Jiu-Fang Lu

Monte Carlo simulation for the hard-core two-Yukawa fluids and test of the two-Yukawa equation of state

A recently proposed analytical equation of state (EOS) for the hard-core two-Yukawa fluids is tested against the results of Monte Carlo (MC) simulation in six cases. One range parameter in the two-Yukawa potential is taken as 1.8 or 2.8647, and another is taken as 2.0, 4.0, 8.0, or 13.5485. Attractive and repulsive dominant cases of the potential outside the core are all considered. The simulation conditions selected ensure that, the interaction between two particles is attractive at enough long distance. Some of the cases can give the similar potential as that between charged colloid particles. The hard-core two-Yukawa fluid with parameters, which are obtained from fitting the Lennard-Jones potential, is also studied. The two-Yukawa EOS results fit very well with that of the MC simulation except for some points at high density (as ρ*=0.9), which is found to be crystal state. It is found that the two-Yukawa EOS can be used to study the fluid–fluid equilibrium of hard-core two-Yukawa fluid.

A new equation of state for real aqueous ionic fluids based on electrolyte perturbation theory, mean spherical approximation and statistical associating fluid theory

Abstract Based on perturbation theory, a new equation of state is developed for real electrolyte aqueous fluids, which is composed of ion–ion, ion–dipole, dipole–dipole, Lennard–Jones dispersion, hydrogen bonding association and hard sphere repulsion terms. The MSA equation from Blum is adopted instead of the perturbed electrostatic term, which simplifies the calculation. The SAFT equation is adopted for the calculation of hydrogen bond association for water molecules. For each electrolyte, only one adjustable parameter, i.e., the cation soft sphere diameter, is regressed. The experimental mean ionic activity coefficient data of 30 aqueous electrolytes (including 1:1, 2:1 and 1:2 electrolytes) were correlated. The parameters thus obtained can be used to predict the densities of these aqueous single electrolyte solutions and the mean ionic activity coefficients in mixed electrolyte aqueous solutions without any additional adjustable mixing parameters.

Study on the activity coefficients and the solubilities of amino acids in water by the perturbation theory

Abstract A non-primitive perturbation model for chain-like molecules has been used to correlate the activity coefficients of amino acids and peptides in aqueous solution. In this model, the mixed segments of hard spheres are taken as the reference and the chain formation, the dipole–dipole and L–J interactions are taken as the perturbation terms. The equation can accurately correlate the activity coefficients of amino acids and peptides in water with four parameters. The solubilities of amino acids in pure water between 0–100°C can be predicted with the model parameters, and the experimental values of standard entropy and enthalpy changes in the dissolving process.

Molecular simulation of liquid-liquid equilibria for Lennard-Jones fluids

Abstract Guo M.X., Li Y.G., Li Z.C. and Lu J.F., 1994. Molecular simulation of liquid-liquid equilibria for Lennard-Jones fluids. Fluid Phase Equilibria , 98: 129-139. In this work, the liquid-liquid phase behavior of Lennard-Jones fluids was simulated by a Gibbs ensemble Monte Carlo (GEMC) method. To improve the convergence of simulation on such high-density systems, an “Internal Identity Interchange” move was proposed and added to the original GEMC method. Five binary mixtures were designed to examine the effects of various molecular parameters on the immiscibility of the two coexistence liquid phases. To study the influences of the two cross-molecular parameters (ϵ 12 and σ 12 ) on the immiscibility of the coexistence liquid phases, the Lorentz-Berthelot combining rules were linearly scaled through multiplication by two constants.

Application of density functional theory for predicting the surface tension of pure polar and associating fluids

Abstract A thermodynamic method has been developed based on the density functional theory (DFT) to predict the surface tension of polar and associating fluids by the authors. The Barker–Henderson (BH) perturbation theory and statistical associating fluid theory (SAFT) are used to establish the equation of state (EOS). The hard sphere repulsion, dispersion, chain formation, and dipole–dipole or association interactions are taken into account. The parameters m, σ and e/k for non-associating polar fluids and m, σ, e/k, eAB/k and κAB for associating fluids are correlated by simultaneously fitting the saturated vapor pressure and the liquid density data with the EOS. The surface region of a pure liquid is divided into many extreme thin layers. The chemical potential in every layer of the surface leads to a constant by optimizing the surface thickness. The density profile is obtained from the optimized surface thickness and the hyperbolic tangent function obtained from molecular simulation. By use of the obtained density profile and the regressed parameters in EOS, the surface tensions for four pure non-associating polar fluids and 11 associating fluids in wide temperature range are predicted satisfactorily.

Prediction of surface tension for pure non-polar fluids based on density functional theory

A method for the prediction of surface tension of non-polar fluids has been developed based on the density functional theory. The Barker–Henderson perturbation theory and statistical associating fluid theory are used to establish an equation of state (EOS). The parameters m, σ and e/k of the EOS are correlated by simultaneously fitting the saturated vapor pressure and the liquid density data. The surface region of a pure liquid is divided into many extreme thin layers. The chemical potentials of different layers in the surface region are equal and lead to a constant by optimizing the surface thickness. The density profile is obtained from the optimized surface thickness and a hyperbolic tangent function. By use of the regressed parameters in the EOS, the surface tensions for 18 pure non-polar fluids are predicted and the results are satisfactory.

Studies on UNIQUAC and SAFT equations for nonionic surfactant solutions

Abstract The segment-based UNIQUAC and SAFT equations are established to calculate the activity coefficients of surfactants in aqueous solutions. The critical micellar concentrations of single aqueous surfactant systems are correlated and predicted. The predicted values are satisfactory. Both equations have good accuracies and good prediction functions, and the SAFT equation is more successful.

Study of the ionic activity coefficients in aqueous electrolytes by the non-primitive mean spherical approximation equation

Abstract In this paper, the ion-dipole non-primitive MSA model is tested with Monte Carlo simulation data, with a brief discussion, and used to calculate the activity coefficients of single strong electrolyte aqueous solutions up to 3 M (mol kg −1 ). The water molecule parameters are obtained by fitting the experimental saturated vapor pressure data from 298.15 to 573.15 K with an equation of state derived from the MSA. The average relative deviation in pressure is within 0.5%. The hard-sphere diameters of cations and anions are treated as the adjustable but concentration-independent parameters and are obtained by fitting simultaneously the experimental mean ionic activity coefficient data of 14 electrolyte aqueous solutions. The fitted ionic diameters are smaller than the Pauling diameters. The average relative deviation of the mean ionic activity coefficient is within 6%.

Study on ionic surfactant solutions by SAFT equation incorporated with MSA

Abstract A statistical associating fluid theory (SAFT) equation of state, incorporated with mean spherical approximation (MSA), has been established to calculate the activity coefficients of surfactants in aqueous solutions. This equation describes molecules as chains of hard sphere segments, and includes the contributions from the hard sphere–hard sphere interaction, the dipole–dipole interaction, the chain formation, the Lennard–Jones interaction, and the charge–charge interaction. The critical micellar concentrations of single aqueous ionic surfactant systems are correlated and predicted. The results are satisfactory, and the adjustable parameters in the model have obvious physical meaning.

Calculation of activity coefficients for systems containing tributyl phosphate, diluents and water by the perturbation theory

Abstract The component activity coefficients in systems of tributyl phosphate (TBP), diluents and water are calculated with a theoretical equation based on perturbation theory. Seven diluents were investigated: n -C 6 H 14 , n -C 7 H 16 , n -C 8 H 18 , C 6 H 6 , cy-C 6 H 12 , CCl 4 and CHCl 3 . Nine segments (H 2 O, CH 3 , CH 2 , CH(ben.), CH 2 (cy.), CCl 4 , CHCl 3 , CH 2 O and PO) are involved in the study. The segment parameters of H 2 O, CH 3 and CH 2 are obtained from PVT data of pure fluids (H 2 O, n -C 4 H 9 OH n -C 8 H 17 OH, and C 2 H 6  n -C 27 H 56 ). The other segment parameters are regressed from the activity coefficients of part of the binary systems. The component activity coefficients for other binary, ternary and quaternary systems composed of TBP, diluents and water are predicted with these parameters. The ARDs for prediction are less than 10% in general.