Search

In this work, an extension is proposed to the standard iterative Boltzmann inversion (IBI) method used to derive coarse-grained potentials. It is shown that the inclusion of target data from multiple states yields a less state-dependent potential, and is thus better suited to simulate systems over a range of thermodynamic states than the standard IBI method. The inclusion of target data from multiple states forces the algorithm to sample regions of potential phase space that match the radial distribution function at multiple state points, thus producing a derived potential that is more representative of the underlying interactions. It is shown that the algorithm is able to converge to the true potential for a system where the underlying potential is known. It is also shown that potentials derived via the proposed method better predict the behavior of n-alkane chains than those derived via the standard IBI method. Additionally, through the examination of alkane monolayers, it is shown that the relative weight given to each state in the fitting procedure can impact bulk system properties, allowing the potentials to be further tuned in order to match the properties of reference atomistic and/or experimental systems.

We present a Fourier space density functional approach for hard particles with attractive interactions, which is based on a previously developed two-dimensional approach [S. Hlushak, W. Rżysko, and S. Sokołowski, J. Chem. Phys. 131, 094904 (2009)] for hard-sphere chains. The interactions are incorporated by means of a three-dimensional Fourier image of the direct correlation function that is obtained from the first-order mean-spherical approximation. In order to improve the computational efficiency, we make extensive use of fast Fourier transforms for calculating density convolution integrals. A two-dimensional implementation of the new density functional approach, based on the expansion of the functional around the bulk fluid density, is used to study structure and adsorption of two model fluids in narrow cylindrical pores. We also investigate two methods that improve the accuracy of the theory as compared to the conventional DFT approach, which expands the free energy functional around the bulk fluid density: One a variant of the reference fluid density functional theory used by Gillespie et al. [Phys. Rev. E 68, 031503 (2003)], and the second a weighted density approach with energy route thermodynamics. Results from these two methods are compared to the conventional approach and also to the results of Monte Carlo simulations. We find that the method of Gillespie et al. and the weighted density approach with energy route thermodynamics yield significant improvement over the conventional approach. [ABSTRACT FROM AUTHOR]

The statistical associating fluid theory for potentials of variable range plus dipole (SAFT-VR+D) is extended to study associating dipolar fluids. In the SAFT-VR+D approach dipolar interactions are taken into account through the use of the generalized mean spherical approximation to describe a reference fluid of dipolar square-well segments. This enables the effect of the dipolar interactions on the thermodynamics and structure of fluids to be explicitly described. Predictions for the thermodynamic properties and phase behavior of dipolar associating square-well monomers with one, two, and four association sites are considered and compared with new isothermal-isobaric and Gibbs ensemble Monte Carlo simulation data. The results show that the SAFT-VR+D equation provides a good description of the phase behavior of dipolar associating fluids. Additionally we have applied the new theoretical approach to study the vapor pressure and saturated liquid density of water. [ABSTRACT FROM AUTHOR]

A statistical associating fluid theory for potentials of variable range to model dipolar fluids is presented. The new theory, termed the SAFT-VR+D equation (the statistical associating fluid theory for potentials of variable range plus dipole), explicitly accounts for dipolar interactions and their effect on the structure of the fluid. This is achieved through the use of the generalized mean spherical approximation (GMSA) to describe a reference fluid of dipolar square-well segments. Isothermal-isobaric and Gibbs ensemble Monte Carlo simulations have been performed in order to test the new theoretical approach. Predictions for the thermodynamic properties and phase behavior of dipolar square-well monomer and chain fluids, in which one or more segments are dipolar, are considered and compared with new computer simulation data. The results show that the equation of state in general provides a good description of the phase behavior of dipolar monomer and chain fluids. Some deviations are seen between the simulation data and theoretical predictions for monomer fluids with large dipole moments and for molecules composed of segments with dipoles in different orientations. This is addressed through the replacement of the GMSA by the linearized exponential approximation. [ABSTRACT FROM AUTHOR]

The accuracy of several theories for the thermodynamic properties of the Yukawa hard-sphere chain fluid are studied. In particular, we consider the polymer mean spherical approximation (PMSA), the dimer version of thermodynamic perturbation theory (TPTD), and the statistical associating fluid theory for potentials of variable attractive range (SAFT-VR). Since the original version of SAFT-VR for Yukawa fluids is restricted to the case of one-Yukawa tail, we have extended SAFT-VR to treat chain fluids with two-Yukawa tails. The predictions of these theories are compared with Monte Carlo (MC) simulation data for the pressure and phase behavior of the chain fluid of different length with one- and two-Yukawa tails. We find that overall the PMSA and TPTD give more accurate predictions than SAFT-VR, and that the PMSA is slightly more accurate than TPTD. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We extend the SAFT-VR+DE equation of state to describe 19 aqueous electrolyte solutions with both a fully dissociated and a partially dissociated model. The approach is found to predict thermodynamic properties such as the osmotic coefficient, water activity coefficient, and solution density, across different salt concentrations at room temperature and pressure in good agreement with experiment using only one or two fitted parameters. At higher temperatures and pressures, without any additional fitting, the theory is found to be in qualitative agreement with experimental mean ionic activities and osmotic coefficients. The behavior of the dielectric constant as a function of salt concentration is also reported for the first time using a statistical associating fluid theory (SAFT)-based equation of state. At high salt concentrations, the stronger electrostatic interactions between the ionic species due to the dielectric decrement, is captured through the inclusion of ion association. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3053-3072, 2015 [ABSTRACT FROM AUTHOR]

Abstract: The high-pressure phase diagram and other thermodynamic properties of the binary systems of water+hydrogen sulphide have been examined using the SAFT-VR+D equation of state [Zhao and McCabe, J. Chem. Phys. 125 (2006) 104504]. The SAFT-VR+D approach is based on a version of the statistical associating fluid theory that was developed to model dipolar fluids by explicitly accounting for dipolar interactions and their effect on the thermodynamics and structure of a fluid. In the SAFT-VR+D equation this is achieved through the use of the generalized mean spherical approximation (GSMA) to describe a monomer fluid of dipolar square-well segments. In the present work, we further develop the SAFT-VR+D equation to study mixtures of dipolar associating fluids of arbitrary size using the analytical solution of the Ornstein–Zernike equation due to Cummings and Blum [J. Chem. Phys. 84, 1833 (1986)]. The high-pressure phase diagram and other thermodynamic properties of the water+hydrogen sulphide binary mixture are then examined. Both components, hydrogen sulphide and water, are naturally modeled as associating spherical molecules with four off-centre sites to mimic hydrogen bonding and an embedded dipole moment (μ) to describe their polarity. Using simple Lorentz–Berthelot combining rules, the theory is able to predict the phase behavior of the H2O+H2S system from pure component parameters. Specifically, type III phase behavior according to the classification of Scott and van Konynenburg is observed, with a three-phase line at low temperatures that terminates at an upper critical end point and two separate critical lines, one at high temperatures that runs continuously from a gas–liquid critical line to a liquid–liquid critical line at high pressures and a second gas–liquid critical line located at low temperatures. The phase behavior predicted by the SAFT-VR+D approach is in excellent agreement with the experimental data, without requiring any fitted binary interaction parameters. In addition, the theory is also able to describe the excess molar enthalpies and the most important features of the Henry''s constants as a function of temperature. [Copyright &y& Elsevier]

The SAFT-VR equation of state is extended to treat heteronuclear chain fluids, focusing, in particular, on symmetric and asymmetric diblock chains. The chain molecules studied are composed of segments of different size and/or energy of interaction. Both symmetric and asymmetric systems are considered. The theoretical predictions are compared with isothermal–isobaric and Gibbs ensemble Monte Carlo simulation data. Excellent agreement is obtained between the hetero-SAFT-VR predictions and the simulation data, validating the use of the SAFT-VR approach for heteronuclear chains in more realistic models of polymers and small molecules composed of different functional groups. [ABSTRACT FROM AUTHOR]

The product-reactant Ornstein-Zernike approach, represented by the polymer mean-spherical approximation (PMSA), is utilized to describe the structure and thermodynamic properties of the fluid of Yukawa hard sphere chain molecules. An analytical solution of the PMSA for the most general case of the multicomponent freely jointed hard sphere multi-Yukawa chain fluid is presented. As in the case of the regular MSA for the hard sphere Yukawa fluid, the problem is reduced to the solution of a set of nonlinear algebraic equations in the general case, and to a single equation in the case of the factorizable Yukawa potential coefficients. Closed form analytical expressions are presented for the contact values of the monomer-monomer radial distribution function, structure factors, internal energy, Helmholtz free energy, chemical potentials and pressure in terms of the quantities, which follows directly from the PMSA solution. By way of illustration, several different versions of the hard sphere Yukawa chain model are considered, represented by one-Yukawa chains of length m, where m = 2,4,8,16. To validate the accuracy of the present theory, Monte Carlo simulations were carried out and the results are compared systematically with the theoretical results for the structure and thermodynamic properties of the system at hand. In general it is found that the theory performs very well, thus providing an analytical route to the equilibrium properties of a well defined model for chain fluids. [ABSTRACT FROM AUTHOR]

The hard-sphere Yukawa chain (HSYC) fluid is a simple model for chain fluids and polymers in which each molecule is composed of freely-jointed tangent spheres. The spheres each have a hard core and an attractive interaction, written as a single Yukawa potential or as a sum of Yukawa potentials. We have calculated the phase equilibria and PVT behaviour of the HSYC fluid both theoretically and by Monte Carlo simulation. The theoretical approach is based on the analytic solution of the polymer mean spherical approximation for the HSYC fluid. By comparing the theory and simulation we are able to evaluate the accuracy of the theory. We find that in general the theory performs very well, thus, it provides an analytic route to an equation of state for a well-defined model of chain fluids. [ABSTRACT FROM AUTHOR]

We broaden the scope of the statistical associating fluid theory for potentials of variable attractive range (SAFT-VR) to treat heteronuclear chain molecules formed from bonded square-well (BSW) segments. The ideas of the bonded hard sphere (BHS) treatment for distributed-site models composed of hard-sphere segments are applied to square-well sites with the SAFT-VR approach. The results of isothermal-isobaric Monte Carlo simulations are reported for heteronuclear square-well diatomics with different sets of energy and range parameters. The SAFT-VR approach provides an excellent description of the equation of state of the diatomic systems for a wide range of densities. The goal of the work is to provide a rigorous treatment of distributed-site models of fluids, and to establish a framework for a group contribution approach with SAFT-VR. [ABSTRACT FROM AUTHOR]

The solubility of xenon in liquid n-pentane and n-hexane has been studied experimentally, theoretically, and by computer simulation. Measurements of the solubility are reported for xenon + n-pentane as a function of temperature from 254 to 305 K. The uncertainty in the experimental data is less than 0.15%. The thermodynamic functions of solvation such as the standard Gibbs energy, enthalpy, and entropy of solvation have been calculated from Henry's law coefficients for xenon + n-pentane solutions and also for xenon + n-hexane, which were reported in previous work. The results provide a further example of the similarity between the xenon + n-alkane interaction and the n-alkane + n-alkane interactions. Using the SAFT-VR approach we were able to quantitatively predict the experimental solubility for xenon in n-pentane and semiquantitatively that of xenon in n-hexane using simple Lorentz−Berthelot combining rules to describe the unlikely interaction. Henry's constants at infinite dilution for xenon + n-pentane and xenon + n-hexane were also calculated by Monte Carlo simulation using a united atom force field to describe the n-alkane and the Widom test particle insertion method. [ABSTRACT FROM AUTHOR]

As part of a combined experimental and theoretical study of the thermodynamic properties of perfluoroalkylalkanes (PFAAs), the liquid density of perfluorobutylpentane (F4H5), perfluorobutylhexane (F4H6), and perfluorobutyloctane (F4H8) was measured as a function of temperature from 278.15 to 353.15 K and from atmospheric pressure to 70 MPa. The liquid densities of n-perfluoropentane, n-perfluorohexane, n-perfluorooctane, and n-perfluorononane were also measured at room pressure over the same temperature range. The PVT behavior of the PFAAs was also studied using the SAFT-VR equation of state. The PFAA molecules were modeled as heterosegmented diblock chains, using different parameters for the alkyl and perfluoroalkyl segments, that were developed in earlier work. Through this simple approach, we are able to predict the thermodynamic behavior of the perfluoroalkylalkanes, without fitting to any experimental data for the systems being studied. Molecular dynamics simulations have also been performed and used to calculate the densities of the perfluoroalkylalkanes studied. [ABSTRACT FROM AUTHOR]

The statistical associating fluid theory is a widely used molecular-based equation of state that has been successfully applied to study a broad range of fluid systems. It provides a framework in which the effects of molecular shape and interactions on the thermodynamics and phase behavior of fluids can be separated and quantified. In the original approach, molecules were modeled as chains composed of identical segments; the heterogeneity of molecules in terms of structure and functional groups was described implicitly through effective parameters. To overcome this limitation, in recent works [Peng et al. Fluid Phase Equilib. 227(2), 131 (2009); Ind. Eng. Chem. Res. 49(3), 1378 (2010)] the GC-SAFT-VR approach has been developed to extend the theory to model chains composed of segments of different size and/or energy of interaction and enable the development of a group-contribution approach within the SAFT-VR framework in which molecular heterogeneity and connectivity is explicitly accounted for. The parameters for several key functional groups (CH3, CH2, CH, CH2=CH, C=O, C6H5, esters, ethers, cis-alkenes and trans-alkenes groups) were determined by fitting to experimental vapor pressure and saturated liquid density data for a number of small molecules containing the functional groups of interest and transferability of the parameters tested by comparing the theoretical predictions with experimental data for pure fluids not included in the fitting process and binary mixtures of both simple fluids and the VLE and LLE of small molecules in polymer systems. In this work, we further extend the applicability of the GC-SAFT-VR approach through the study of the vapor–liquid phase behavior of associating systems, such as linear and branched alcohols, primary and secondary amines, aldehydes, and carboxylic acids, and their mixtures. In the study of these new molecules several new functional groups (OH (linear and branched), HC=O, NH2, NH and C=OOH) are defined and their molecular parameters characterized. The transferability of the parameters is again tested by comparing the theoretical predictions with experimental data for pure fluids and binary mixtures not included in the fitting process. The GC-SAFT-VR approach is found to predict the phase behavior of the systems studied in most cases in good agreement with experimental data and accurately captures the effects of changes in structure and molecular composition on phase behavior. [Copyright &y& Elsevier]

The recently developed heteronuclear group contribution SAFT-VR equation (GC-SAFT-VR) [Penget al. Fluid Phase Equilib.2009, 227(2), 131−144] enables the predictive study of the thermodynamic properties of fluids and their mixtures by using a molecular-based model in which the molecules are described by heterosegmented chains in which each type of segment represents a functional group present in the molecule. Given the success of the GC-SAFT-VR approach in predicting the fluid phase equilibria of mixtures without fitting to any mixture data, and the known difficulties in determining equation-of-state parameters for polymers because of the lack of coexistance data, in this work, we extend the GC-SAFT-VR equation to study the phase equilibria of small molecules in polymer systems. The results demonstrate the ability of the GC-SAFT-VR equation of state to predict the vapor−liquid and liquid−liquid equilibria of polymer solutions and accurately capture the effects of polymer molecular weight and molecular topology on phase behavior. [ABSTRACT FROM AUTHOR]

Abstract: The Gibbs free energy of solvation (ΔGsolv ) for C60, and six other idealized, non-functionalized, fullerene particles of differing size and shape has been determined in octanol and water solvents from molecular dynamics simulations using thermodynamic integration. In particular, we have studied Buckminster fullerene (C60) and open and capped carbon nanotubes of different aspect ratios and solvent accessible surface areas. Knowledge of the ΔGsolv of a molecule in octanol and water can be used to understand the partitioning of the molecule between organic and aqueous phases and is one of several parameters used to model the fate of chemicals in the natural environment. The simulations were performed at ambient conditions, i.e., a temperature of 25°C and a pressure of 1bar. The fullerene molecules are all found to have a very high ΔGsolv in water, and a very low ΔGsolv in octanol, suggesting a strong preference for the organic phase. From a comparison of the results for capped and uncapped carbon nanotubes we found that the uncapped tubes exhibit significantly higher ΔGsolv than capped tubes. Furthermore, for capped carbon nanotubes, hydrophobic/organophilic shifts are observed with increasing excluded volume and solvent accessible surface area. [Copyright &y& Elsevier]

Abstract: The hetero-segmented version of the statistical associating fluid theory for potentials of variable range (hetero-SAFT-VR) is used to develop a predictive molecular-based group-contribution SAFT-VR equation of state (GC-SAFT-VR). The hetero-SAFT-VR approach models molecules composed of segments of different size and/or energies of interaction enabling an accurate description of real molecules composed of different functional groups. The differences in interactions between functional groups are maintained throughout the theory in contrast to other approaches in which the parameters for functional groups are averaged in order to model a molecule as a homonuclear chain with “group-averaged” parameters. Through the GC-SAFT-VR approach we can study the effect of molecular functionality and topology on the thermodynamic properties of real fluid systems, since parameters are determined for each functional group and chain connectivity is explicitly specified. In this initial study GC-SAFT-VR parameters are developed for key organic functional groups (CH3, CH2, CH2 glyph name="dbnd" />O, C6H5, OCH3 and OCH2) by fitting to experimental vapor pressure and saturated liquid density data for a selected group of compounds that contain these functional groups. The transferability of the parameters obtained is tested by comparing theoretical predictions with experimental data for pure fluids not included in the fitting process and binary mixtures. Using the GC-SAFT-VR approach good agreement is obtained between experimental data and the theoretical predictions for pure substances, including isomers, and their mixtures. The GC-SAFT-VR approach is able to accurately predict the effect of molecular functionality on mixture phase behavior without fitting to any experimental data for the system being studied. [Copyright &y& Elsevier]