due to a high dissociative barrier of 0.89 eV. We found that a water molecule dissociates on AlP monolayer with a dissociative energy barrier of 0.42 eV, but it can be stably adsorbed on the BP, GaP and InP monolayers without the breakage of O-H bonds. Nevertheless, with the increase of the number of water molecules, the formed intermolecular H-bonding can promote interactions between water molecules and the monolayers, leading to structural distortion in the GaP and InP monolayers. When putting a water molecule near the oxygen molecule, the oxidation barrier of the BP monolayer is reduced to 0.75 eV, which is still high enough to maintain its stable state. This investigation theoretically provides environmental conditions for the experimental fabrication and practical application of semiconductor monolayers.

Silicene has recently shown high electrochemical performance with discharging product Li2O(s) and high stability, avoiding discharging byproducts for nonaqueous lithium–oxygen batteries. At the fun...

Group-IV phosphide monolayers (MP, M=C, Si, Ge and Sn) provide a versatile platform for photocatalysts, as well as optoelectronic and nanoelectronic devices. Herein, comprehensive first-principles calculations and ab initio molecular dynamics (AIMD) simulations were performed to explore their stabilities in the air. We identified that the MP monolayers have excellent mechanical properties and their carrier mobilities are higher than that of phosphorene. The MP monolayers were predicted to possess superior oxidation resistance than the boron phosphide (BP) monolayer based on the proposed donation-backdonation theory. It was observed that the dissociation and chemisorption of a water molecule on the monolayers are kinetically difficult both in the water and in oxygen-water environments involving energy barriers of 1.28-3.48 eV. We also performed AIMD simulations at 300, 1000, 1200 and 1500 K. It is noteworthy that only the carbon phosphide (CP) monolayer can retain an intact structure at 1500 K, while the other three monolayers can just sustain to 1200 K. These results provide a guidance for their practical application and experimental fabrication.

The boundary between quantum capacitance and pseudocapacitance is blurred due to their similarity in electronic populations. Here we propose the rules to classify the contributions of quantum capacitance and pseudocapacitance and their effects on the intrinsic ion transport kinetics based on the applied voltages and electronic structures. Systems including graphite, LiFePO4, LiTiS2, LiCoO2, and Ti3C2Tx MXene are employed to illustrate their different intrinsic kinetics according to the classifications based on the density functional theory calculations and the reported experimental measurements. These results can facilitate the step to screen the electrode materials with different kinetics through calculations.

Sodium storage capacity, mobility, and volume change during sodiation on the surfaces of interlayer-expanded Ti3C2 MXenes are investigated using ab initio density functional theory. The theoretical results reveal that the interlayer-expanded bare, F-, O-, and OH-functionalized Ti3C2 MXenes exhibit low barriers for sodium diffusion and small changes of lattice constant during sodiation. In addition, enlarged interlayer distance enables the stable multilayer adsorption on the bare and O-functionalized Ti3C2 MXenes and therefore significantly enhances their theoretical capacities. Both bare and O-functionalized Ti3C2 MXenes are predicted to be prospective anode materials for sodium-ion batteries with high theoretical capacities, fast discharge/charge rates, and good cycling performances. The present results provide a new route to improve the battery performances of anode materials based on MXene intercalation hosts.

Ion adsorption and electroosmotic flow induced by an external electric field have a variety of practical applications, especially for membrane technology. In this work, a partially perturbative density functional theory (DFT) based on the modified fundamental measure theory was applied to investigate the ion density distributions and partitions in a charged cylindrical pore. Different types of electrolyte solutions, including both charge symmetric and asymmetric, were examined using the proposed theory with various pore diameters, bulk densities, ion valencies and surface charge densities. The ion concentration profiles calculated with the theory exhibit good agreements with the results of the Monte Carlo simulations, while the results of the Poisson–Boltzmann equation deviate greatly especially for the high valence electrolytes in narrow cylindrical pores. Some interesting phenomena discovered in both experiments and simulations, such as the reverse distribution of the ions and charge inversion, ...

Low contact barrier electrodes and various field-emitting devices require a tunable work function, and graphene is a dream material for these applications. In this work, the theoretical investigations on the variation of the work function for monolayer graphene doped with different kinds of atoms from groups IIA–VIA of the Periodic Table are reported. The geometry, density of states, dipole moment, and work function of each heteroatom-doped graphene are calculated using ab initio density functional theory with a dispersion correction. The obtained formation energy of the heteroatom-doped graphenes is in the order: N < B < P < O < S < Si < As < Se < Ge < Al < Ga. The work functions without an electric field abide by a periodic law in terms of doping atoms except for O-doped graphene. The calculated results demonstrate that the work functions of all heteroatom-doped graphenes are a linear function of the applied external electric field intensity, and the slopes of the lines deviate from the ideal value to a...

In this work, we relate the self-diffusion coefficient to the residual entropy of the system according to the free volume theory and scaling principle. The viscosity equation for a freely jointed Lennard-Jones chain fluid is then obtained from the expression of self-diffusion coefficient by applying the Stokes–Einstein equation. The real polyatomic compounds are modeled as chains of tangent Lennard-Jones segments. The segment size and energy parameter as well as chain length (expressed by the number of segments) are obtained from the experimental viscosity data. The proposed viscosity equation reproduces the experimental viscosity data with an average absolute deviation of 5.12% for 18 polyatomic compounds (1600 data points) over wide ranges of temperature and pressure. For engineering applications, the generalized model parameters for normal alkanes with the number of carbon atoms n > 3 are proposed. The segment energy parameter is suggested to be evaluated from the critical temperature, and the segment size parameter and chain length are correlated with the number of carbon atoms in an alkane molecule.

The Gibbs ensemble Monte Carlo simulation is used to investigate the vapor–liquid phase behavior of a binary Lennard–Jones mixture confined in a slit-like pore at reduced temperature T* = 1.0. Our simulation program is tested by comparing the simulated phase diagram with the previous simulation results from various methods. Then phase diagrams of a binary asymmetric Lennard–Jones mixture confined in the slit-like pores are investigated by varying the pore width and fluid–pore wall interaction. The asymmetric system studied exhibits azeotropic behavior. The pore walls are symmetric but selective to the two components. The simulated results indicate that both the pore width and interactions between fluid and the pore wall have significant influence on the vapor–liquid phase diagram for the binary Lennard–Jones mixture. When the interaction energy parameter between component 1 and the pore wall is fixed, the azeotropic point is shifted from the bulk azeotropic composition to component 2-rich side of the phase diagram as the interaction strength between component 2 and the pore wall is increased. In the meantime, both the azeotropic pressure and the relative volatility increase significantly. As the pore becomes narrower, it is found that the azeotropic pressure increases significantly while the azeotropic composition remains almost unchanged. The phase selectivity decreases as the pore becomes narrower in the whole range of composition. All the simulated results suggest that the azeotropic point of a binary mixture can be shifted or even be removed by the confinement and the relative volatility for vapor–liquid equilibria can be changed greatly by changing the pore width or fluid–pore wall interactions.

Structural and kinetic properties of vitamin E in biomembranes provide the key to understanding the biological functions of this lipophilic vitamin. We report a series of molecular dynamics simulations of two alpha-tocopherol/phosphatidylcholine systems and two alpha-tocopherol/phosphatidylethanolamine systems in water at 280, 310, and 350 K. The preferential position, hydrogen bonding, orientation, and dynamic properties of the alpha-tocopherol molecule in the bilayers have been examined. In all the four systems simulated, the vitamin remains in one leaflet of lipid bilayer at 280 and 310 K but flips over from one side to the other at 350 K within 200 ns of the simulation. The hydroxyl oxygen in the headgroup of alpha-tocopherol preferred a location between the third and the fifth carbon atom in the sn-2 acyl chains of the lipids. Hydrogen bonding analysis shows that the hydrogen bonds are mainly with the oxygens of the fatty acid esters rather than with the phosphate oxygens of the lipid molecule, and those with the amino groups are trivial in the case of phosphatidylethanolamines, at all three temperatures. The hydrogen bonds with phosphatidylethanolamines are more stable than those with phosphatidylcholines at low temperatures. The orientation of alpha-tocopherol in the bilayers is relatively flexible: the chromanol ring takes various tilt angles with respect to the bilayer normal, and the isoprenyl chain is mobile and able to adopt many different conformers. Calculation of lateral diffusion coefficients of alpha-tocopherol and phospholipid molecules shows that alpha-tocopherol has a comparable diffusion rate with phospholipid molecules at the gel phase but diffuses more rapidly than lipid molecules at the liquid-crystal phase.

The structural properties of fully hydrated distearoylphosphatidylcholine (DSPC) and distearoylphosphatidylethanolamine (DSPE) bilayers near the main phase transition were investigated using molecular dynamics simulations on the basis of a united-atom model. Although largely similar in their molecular structures, the two lipids were found with different molecular packing modes at temperatures below the phase transition. For DSPC, three packing modes, namely, cross-tilt, partially interdigitated, and "mixed" gel phases, were observed, while, for DSPE, the lipid tails were almost perpendicular to the lipid surface. Above the main transition temperature, both lipid bilayers transformed into a disordered liquid-crystal phase with marked greater area per lipid and gauche % of the acyl chains and smaller bilayer thickness and order parameter, in comparison with the gel phase. The transformation process of liquid-crystal to gel phase was proved to experience the nucleation and growth stages in a hexagonal manner. The electron density profiles of some major components of both lipid bilayers at various temperatures have been calculated, and the results reveal that both lipid bilayers have less interdigitation around the main transition temperature.

Mutual diffusion coefficients of concentrated 1:1 electrolyte solutions are investigated using a modified mean spherical approximation (MSA), in which the effective cationic diameter is a function of total ionic strength. We apply this improved MSA to calculate the equilibrium pair correlation function, and the Smoluchowski equation is used for the dynamics. We compare the calculated mutual diffusion coefficients of 18 uni-valence electrolyte solutions for molar concentrations up to 4.0 mol/L known experimental data. Good agreement between theoretical results and the experimental mutual diffusion coefficients are found for all the cases studied.

The ion density profiles and mean electrostatic potentials around DNA from the Monte Carlo simulations are compared with the predictions of the Poisson–Boltzmann equation and the density functional theory. The DNA molecules are modeled as a charged cylinder while the ions are represented as charged hard spheres with different diameter. In the density functional theory, the Helmholtz free energy functional due to hard-sphere repulsion and electrostatic interaction are obtained from the modified fundamental measure theory and a quadratic functional Taylor expansion, respectively. The results show that due to the inclusion of the ion–ion correlation, the density functional theory is more accurate than the Poisson–Boltzmann equation. The density functional theory gives accurate ion structures and mean electrostatic potentials near the surface of DNA. When the established density functional theory is combined with the cell model, the osmotic coefficients of aqueous DNA–electrolyte solutions can be predicted. The results show that the DFT-cell model captures the essential features of the experimental osmotic coefficient, but fails to give a quantitative description. Possible reasons for this discrepancy are discussed.

The space-charge (SC) model and the Teorell–Meyer–Sievers (TMS) model are widely employed to calculate the membrane potential of charged porous membranes in 1-1 electrolyte solutions, but few works with the other kinds of electrolyte solutions have been reported. In this article, membrane potentials for 1-1, 1-2, 2-1 and 2-2 electrolyte solutions have been calculated numerically based on the two models using parameters of concentration, pore radius and surface charge density. The results suggest that equivalent electrolyte concentration is more suitable to characterize the membrane potential in different kinds of electrolyte solutions than electrolyte concentration. Membrane potential approaches to the Nernst potential in the low concentration region, which indicates membrane potential in monovalent cation electrolyte solution is twice as large as that in divalent cation electrolyte solution. In the high concentration region, membrane potential is close to the diffusion potential, which implies membrane potential would be reversed if the diffusion coefficient ratio of coion and counterion is larger than 1.0. While in the intermediate concentration region, membrane potential as a function of electrolyte concentration is almost linear. In the comparison of the SC and TMS models, theoretical predictions by the two models for 1-2, 2-1 and 2-2 electrolyte solutions appear to be similar to those for 1-1 electrolyte solutions reported earlier by Westermann-Clark and Christoforou [3]. The two models coincide with each other when the radius, rp, is less than 5.0 nm and the dimensionless charge density in the pore wall, q0 is less than 1.0. Otherwise, the TMS model would overestimate the membrane potential. Furthermore, the membrane potential shows dependence on the ratio of the volume charge density to equivalent electrolyte concentration, ξf, q0 and rp by the SC model, but only varies with ξf irrespective of q0 and rp by the TMS model. These findings have a significant influence on the establishment of an appropriate theory to evaluate membrane potentials in different electrolyte solutions.

A density functional theory is proposed to investigate the effects of polymer monomer-monomer and monomer-wall attractions on the density profile, chain configuration, and equilibrium capillary phase transition of a freely jointed multi-Yukawa fluid confined in a slitlike pore. The excess Helmholtz energy functional is constructed by using the modified fundamental measure theory, Wertheim's first-order thermodynamic perturbation theory, and Rosenfeld's perturbative method, in which the bulk radial distribution function and direct correlation function of hard-core multi-Yukawa monomers are obtained from the first-order mean spherical approximation. Comparisons of density profiles and bond orientation correlation functions of inhomogeneous chain fluids predicted from the present theory with the simulation data show that the present theory is very accurate, superior to the previous theory. The present theory predicts that the polymer monomer-monomer attraction lowers the strength of oscillations for density profiles and bond orientation correlation functions and makes the excess adsorption more negative. It is interesting to find that the equilibrium capillary phase transition of the polymeric fluid in the hard slitlike pore occurs at a higher chemical potential than in bulk condition, but as the attraction of the pore wall is increased sufficiently, the chemical potential for equilibrium capillary phase transition becomes lower than that for bulk vapor-liquid equilibrium.

The equilibrium selective adsorption and fluxes of oxygen/nitrogen binary gas mixtures through carbon membranes are investigated at 303 K, respectively, using a grand canonical Monte Carlo simulation and a dual control volume grand canonical molecular dynamics method. The carbon membrane pores are modeled as slit-like pores with a two-dimensional structure where carbon atoms are placed according to the structure of graphite layers. The effect of the membrane thickness, bulk pressure, and pore width on the equilibrium selective adsorption and dynamic separation factor is discussed. Meanwhile a new iteration approach to calculate the flux and dynamic separation factor of binary gas mixtures through membranes is proposed, by which we can simulate the permeation and fluxes of gases through the membranes in the presence of pressure gradient and consider the effect of pressure and composition of low-pressure side in the meantime. The simulated results show that bulk pressure and membrane thickness have no effect on the equilibrium selectivity, but they have a great effect on the fluxes and dynamic separation factors of gases. The pore width impacts the equilibrium selectivity and dynamic separation factors strongly, especially when the pore width is very small. Molecular sieving dominates the separation of oxygen/nitrogen in non-equilibrium simulations. But due to the comparable molecular size of oxygen and nitrogen, we have to modify the carbon membranes in order to improve dynamic separation of atmosphere.

Because of the increasing interest in studying the phenomenon exhibited by charge-stabilized colloidal suspensions in confining geometry, we present a density functional theory (DFT) for a hard-core multi-Yukawa fluid. The excess Helmholtz free-energy functional is constructed by using the modified fundamental measure theory and Rosenfeld's perturbative method, in which the bulk direct correlation function is obtained from the first-order mean spherical approximation. To validate the established theory, grand canonical ensemble Monte Carlo (GCMC) simulations are carried out to determine the density profiles and surface excesses of multi-Yukawa fluid in a slitlike pore. Comparisons of the theoretical results with the GCMC data suggest that the present DFT gives very accurate density profiles and surface excesses of multi-Yukawa fluid in the slitlike pore as well as the radial distribution functions of the bulk fluid. Both the DFT and the GCMC simulations predict the depletion of the multi-Yukawa fluid near a nonattractive wall, while the mean-field theory fails to describe this depletion in some cases. Because the simple form of the direct correlation function is used, the present DFT is computationally as efficient as the mean-field theory, but reproduces the simulation data much better than the mean-field theory.

Monte Carlo simulations have been carried out to investigate supercritical ethanol using the OPLS–UA [W.L. Jorgensen, Optimized intermolecular potential functions for liquid alcohols, J. Phys. Chem. 90 (1986) 1276–1284] and TraPPE–UA [B. Chen, J.J. Potoff, J.I. Siepmann, Monte Carlo calculations for alcohols and their mixtures with alkanes. Transferable potentials for phase equilibria. Part 5: united-atom description of primary, secondary, and tertiary alcohols, J. Phys. Chem. B 105 (2001) 3093–3104] force fields. Two potentials were compared and TraPPE potential performed better in reproducing densities as a function of pressure, so, it was selected in further structural analysis. The pair distribution functions were obtained and their pressure and temperature dependence were analyzed. In addition, a serial two dimensional distribution functions were plotted to depict the detailed structure of liquid and supercritical ethanol. Based on a widely used geometric criterion, hydrogen bonding statistical results showed that the hydrogen bonds become weaker but still exist under supercritical condition, which are in good agreement with recent experimental measurements. The hydrogen bonds of water, methanol and ethanol were compared and ethanol has nearly the same behavior as methanol under supercritical conditions.

The structural properties and transport properties of electrolyte solutions are investigated by smart Brownian dynamics simulation based on the primitive model. To test the simulation program, the radial distribution functions of ions g +- (r) and g ++ (r) for KCl solution are calculated and compared with the results of previous simulations. And then, the effects of ions diameter and concentration on the structural properties and transport properties of electrolyte solution at 298 K are extensively simulated. The results indicate that both the radial distribution function and the self-diffusion coefficient are more sensitive to the ion size than salt concentration. As the radius ratio of cation to anion increases, self-diffusion coefficients of both cation and anion become larger.

A density-functional theory is proposed to describe the density profiles of small ions around an isolated colloidal particle in the framework of the restricted primitive model where the small ions have uniform size and the solvent is represented by a dielectric continuum. The excess Helmholtz energy functional is derived from a modified fundamental measure theory for the hard-sphere repulsion and a quadratic functional Taylor expansion for the electrostatic interactions. The theoretical predictions are in good agreement with the results from Monte Carlo simulations and from previous investigations using integral-equation theory for the ionic density profiles and the zeta potentials of spherical particles at a variety of solution conditions. Like the integral-equation approaches, the density-functional theory is able to capture the oscillatory density profiles of small ions and the charge inversion (overcharging) phenomena for particles with elevated charge density. In particular, our density-functional theory predicts the formation of a second counterion layer near the surface of highly charged spherical particle. Conversely, the nonlinear Poisson-Boltzmann theory and its variations are unable to represent the oscillatory behavior of small ion distributions and charge inversion. Finally, our density-functional theory predicts charge inversion even in a 1:1 electrolyte solution as long as the salt concentration is sufficiently high.

Canonical-ensemble Monte Carlo simulation and an improved fundamental-measure theory are applied to calculating the structures and chemical potentials of neutral and associating spherical particles confined in rectangular or corrugated microchannels. It is found that the confinement significantly affects the distributions of neutral spheres in the microchannels, especially at high densities or near the confining surfaces. However, for associating particles, the combined effects of packing and association lead to virtually uniform density distributions. The density profiles calculated from the density functional theory agree well with simulation results for neutral hard spheres in both rectangular and corrugated microchannels except when the average packing density inside the channel is near the freezing point.

In substitution of path integral isomorphism of the quantum particle, an effective polymer ring model is proposed in the density functional calculation for hydrogen adsorption in single-walled carbon nanotubes. The excess intrinsic Helmholtz energy for quantum particles includes contributions from hard-sphere repulsion, interatomic bonding and soft attraction. The first two contributions are considered through the method developed by Yu and Wu [J. Chem. Phys. 117, 2368 (2002)], and the last contribution is obtained from mean field approximation using Weeks–Chandler–Anderson potential. The theoretical predictions are in good agreement with Monte Carlo simulation data for the density distributions of the hydrogen molecule inside the tube. In addition, the proposed model is applied to the calculation of the adsorption isotherms of hydrogen at 100 and 150 K. The present model is simpler than the current existing theories for quantum fluids.

Solid–liquid equilibria in mixed electrolyte aqueous solution have been investigated using available thermodynamic data for solids and for aqueous electrolyte solutions. The mean spherical approximation (MSA) modified by Lu et al. [Fluid Phase Equilib. 85 (1993) 81] is used to calculate the mean ionic activity coefficients in mixed electrolyte solutions at saturation conditions. Solid–liquid equilibria of seven mixed electrolyte systems at 298.15 K are successfully predicted using the modified MSA method with the parameters obtained from activity coefficient data of corresponding single electrolyte solutions. The total average absolute deviation between predicted and experimental values is 5.58%. Furthermore, the predicted results of solid–liquid equilibria for four mixed electrolyte solutions over a range of temperature indicate that the modified MSA method can fairly be used to predict solid–liquid equilibria for mixed electrolyte aqueous solutions at various temperatures.

The Percus’ test-particle method is extended to predict the inter- and intramolecular correlation functions of polymeric fluids using a density functional theory developed earlier [J. Chem. Phys. 117, 2368 (2002)]. The calculated inter- and intramolecular distribution functions as well as the site–site correlation functions agree well with the results from Monte Carlo simulation for freely jointed hard-sphere chains. Compared with the integral-equation approaches and alternative density functional theories, the present method is free of molecular simulations as input and has the advantage of self-consistency among inter- and intramolecular correlation functions and thermodynamic properties.

We reformulate Rosenfeld’s fundamental-measure theory using the excess Helmholtz energy density from the Boublik–Mansoori–Carnahan–Starling–Leland equation of state instead of that from the scaled-particle theory. The new density functional theory yields improved density distributions, especially the contact densities, of inhomogeneous hard-sphere fluids as well as more accurate direct and pair correlation functions of homogeneous hard spheres including those of highly asymmetric mixtures. This new density functional theory will provide an accurate reference for the further development of a statistical-thermodynamic theory of complex fluids at uniform and at inhomogeneous conditions.

A new density functional theory is developed for inhomogeneous mixtures of polymeric fluids by combining Rosenfeld’s fundamental-measure theory for excluded volume effects with Wertheim’s first-order thermodynamic perturbation theory for chain connectivity. With no adjustable parameters, theoretical predictions are in excellent agreement with Monte Carlo simulation data for the density distributions and for the adsorption isotherms of hard-sphere chains near hard walls or in slit-like pores. This theory is applied to calculate the force between two parallel hard walls separated by hard-sphere chains at different densities. Calculated results indicate that the chain-mediated force is attractive and decays monotonically with separation at low chain densities, it oscillates at high chain densities and in between, it is attractive at small separation and repulsive at large separation. This new density functional theory is simpler than similar theories in the literature and is directly applicable to mixtures.

The fundamental-measure theory (FMT) of Rosenfeld for hard spheres is extended to inhomogeneous associating fluids on the basis of Wertheim’s first-order thermodynamic perturbation theory (TPT1). The excess intrinsic Helmholtz energy, which includes contributions from hard-sphere repulsion and from intermolecular bonding, is represented as a functional of three weighted densities that are related to the geometry of spherical particles. In the absence of association, this theory is the same as the original FMT, and at bulk conditions it reduces to TPT1. In comparison with Monte Carlo simulation results, the extended fundamental-measure theory provides good descriptions of the density profiles and adsorption isotherms of associating hard spheres near a hard wall. Calculated results indicate that the critical temperatures for the vapor–liquid equilibria of associating fluids in hard slit pores are suppressed compared with that for the bulk fluid and the confinement has more significant impact on the liquid side than the vapor side of the coexistence curve. Unlike nonpolar fluids at similar conditions, saturated associating liquids in hard slit pores do not exhibit strong layering near the solid surface.

In this paper, the separation of binary gas mixture carbon monoxide and hydrogen using single-walled carbon nanotubes (SWNTs) is studied by grand canonical Monte Carlo (GCMC) simulation. All of the particle–particle interactions between hydrogen, carbon monoxide and carbon are modeled with Lennard–Jones potential. Widom test particle method in NVT ensemble is used in determining chemical potentials of the two components of the fluid. It can be concluded from the adsorption isotherms at 293.15 K that the selectivity of carbon monoxide decreases with the increase of the bulk pressure. In the adsorption isostatics at 0.157 MPa, the selectivity has a maximum along with the increase of temperature. However, to given pressure and temperature, the selectivity fluctuates violently with different van der Waals (VDW) distances or diameters of carbon nanotubes. For example, at 0.157 MPa, 293.15 K, diameter of 0.892 nm and VDW distance of 1 nm, with the feed containing hydrogen and carbon monoxide both 50 mol%, carbon monoxide could be enriched up to 98 mol% in the adsorbate, so that the selectivity comes to 60. It can be indicated that carbon nanotube, as a novel material developed in the past 10 years, can make a very effective separation tool of certain gas mixtures.

The nucleation path of graphene growth on the Cu(111) surface is investigated by importing carbon atoms step-by-step using density functional theory (DFT) calculations. An overall path of graphene nucleation has been proposed based on configuration and energy analysis. At the very first stage, linear chains will be formed and dominate the copper surface. Then, Y-type (furcate) carbon species will be shaped when new carbon atoms are absorbed aside the linear chains. Finally, ring-containing carbon species and graphene islands will be formed stepwise, with energetic preference. We find that the Y-type and ring-containing carbon species are not likely formed directly at the initial stage of graphene nucleation, but should be formed starting from linear chains. The nucleation limiting step is the formation of the Y-type species, which must pass an energy barrier of about 0.25 eV. These underlying observations are instructive to stimulate future experimental efforts on graphene synthesis.

Boiling points have been determined at 101.33 kPa for the binary mixtures of sulfolane+o-xylene, sulfolane+m-xylene, sulfolane+p-xylene, sulfolane+ethylbenzene and sulfolane+1,2,4-trimethylbenzene. Calculations of the non-ideality of the vapor phase were made with the second virial coefficients evaluated from the Hayden–O’Connell method. The binary parameters for five activity coefficient models (Margules, van Laar, Wilson, NRTL and UNIQUAC) have been fitted with the experimental boiling points measured in this work. A comparison of model performances has been carried out using the criterion of the average absolute deviations in boiling point. The activity coefficient of the component in the liquid phase is discussed based on the UNIFAC model with the consideration of the dipole–dipole interactions.

The solubilities of oxygen in 0.2, 0.5, 0.7 and 1.0 M Na 2 CO 3 solution have been measured at 300.15 K and under pressures up to 10 MPa using a magnetically stirred autoclave and a direct sampling technique. The accuracy of apparatus was verified by duplicating the solubility of oxygen in pure water in literature. The experimental data of the solubility of oxygen in aqueous sodium carbonate solution were shown that the solubility of oxygen increases with increasing pressure and decreases with increasing salt concentration due to salting-out effect. The experimentally measured data were satisfactorily compared with the predicted values by our model based on non-primitive mean spherical approximation (MSA) and perturbation theory.

An attempt is made in this work to combine the Lennard‐Jones chain model (LJC) of self-diffusion coefficient with the statistical associating fluid theory (SAFT). The real non-spherical associating molecules are modeled as chains of tangent Lennard‐Jones segments with association sites. An equation for the self-diffusion coefficient in a polyatomic associating fluid is presented as a product of a non-hydrogen-bond contribution and a hydrogen-bond contribution. The SAFT equation provides the density and temperature dependence of an average number of hydrogen bonds in a molecule, and the LJC equation is used to calculate the self-diffusion coefficient for a non-associating fluid. The segment‐segment interaction energy " is obtained from the critical temperature for alcohols and hydrogen fluoride, and, the segment diameter , the chain length N (the number of segments), association energy " HB and volume HB are determined from the experimental diffusion data. The equation reproduces the experimental self-diffusion coefficient with total average absolute deviation of 6.69% for water, 6% alcohols and hydrogen fluoride over wide ranges of temperature and pressure, including the super-critical water. © 2001 Elsevier Science B.V. All rights reserved.

The self-diffusion coefficients for a dense fluid of particles interacting with a square-well potential are described. The expression for self-diffusion coefficients obtained by the Chapman–Enskog method of solution is corrected by the molecular dynamics simulation data. The structural data for the square-well fluid required in the calculation are obtained from explicit analytic equations for the radial distribution function. The corrected equation represents the self-diffusion coefficients with an average absolute deviation of 4.63% for the square-well fluid. The results are compared with those of other expressions. In addition, the self-diffusion coefficients for real fluids composed of simple, near spherical molecules are correlated using the square-well potential, and acceptable agreement is obtained between the calculated and experimental diffusion data.

The concentration dependence of the surface tension of single and mixed electrolyte aqueous solutions is studied, based on the assumption that the surface layer can be treated as a separate phase located between vapor and bulk liquid phases. The mean spherical approximation modified by Lu et al. [J.-F. Lu, Y.-X. Yu, Y.-G. Li, Fluid Phase Equilibria 85 (1993) 81–100] is used to calculate the activity coefficients of water in the surface and bulk liquid phases. The relation between the electrolyte concentration in the surface and bulk liquid phases is established and only one parameter needs to be determined. The surface tensions for 31 single electrolyte aqueous solutions are correlated and the overall average absolute deviation is 0.70%. The surface tensions at different temperatures are predicted with the parameters obtained at one fixed temperature. By introducing the proper mixing rules, the surface tensions for 14 mixed electrolyte aqueous solutions are predicted without any mixing parameters, and the total average absolute deviation is 0.63%. All the calculated results are compared with that of the surface tension model for aqueous electrolyte solutions proposed by Li et al. [Z.-B. Li, Y.-G. Li, J.-F. Lu, Ind. Eng. Chem. Res. 38 (1999) 1133–1139].

An equation for the self-diffusion coefficient in a polyatomic fluid is presented as a sum of three friction coefficient terms: the temperature-dependent hard-sphere contribution, the chain contribution and the soft contribution. This equation has been developed by using the molecular dynamics simulation data for the HS chain fluid and the expression for the Lennard–Jones (LJ) fluid proposed by Ruckenstein and Liu. The real nonspherical compounds are modeled as chains of tangent LJ segments. The segment diameter σ LJ , segment–segment interaction energy e LJ and chain length N (the number of segments) are obtained from the experimental diffusion data. The equation reproduces the experimental self-diffusion coefficients with an average absolute deviation (AAD) of 3.72% for 22 polyatomic compounds (1081 data points) over wide ranges of temperature and pressure. The results have been compared with that of the rough LJ (RLJ) equation. To minimize the number of the fitting parameters, the energy parameter e LJ is estimated using a correlation obtained from viscosity data. The equation with two parameters gives an AAD of 4.72%.

A new model for calculating high-pressure solubility of gas in aqueous electrolyte solution is proposed based . on non-primitive mean spherical approximation MSA equation and perturbation theory. The non-primitive MSA is used to describe the interactions of ion-ion, ion-dipole and dipole-dipole. The Lennard-Jones potential energy as a perturbation term is introduced to represent the dispersion effects among particles including both ion and molecule species. The water molecule parameters are derived from the experimental saturated vapor pressure data of pure water and the ion parameters of cations and anions are obtained by fitting experimental mean ionic activity coefficients. The parameters of gas solved in aqueous salt solution are only derived from the experimental solubility data of gas in pure water. The equation of state derived from the previous procedures have been used successively to predict gas solubility in aqueous salt solution using the pure-component parameters. q 1999 Elsevier Science B.V. All rights reserved.

Isobaric vapor–liquid equilibrium data have been determined at 101.33 kPa for the binary mixtures of benzene-tetraethylene glycol (TeEG), toluene-TeEG and o-xylene-TeEG. The vapor-phase fugacity coefficients were calculated from the virial equation. The thermodynamic consistency of the data has been tested via Herington analysis. The binary parameters for four activity coefficient models (van Laar, Wilson, NRTL and UNIQUAC) have been fitted with the experimental data. A comparison of model performances has been made by using the criterion of root mean square deviations in boiling point and vapor-phase composition.

Excess molar volume data on mixing for binary mixtures of sulfolane with toluene, o-xylene, m-xylene, p-xylene, ethylbenzene and 1,2,4-trimethyl benzene have been measured over the entire composition range at 298.15 K and atmospheric pressure in order to investigate interactions between molecules. A vibrating tube density meter was used. All mixtures exhibit negative excess volumes with a minimum which occurs approximately at x=0.5. The experimental results have been correlated using the Redlich–Kister equation and have been qualitatively discussed.

An apparatus for measuring high pressure liquid–liquid equilibria was established. In order to diminish disturbance for the equilibration, the capillaries of stainless steel with 0.7 mm in inside diameter were used to take samples. The taken samples were totally evaporated into gas chromatography. The accuracy of the apparatus was verified by duplicating the literature data. Liquid–liquid equilibrium data for pseudo-ternary mixture of water–2-butanol–C4-hydrocarbon (the mixture of n-butane+trans-2-butene+cis-2-butene) were experimentally determined under pressure 8 MPa and at temperatures from 403.2 to 443.2 K. The measured liquid–liquid equilibrium data could be correlated by modified Redlich–Kwong (R–K) equation of state.

The electronic and adsorption properties of graphene can be changed significantly through substitutional doping with nitrogen and nitrogen decoration of vacancies. Here ab initio density functional theory with a dispersion correction was used to investigate the stability, magnetic and adsorption properties of nine defects in graphene, including both nitrogen substitutional doping and nitrogen decoration of vacancies. The results indicate that only pyridinic N2V2 defect in graphene shows a ferromagnetic spin structure with high magnetic moment and magnetic stabilization energy. Not all nitrogen-doped defects can improve the capacity of the lithium-ion batteries. The adsorption energies of a lithium atom on nitrogen-substituted graphenes are more positive, indicating that they are meta-stable and no better than the pristine graphene as anode materials of lithium-ion batteries. Nitrogen-decorated single and double vacancy defects, especially for the pyridinic N2V2 defect in graphene, can greatly improve the reversible capacity of the battery in comparison with the pristine graphene. The theoretical prediction of the reversible capacity of the battery is 1039 mA h g(-1) for the nitrogen-doped graphene material synthesized by Wu et al., which is in good agreement with the experimental data (1043 mA h g(-1)). The theoretical computations suggest that nitrogen-decorated single and double vacancy defects in graphene are the promising candidate for anode materials of lithium-ion batteries. Each nitrogen atom in the decoration can improve the reversible capacity of the battery by 63.3-124.5 mA h g(-1) in a 4 × 4 supercell of graphene. The present work provides crucial information for the development of N-doped graphene-based anode materials of lithium-ion batteries.

An equation of state for homonuclear chain molecules is formulated on the basis of thermodynamic perturbation theory of Wertheim. It yields a very accurate prediction of the compressibility factor of different chain molecules including hard spheres and hard dumbbells as well as flexible chains with up to 200 bonds in a molecule. In addition, a large number of molecular simulation results are used to test the thermodynamic first-order perturbation theory (TPT1), the thermodynamic second-order perturbation theory (TPT2), the scaled particle theory (SPT), the Percus-Yevick and Carnahan-Starling (PY-CS) equation, the generalized Flory-Dimer equation (GF-D) and our equation. For short chain molecules ( m ≥ 4), good agreement between the calculated and computer simulation data can be obtained from all of the above equations, but for long chain molecules the equation proposed in this work is better than the other equations.

Lu, J.-F., Yu, Y.-X. and Li, Y.-G., 1993. Modification and application of the mean spherical approximation method. Fluid Phase Equilibria, 85: 81-100. The mean spherical approximation (MSA) method is improved by the introduction of the effective diameter of the cation. The dependence of the ionic strength on the effective diameters is mainly attributed to the solvation effect. This modified model has been used to correlate the mean ionic activity coefficients for 85 single-electrolyte solutions. The results show that the modified MSA gives smaller deviations than the original MSA and Pitzer models. The modified MSA was also applied to calculate 32 mixed-electrolyte solutions without any mixing parameters. The calculated results indicate that the modified MSA can be used to calculate the mixture properties in terms of the parameters of single-electrolyte solutions. Furthermore, the mean ionic activity coefficients of some single-electrolyte solutions can be calculated for other temperatures by using the parameters obtained at 298.15 K.

A new density functional theory (DFT) for an inhomogeneous 12-6 Lennard-Jones fluid is proposed based on the modified fundamental measure theory for repulsive interaction and a weighted density functional for attractive interaction. The Helmholtz free energy functional for the attractive part is constructed using the modified Benedict-Webb-Rubin equation of state with a mean-field weight function. Comparisons of the theoretical results with molecular simulation data suggest that the new DFT yields accurate bulk surface tension, density distributions, adsorption-desorption isotherms, pore pressures, and capillary phase transitions for the Lennard-Jones fluid confined in slitlike pores with different widths and solid-fluid interactions. The new DFT reproduces well the vapor-liquid critical temperatures of the confined Lennard-Jones fluid, whereas the mean-field theory always overestimates the critical temperatures. Because the new DFT is computationally as simple and efficient as the mean-field theory, it will provide a good reference for further development of a statistical-thermodynamic theory of complex fluid under both homogeneous and inhomogeneous conditions when disperse force has to be considered.

A density functional theory (DFT) in the framework of cell model is proposed to calculate the structural and thermodynamic properties of aqueous DNA-electrolyte solution with finite DNA concentrations. The hard-sphere contribution to the excess Helmholtz energy functional is derived from the modified fundamental measure theory, and the electrostatic interaction is evaluated through a quadratic functional Taylor expansion around a uniform fluid. The electroneutrality in the cell leads to a variational equation with a constraint. Since the reference fluid is selected to be a bulk phase, the Lagrange multiplier proves to be the potential drop across the cell boundary (Donnan potential). The ion profiles and electrostatic potential profiles in the cell are calculated from the present DFT-cell model. Our DFT-cell model gives better prediction of ion profiles than the Poisson-Boltzmann (PB)- or modified PB-cell models when compared to the molecular simulation data. The effects of polyelectrolyte concentration, ion size, and added-salt concentration on the electrostatic potential difference between the DNA surface and the cell boundary are investigated. The expression of osmotic coefficient is derived from the general formula of grand potential. The osmotic coefficients predicted by the DFT are lower than the PB results and are closer to the simulation results and experimental data.