Yi-Gui Li

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.

First-order mean spherical approximation for attractive, repulsive, and multi-Yukawa potentials

The first-order mean spherical approximation (FMSA) theory proposed by Tang et al. [Fluid Phase Equilib., 134, 21(1997)] is applied for studying several typical Yukawa fluids, including attractive, repulsive, and multi-Yukawa cases. The FMSA study is particularly advantageous in providing thermodynamics and structure information in a simple, analytical, and consistent manner. Comparisons with the latest reported computer simulation data for compressibility factor, internal energy, and radial distribution function show that FMSA performs very well and the performance is very close to the full MSA and to several other theories, developed individually for the above-mentioned cases or properties. The present study provides solid evidence to support FMSA applications to more complex fluids.

Prediction of phase equilibria for CO2-C2H5OH-H2O system using the SAFT equation of state

Abstract The SAFT equation of state is applied to the correlation of thermodynamic properties of the binary systems consisting of water–ethanol, water–carbon dioxide and ethanol–carbon dioxide. Based on this, the liquid phase compositions of ternary system are calculated from vapor phase compositions using SAFT equation in the range of 308.15–338.15 K and 10.1–17.0 MPa, in which carbon dioxide acts as a supercritical extraction solvent. The phase equilibria predicted by SAFT agree with the experimental data from literatures accurately.

Crossover SAFT-BACK equation of state for pure CO2 and H2O

Abstract In this paper, the crossover statistical associating fluid theory and Boublik–Alder–Chen–Kreglewski (SAFT–BACK) equation of state (EOS) is established. The system-dependent parameters for pure carbon dioxide and water are regressed and their thermodynamic properties are calculated. Then the classical and crossover SAFT–BACK EOSs are compared. For pure carbon dioxide, their calculated deviations are both small. For pure water, the calculated accuracy with the crossover EOS is better than that with the classical EOS. Then some rules about the crossover method are revealed.

Crossover SAFT equation of state for pure supercritical fluids

Abstract A crossover statistical associating fluid theory (SAFT) equation of state (EOS) is used to fit the parameters of eight common pure supercritical fluids (water, ammonia, carbon dioxide, R134a, ethane, propane, ethene and propene) and calculate their thermodynamic properties. Over a wide range including the critical region, the EOS reproduces the saturated pressure data with an average absolute deviation (AAD) of about 1% and the saturated densities with an AAD of about 2%. In the one-phase region, the EOS represents the experimental values of pressure with an AAD of about 1–3%. The results are satisfactory.

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.

Comparison of equations of state for pure Lennard–Jones fluids and mixtures with molecular simulation data

Abstract Several equations of state for Lennard–Jones (LJ) fluids and mixtures are used in this work, including Cotterman’s expresssion and SAFT-VR. A new set of coefficients for Cotterman equation is re-regressed by fitting both the reduced pressure and internal energy data from molecular simulations. Comparisons between the two sets of coefficients are carried out. The SAFT-VR EOS for LJ fluids and its simplified form by mean-field approximation are compared with the Cotterman EOS. Based on SAFT, the contribution due to the formation of chains is expressed as a function of background correlation function for reference fluid. Two reference fluids (the hard sphere and the LJ fluid) and three methods to calculate the background correlation function are adopted. The EOS is extended to mixtures by use of the van der Waals one-fluid (VDW1) mixing rules. Comparisons for the pressures and residual chemical potentials with molecular simulation data are reported. The accuracy of predictions for liquid–liquid equilibria (LLE) of binary LJ mixtures is also examined.

Study on Surface Tension for Non-polar and Associating Fluids Based on Density Functional Theory

A method for the prediction of surface tension of non-polar and associating fluids has been developed based on the density functional theory (DFT). The WCA perturbation theory and the SAFT are used to establish the equation of state (EOS). The adjustable parameters are correlated by simultaneously fitting the saturated vapor pressure and the liquid density data with the EOS. The local density approximation (LDA) is applied to the reference term and the mean field approximation (MFA) is used for perturbation term. The density profile is obtained by minimizing the grand potential functional. The surface thickness is calculated by “10%-to-90%-width” method. By use of the regressed parameters in EOS and the obtained density profile, the surface tensions for 13 non-polar fluids and 10 associating fluids are predicted satisfactorily.

Comparison of perturbation theory and mean spherical approximation for polar fluids and ion-dipole mixtures based on molecular simulation data

Abstract A comprehensive study on various internal energies, pressures and chemical potentials for the pure dipolar hard sphere fluids, pure Stockmayer fluids, the Lennard–Jones and Stockmayer mixtures, the Stockmayer and Stockmayer mixtures and the ion–dipole mixtures is reported based on the perturbation theory (PT) and mean spherical approximation (MSA). Compared with the results of molecular simulations, it is shown that the PT is superior to MSA in most cases.

Study on the analytical solution of the MSA for a one-component two-Yukawa potential in bovine serum albumin—NaC1 aqueous solution

Using the mean spherical approximation (MSA), an explicit analytical equation of state (EOS) with non-dimensional variables for one-component two-Yukawa fluid is established based on the work of Blum, L., and Ubriaco, M., 2000, Molec. Phys., 98, 829. A simple and directly iterative method is found for obtaining an acceptable solution. The strict two-Yukawa EOS is used to correlate the experimental osmotic pressure data of aqueous bovine serum albumin (BSA)-NaCl solutions in the one-component assumption. Considering the experimental error, the correlation results are good, with only one regressed parameter (disperse energy parameter ε). The deviation of correlation is discussed in detail. A concept of effective diameter, which obviously can decrease the deviation of correlation, is given.