Isobaric vapor–liquid equilibrium for methyltrichlorosilane–dimethyldichlorosilane–benzene system

Isobaric vapor–liquid equilibrium (VLE) for the system methyltrichlorosilane–dimethyldichlorosilane–benzene and that of the three binary systems were measured with a new pump-ebulliometer at the pressure of 101.325 kPa. These binary compositions of the equilibrium vapor were calculated according to the Q function of molar excess Gibbs energy by the indirect method and the resulted VLE data agreed well with the thermodynamic consistency. Moreover, the experimental data were correlated with the Wilson, NRTL, Margules and van Laar equations by means of the least-squares fit, the acquired optimal interaction parameters were fitted to experimental vapor–liquid equilibrium data for binary systems. The binary parameters of Wilson equation were also used to calculate the bubble point temperature and the vapor phase composition for the ternary mixtures without any additional adjustment. The predicted vapor–liquid equilibrium for the ternary system accorded well with the experimental results. The separation factor of methyltrichlorosilane against dimethyldichlorosilane in benzene was also reported. The VLE of binary and multilateral systems provided essential theory for the production of the halogenated silane.     

Isobaric vapor–liquid equilibria of binary system ethyl acetate + ethyl benzene + lithium bromide

The isobaric vapor–liquid equilibrium (VLE) behaviors for binary system, ethyl acetate + ethyl benzene, and ethyl acetate + ethyl benzene + LiBr (at saturation) were studied at the local ambient pressure (707 ± 1 mmHg). Equilibrium still was used where both liquid and vapor were continuously circulated. The experimental results showed that salt-free ethyl acetate + ethyl benzene system does not form an azeotrope point. The experimental results for ethyl acetate + ethyl benzene system were in a very good agreement with the predicted results using UNIFAC, UNIQUAC, NRTL, and Wilson models. Adding LiBr, as a salt, did show slight effects on the VLE behavior of ethyl acetate + ethyl benzene system.     

Vapor–liquid equilibria for pentafluoroethane + propane and difluoromethane + propane systems over a temperature range from 253.15 to 323.15 K

This paper provides vapor–liquid equilibrium (VLE) data obtained for two binary systems of pentafluoroethane (R125)+propane (R290) and difluoromethane (R32)+R290 over a temperature range from 253.15 to 323.15 K. The measurement of VLE was performed at isothermal conditions in a vapor recirculation apparatus. Both systems form azeotropes in the temperature range of this study. The experimental results were well correlated with the Peng–Robinson equation of state (PR EoS) using one parameter van der Waals one fluid model. The binary interaction parameters were optimized using the experimental data of bubble point pressure. A comparison with published experimental VLE data has been carried out by means of the PR equation of state.     

苯—正庚烷—乙醇三元体系加压汽液平衡的研究

用双循环武加压汽液平衡装置测定了苯-正庚烷-乙醇三元体系在101.3、302.5、506.8、709.3、810.6 kPa下的汽液平衡数据。根据有关二元体系在对应压力下的汽液平衡数据所求得的能量配偶参数,对该体系的汽液平衡进行了预测,计算值与实验值符合良好。     

Isobaric vapor–liquid equilibria for mixtures of acetone,ethanol, and 2,2,4-trimethylpentane at 101.3 kPa

Isobaric vapor–liquid equilibrium (VLE) data were determined at the pressure of 101.3 kPa for binary and ternary systems composed of acetone, ethanol, and 2,2,4-trimethylpentane (isooctane). Minimum boiling azeotropes were found in the acetone + 2,2,4-trimethylpentane and ethanol + 2,2,4-trimethylpentane systems. Azeotropic behavior was not found for the ternary system. Thermodynamic consistency tests were performed for all VLE data. The activity coefficients of the binary mixtures were satisfactorily correlated as function of the mole fraction using the Wilson, NRTL, and UNIQUAC models. The models with their best-fitted parameters were used to predict the ternary vapor–liquid equilibrium. The Wilson model appears to yield the best prediction in boiling temperatures.     

On use of the Amber potential with the Langevin dipole method

Inclusion of solvent effects in biomolecular simulations is most ideally done using explicit methods, as they are able to capture the heterogeneous environment typical of biomolecules and systems involving them (e.g., proteins at solid interfaces). Common explicit methods based on molecular solvent models (e.g., TIP and SPC models) and molecular dynamic or Monte Carlo simulation are computationally expensive and are, therefore, not well-suited to situations where many simulations are required (e.g., in the ab initio structure prediction or design contexts). In such cases, more coarse-grained explicit approaches such as the Langevin dipole (LD) method of Warshel and co-workers are more appropriate. The recent incarnations of the LD method appear to produce good solvation free energy estimates. These incarnations use charges and solute structures obtained from high-level quantum mechanics simulations. As such an approach is clearly not possible for larger solutes or when many structures are to be considered, an alternative must be sought. One possibility is to use structures and charges derived from an existing analytical potential model-we report on such a coupling here with the Amber potential model. The accuracy and computational performance of this hybrid approach, which we term LD-Amber to distinguish it from previous incarnations of the LD method, was assessed by comparing results obtained from the approach with those from experiment and other theoretical methods for the solvation of 18 amino acid analogues and the alanine dipeptide. This comparison shows that the LD-Amber approach can yield results in line with experiment both qualitatively and quantitatively and is as accurate as other explicit methods while being computationally much cheaper.     

Isobaric vapor–liquid equilibria of the binary system maleic anhydride and di-isobutyl hexahydrophthalate at 2.67, 5.33 and 8.00 kPa

Isobaric vapor–liquid equilibrium (VLE) data for the binary system maleic anhydride (MAN) + di-isobutyl hexahydrophthalate (DIBE) at 2.67, 5.33 and 8.00 kPa were measured with a modified ebulliometer. Also, saturated vapor pressures of pure DIBE were measured and Antoine constants were obtained. These data were used to calculate equilibrium vapor and liquid compositions for this binary system using the NRTL model. Prediction of the VLE was done with the universal quasi-chemical functional-group activity coefficients (UNIFAC) model. The predicted results generally agree with those from experiment.     

Does water condense in carbon pores?

Using grand canonical Monte Carlo (GCMC) simulations of molecular models, we investigate the nature of water adsorption and desorption in slit pores with graphitelike surfaces. Special emphasis is placed on the question of whether water exhibits capillary condensation (i.e., condensation when the external pressure is below the bulk vapor pressure). Three models of water have been considered. These are the SPC and SPC/E models and a model where the hydrogen bonding is described by tetrahedrally coordinated square-well association sites. The water-carbon interaction was described by the Steele 10-4-3 potential. In addition to determining adsorption/desorption isotherms, we also locate the states where vapor-liquid equilibrium occurs for both the bulk and confined states of the models. We find that for wider pores (widths >1 nm), condensation does not occur in the GCMC simulations until the pressure is higher than the bulk vapor pressure, P0. This is consistent with a physical picture where a lack of hydrogen bonding with the graphite surface destabilizes dense water phases relative to the bulk. For narrow pores where the slit width is comparable to the molecular diameter, strong dispersion interactions with both carbon surfaces can stabilize dense water phases relative to the bulk so that pore condensation can occur for P < P0 in some cases. For the narrowest pores studied--a pore width of 0.6 nm--pore condensation is again shifted to P > P0. The phase-equilibrium calculations indicate vapor-liquid coexistence in the slit pores for P < P0 for all but the narrowest pores. We discuss the implications of our results for interpreting water adsorption/desorption isotherms in porous carbons.     

Molecular dynamics simulations of vapor/liquid coexistence using the nonpolarizable water models

The surface tension, vapor-liquid equilibrium densities, and equilibrium pressure for common water models were calculated using molecular dynamics simulations over temperatures ranging from the melting to the critical points. The TIP4P/2005 and TIP4P-i models produced better values for the surface tension than the other water models. We also examined the correlation of the data to scaling temperatures based on the critical and melting temperatures. The reduced temperature (T/T(c)) gives consistent equilibrium densities and pressure, and the shifted temperature T + (T(c, exp) - T(c, sim)) gives consistent surface tension among all models considered in this study. The modified fixed charge model which has the same Lennard-Jones parameters as the TIP4P-FQ model but uses an adjustable molecular dipole moment is also simulated to find the differences in the vapor-liquid coexistence properties between fixed and fluctuating charge models. The TIP4P-FQ model (2.72 Debye) gives the best estimate of the experimental surface tension. The equilibrium vapor density and pressure are unaffected by changes in the dipole moment as well as the surface tension and liquid density.     

Molecular models for 267 binary mixtures validated by vapor–liquid equilibria: A systematic approach

By assessing a large number of binary systems, it is shown that molecular modeling is a reliable and robust route to vapor–liquid equilibria (VLE) of mixtures. A set of simple molecular models for 78 pure substances from prior work is taken to systematically describe all 267 binary mixtures of these components for which relevant experimental VLE data is available. The mixture models are based on the modified Lorentz–Berthelot combining rule. Per binary system, one state independent binary interaction parameter in the energy term is adjusted to a single experimental vapor pressure. The unlike energy parameter is altered usually by less than 5% from the Berthelot rule. The mixture models are validated regarding the vapor pressure at other state points and also regarding the dew point composition, which is a fully predictive property in this work. In almost all cases, the molecular models give excellent predictions of the mixture properties.     

Set of molecular models based on quantum mechanical ab initio calculations and thermodynamic data

A parametrization strategy for molecular models on the basis of force fields is proposed, which allows a rapid development of models for small molecules by using results from quantum mechanical (QM) ab initio calculations and thermodynamic data. The geometry of the molecular models is specified according to the atom positions determined by QM energy minimization. The electrostatic interactions are modeled by reducing the electron density distribution to point dipoles and point quadrupoles located in the center of mass of the molecules. Dispersive and repulsive interactions are described by Lennard-Jones sites, for which the parameters are iteratively optimized to experimental vapor-liquid equilibrium (VLE) data, i.e., vapor pressure, saturated liquid density, and enthalpy of vaporization of the considered substance. The proposed modeling strategy was applied to a sample set of ten molecules from different substance classes. New molecular models are presented for iso-butane, cyclohexane, formaldehyde, dimethyl ether, sulfur dioxide, dimethyl sulfide, thiophene, hydrogen cyanide, acetonitrile, and nitromethane. Most of the models are able to describe the experimental VLE data with deviations of a few percent.     

Molecular dynamics study of polarizable point dipole models for molten sodium iodide

The structure, the ionic transport properties, and the dynamics of long-wavelength charge-density fluctuations, for two polarizable point dipole models of molten NaI, have been studied by molecular dynamics simulations. These models are based on a rigid ion potential to which the induced dipole polarization of the anions is added. The polarization is added in such a way that point dipoles are induced on the anions by both local electric field and short-range damping interactions that oppose the electrically induced dipole moments. The two polarizable ion models differ only in the range of the damping polarization interactions. The influence of the induced anion polarization on the different properties of simulated molten NaI is discussed.     

An intramolecular induction correction model of the molecular dipole moment

A model for intramolecular polarization is presented. It is used to describe the changes in the molecular charge distribution occurring as a response to changes of dihedral angles in the molecule. The model is based on a multicenter multipole distribution of the molecular charge distribution. The electric field from this charge distribution induce dipole moments in the same molecule. The model contains atom type parameters to describe the damping of the electric field. A total of four atom types are used. The parameters are fitted to a calibration set with various functional groups, and tested against a validation set. The error obtained for the calibration set is reduced by 92% and by 88% for the validation set, if compared to an accurate state-of-the-art force field. It is shown that rotating the non-polarizable multicenter multipole distribution for the equilibrium geometry gives too large dipole moments for dihedral angles deviating from the equilibrium geometry. This will lead to too large long-range attractions in simulations. This problem is overcome by using the dipole polarizability correction suggested here, which gives dipole moments very close to the Hartree-Fock dipole moments obtained from reference calculations.     

A review of Barker’s activity coefficient method and VLE data reduction

The method of Barker is a popular scheme for determination of activity coefficients from total pressure measurements. A comprehensive review of this method is presented in this study. While discussing this technique various aspects of (vapor + liquid) equilibrium (VLE) data reduction process including types of algorithms applied, roles of saturated vapor pressures and equilibrium vapor compositions data, and types of objective functions used are analyzed. Activity coefficient or liquid state models frequently used in VLE data reduction are shown and their comparisons are investigated. More so, advantages and limitations of Barker’s method are demonstrated.     

Isobaric Vapor–Liquid Equilibrium for the Mixture of Neohexane,Cyclopentane and N,N-Dimethylformamide at 101.3 kPa

The vapor–liquid phase equilibrium (VLE) data for binary systems of neohexane?+?cyclopentane, neohexane?+?N,N-dimethylformamide (DMF), cyclopentane?+?DMF and ternary system of neohexane?+?cyclopentane?+?DMF were determined with a modified Rose still at 101.3 kPa, and all the binary data passed the Wisniak’s test (D?<?5), which accorded with the thermodynamic consistency. Three activity coefficient models namely, Wilson, NRTL and UNIQUAC were used to correlate VLE data and get binary interaction parameters, then the ternary VLE data of neohexane?+?cyclopentane?+?DMF were estimated based on these model parameters using Aspen Plus software. The estimation values of the three models agree well with the experimental data (σ(T)?<?0.5 K). Moreover, the analysis of the effect of DMF on the vapor–liquid phase equilibrium shows that DMF can act as an effective extractant for the system studied.

     

Modification of the Furter equation and correlation of the vapor–liquid equilibrium for mixed-solvent electrolyte systems

Isobaric vapor–liquid equilibrium values at 1 atm pressure were measured for the systems 1-propanol–water–potassium acetate and 2-propanol–water–potassium acetate under fixed salt mole fractions using a modified Othmer recirculation still. A modified Furter equation, ln(α_s/α)=k₁z+k₂z², was proposed for correlating the effect of dissolved salts on vapor–liquid equilibrium (VLE). The modified equation contains two parameters that are applicable to the entire salt/solvent composition range. Correlation of VLE for 15 mixed-solvent electrolyte systems was made by means of the proposed modified equation with better results than those obtained from the original equation.     

Nanoporous Materials Can Tune the Critical Point of a Pure Substance

Molecular simulations and NMR relaxometry experiments demonstrate that pure benzene or xylene confined in isoreticular metal–organic frameworks (IRMOFs) exhibit true vapor–liquid phase equilibria where the effective critical point may be reduced by tuning the structure of the MOF. Our results are consistent with vapor and liquid phases extending over many MOF unit cells. These results are counterintuitive since the MOF pore diameters are approximately the same length scale as the adsorbate molecules. As applications of these materials in catalysis, separations, and gas storage rely on the ability to tune the properties of adsorbed molecules, we anticipate that the ability to systematically control the critical point, thereby preparing spatially inhomogeneous local adsorbate densities, could add a new design tool for MOF applications.     

Slip flow in graphene nanochannels

We investigate the hydrodynamic boundary condition for simple nanofluidic systems such as argon and methane flowing in graphene nanochannels using equilibrium molecular dynamics simulations (EMD) in conjunction with our recently proposed method [J. S. Hansen, B. D. Todd, and P. J. Daivis, Phys. Rev. E 84, 016313 (2011)]. We first calculate the fluid-graphene interfacial friction coefficient, from which we can predict the slip length and the average velocity of the first fluid layer close to the wall (referred to as the slip velocity). Using direct nonequilibrium molecular dynamics simulations (NEMD) we then calculate the slip length and slip velocity from the streaming velocity profiles in Poiseuille and Couette flows. The slip lengths and slip velocities from the NEMD simulations are found to be in excellent agreement with our EMD predictions. Our EMD method therefore enables one to directly calculate this intrinsic friction coefficient between fluid and solid and the slip length for a given fluid and solid, which is otherwise tedious to calculate using direct NEMD simulations at low pressure gradients or shear rates. The advantages of the EMD method over the NEMD method to calculate the slip lengths/flow rates for nanofluidic systems are discussed, and we finally examine the dynamic behaviour of slip due to an externally applied field and shear rate.