Guang-Hua Gao

Density-functional theory of spherical electric double layers and ζ potentials of colloidal particles in restricted-primitive-model electrolyte solutions

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.

Simulation study of hydrogen storage in single walled carbon nanotubes

Abstract Hydrogen storage in single-walled carbon nanotubes (SWNTs) is studied by grand canonical Monte Carlo (GCMC) simulation. Hydrogen–hydrogen and hydrogen–carbon interactions are both modeled with Lennard–Jones potential. Hydrogen–carbon interactions are integrated over the whole nanotube to get molecule–tube interactions. Three adsorption isotherms of different diameters at 293.15 K , one adsorption isostatics at 2.66 MPa with radius of 0.587 nm , the amount of adsorption as a function of van der Waals (VDW) distance of nanotubes with the three diameters at 3 MPa (where the VDW distance is defined as the distance between the walls of the nearest neighbor tubes in the bundle, as measured from the carbon centers) and the adsorption as function of continuously changing diameter are displayed. Finally, the influences of pressures, temperatures, the diameters and VDW distances of SWNTs on adsorption are discussed.

A simple correlation to evaluate binary interaction parameters of the Peng-Robinson equation of state: binary light hydrocarbon systems

Abstract Gao, G., Daridon, J.-L., Saint-Guirons, H., Xans, P. and Montel, F. 1992. A simple correlation to evaluate binary interaction parameters of the Peng-Robinson equation of state: binary light hydrocarbon systems. Fluid Phase Equilibria , 74:85-93 A simple correlation to evaluate binary interaction parameters k ij of the Peng-Robinson equation of state for light hydrocarbons mixtures is proposed, as a function only of critical temperature and compressibility factor. The method yields results with an accuracy bordering on that obtained with methods requiring specific adjustments for each binary mixture coefficient k ij .

Surface tension for aqueous electrolyte solutions by the modified mean spherical approximation

Abstract 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].

Structures and correlation functions of multicomponent and polydisperse hard-sphere mixtures from a density functional theory

The structures of nonuniform binary hard-sphere mixtures and the correlation functions of uniform ternary hard-sphere mixtures were studied using a modified fundamental-measure theory based on the weight functions of Rosenfeld [Rosenfeld, Phys. Rev. Lett. 63, 980 (1989)] and Boublik-Mansoori-Carnahan-Starling-Leland equation of state [Boublik, J. Chem. Phys. 53, 471 (1970); Mansoori et al., J. Chem. Phys. 54, 1523 (1971)]. The theoretical predictions agreed very well with the molecular simulations for the overall density profiles, the local compositions, and the radial distribution functions of uniform as well as inhomogeneous hard-sphere mixtures. The density functional theory was further extended to represent the structure of a polydisperse hard-sphere fluid near a hard wall. Excellent agreement was also achieved between theory and Monte Carlo simulations. The density functional theory predicted oscillatory size segregations near a hard wall for a polydisperse hard-sphere fluid of a uniform size distribution.

Self-diffusion coefficient equation for polyatomic fluid

Abstract 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%.

Density-functional theory and Monte Carlo simulation study on the electric double layer around DNA in mixed-size counterion systems

A density-functional approach and canonical Monte Carlo simulations are presented for describing the ionic microscopic structure around the DNA molecule immersed in mixed-size counterion solutions. In the density-functional approach, the hard-sphere contribution to the Helmholtz energy functional is obtained from the modified fundamental measure theory [Y.-X. Yu and J. Z. Wu, J. Chem. Phys. 117, 10156 (2002)], and the electrostatic contribution is evaluated through a quadratic functional Taylor expansion. The new theory is suitable to the systems containing ions of arbitrary sizes and valences. In the established canonical Monte Carlo simulation, an iterative self-consistent method is used to evaluate the long-range energy, and another iterative algorithm is adopted to obtain desired bulk ionic concentrations. The ion distributions from the density-functional theory (DFT) are in good agreement with those from the corresponding Monte Carlo (MC) simulations. It is found that the ratio of the bulk concentrations of two species of counterions (cations) makes significant contribution to the ion distributions in the vicinity of DNA. Comparisons with the electrostatic potential profiles from the MC simulations show that the accuracy of the DFT becomes low when a small divalent cation exists. Both the DFT and MC simulation results illustrate that the electrostatic potential at the surface of DNA increases as the anion diameter or the total cation concentration is increased and decreases as the diameter of one cation species is increased. The calculation of electrostatic potential using real ion diameters shows that the accuracy of DFT predictions for divalent ions is also acceptable.

Structures and adsorption of binary hard-core Yukawa mixtures in a slitlike pore: Grand canonical Monte Carlo simulation and density-functional study

The grand canonical ensemble Monte Carlo simulation and density-functional theory are applied to calculate the structures, local mole fractions, and adsorption isotherms of binary hard-core Yukawa mixtures in a slitlike pore as well as the radial distribution functions of bulk mixtures. The excess Helmholtz energy functional is a combination of the modified fundamental measure theory of Yu and Wu [J. Chem. Phys. 117, 10156 (2002)] for the hard-core contribution and a corrected mean-field theory for the attractive contribution. A comparison of the theoretical results with the results from the Monte Carlo simulations shows that the corrected theory improves the density profiles of binary hard-core Yukawa mixtures in the vicinity of contact over the original mean-field theory. Both the present corrected theory and the simulations suggest that depletion and desorption occur at low temperature, and the local segregation can be observed in most cases. For binary mixtures in the hard slitlike pore, the present corrected theory predicts more accurate surface excesses than the original one does, while in the case of the attractive pore, no improvement is found in the prediction of a surface excess of the smaller molecule.

Density functional study of the adsorption and separation of hydrogen in single-walled carbon nanotube

Abstract In this work, we report an investigation by means of density functional theory (DFT) of the adsorption of hydrogen and the separation of hydrogen–carbon monoxide mixture in an isolated single-walled carbon nanotube. The theory is based on a perturbative construction of free energy functional for inhomogeneous pure fluid and binary fluid mixture. The reformulated Rosenfelds fundamental-measure theory using the excess Helmholtz energy density from the Boublik–Mansoori–Carnahan–Starling–Leland equation of state proposed by Yu and Wu (J. Chem. Phys. 117 (2002) 10156) is applied to represent the pure and binary hard-sphere repulsive interaction, and Weeks–Chandler–Andersen perturbation theory is used to build the attractive contribution. The density profiles in three sizes of tubes at 300 K and reduced bulk density from 0.2 to 0.7 for pure hydrogen and hydrogen–carbon monoxide mixture are obtained. The theoretical calculations are in good agreement with the simulation results in this work and other data available in literature. The adsorption of hydrogen and the selectivity of hydrogen–carbon monoxide mixture are predicted from DFT and the adsorption characteristics of the isolated cylindrical wall is discussed.

Density functional study on the structural and thermodynamic properties of aqueous DNA-electrolyte solution in the framework of cell model

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.