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Wertheim's multi-density formalism is extended for patchy colloidal fluids with two multiple bonding patches. The theory is developed as a density functional theory to predict the properties of an associating inhomogeneous fluid. The equation of state developed for this fluid depends on the size of the patch, and includes formation of cyclic, branched and linear clusters of associated species. The theory predicts the density profile and the fractions of colloids in different bonding states versus the distance from one wall as a function of bulk density and temperature. The predictions from our theory are compared with previous results for a confined fluid with four single bonding association sites. Also, comparison between the present theory and Monte Carlo simulation indicates a good agreement.

Cavity correlation functions, pair correlation functions, and bridge functions for the Lennard-Jones fluid are calculated from first Percus–Yevick (PY) theory and second-order Percus– Yevick (PY2) theory, molecular dynamics, and grand canonical Monte Carlo techniques. We find that the PY2 theory is significantly more accurate than the PY theory, especially at high density and near the critical point. The pair correlation function near the critical point has the expected slowly decaying long-range behaviour. However, we do not observe any long-range behaviour in the bridge function for the state points near the critical point we have simulated. However, we do note that the bridge function, which is usually negative near r = 0, becomes positive as r → 0. This behaviour is seen for the bridge functions computed from both PY2 and molecular dynamics, but not from PY.

The mean-field intermolecular interaction contribution in interfacial statistical associating fluid theory (iSAFT) is extended to include correlation functions, which could be evaluated from molecular simulation and the density functional theory (DFT) itself. Order–disorder transitions (ODTs) between lamellar and disordered phases for pure block copolymers and copolymer nanoparticle composites are reported, and the extended theory has been shown to give good agreement with molecular simulations and considerable improvement over the previous mean-field dispersion description. The interaction parameters at ODT are also correlated with the length of copolymers. Concentration profiles of different species are then calculated and compared quantitatively with simulation. The scaled microstructures predicted by the theory agree with simulation for all the nanoparticle size and surface chemistry investigated. Improvements over previous DFT reports have also been observed for large selective nanoparticles. iSAFT h...

In this work, classical density functional theory (DFT) is extended to model the n-alkane/water interface. The iSAFT density functional theory is applied to the n-alkane/water interface for even numbered alkanes. To apply the DFT, a simple parameter model is developed and all parameters are obtained from bulk property data only. It is shown that the iSAFT DFT combined with the simple parameter model yields accurate interfacial tensions, density profiles, alkane order parameters, and interfacial widths as compared to experiment and atomistic molecular dynamics simulation. A prediction of the DFT is that hexane dissolved in the water phase near the interface prefers a normal orientation to the interface.

Asphaltenes tend to deposit in reservoir, well tubing, flow lines, separators, etc., causing significant production losses. Asphaltenes are originally stable in crude oil at reservoir conditions. However, changes in temperature, pressure, and/or composition may cause asphaltenes to precipitate and potentially deposit onto the surfaces of a flowing conduit. There are several publications in the literature that discuss modeling of asphaltene phase behavior in oil as well as development of deposition models to simulate asphaltene deposition profiles along a flow path. In this paper, a previously reported asphaltene deposition tool (ADEPT) is used to study the deposition in a subsea pipeline in the Gulf of Mexico. This is the first demonstration of an asphaltene deposition simulator that has been used in a fully predictive manner. All of the required kinetic parameters used for deposition predictions were experimentally measured. A new methodology to scale up the deposition constant measured from a small-scal...

Asphaltenes are the heaviest and most polarizable components of crude oil. The phase behavior of these polydisperse components is important in both the upstream and downstream processing of crude oil because of their potential to precipitate, deposit and plug pipelines and production equipment. Predicting flow assurance issues caused by asphaltenes requires the ability to model the phase behavior of asphaltenes as a function of temperature, pressure, and composition. In this work we present a detailed procedure to characterize crude oil and plot asphaltene phase envelope, using the Perturbed Chain form of the Statistical Associating Fluid Theory (PC-SAFT). This work also demonstrates that the proposed procedure can model the asphaltene thermodynamic phase behavior better than using a cubic equation of state typically used in the industry, even with compositional data as low as C9+. The results obtained with the proposed characterization method show a remarkable matching with the experimental data points for both the bubble point and asphaltene precipitation onset curves. A wide range of temperatures, pressures and gas injection percentages have been tested. In this work, the concept of lower asphaltene onset pressure is also clarified and a new representation of asphaltene phase plot is presented. The results obtained in this work are very promising in providing better tools to model asphaltene phase behavior.

Second-order classical density functional theory is applied to assess the effect of surfactant properties on the interfacial structure and interfacial tension of a planar oil/water interface. Specifically the affect of the relative locations of the hydrophobic and hydrophilic portions, rigidity vs flexibility, and bond angle of the surfactant are investigated. It is found that bond angle and branching significantly affect the tendency of a surfactant to adsorb on the interface and the degree to which the interfacial tension is lowered.

We develop a general density functional theory for polyatomic molecules by taking the complete association limit of Wertheim's thermodynamic perturbation theory in a general way. By enforcing the complete association limit at an early point in the derivation and introducing the concept of molecular graphs, a general form for the free energy functional and segment densities are developed for both rigid and semiflexible molecules. The theory is applicable to branched chains, rings, or any other molecular structure. As an example the theory is applied as a second order perturbation theory to the case of molecules with a rigid three segment head and a fully flexible tail in a slit pore where bond angle of the rigid portion is an independent variable.

Gas hydrates have been suggested as a carbon source for Palaeogene hyperthermal events, but warm seafloor temperatures are thought to have limited their accumulation. Numerical simulations suggest that enhanced organic carbon sedimentation and methanogenesis could have compensated for the smaller area of hydrate stability. Abrupt periods of global warming between 57 and 50 million years ago—known as the Early Palaeogene hyperthermal events—were associated with the repeated injection of massive amounts of carbon into the atmosphere1,2,3,4. The release of methane from the sea floor following the dissociation of gas hydrates is often invoked as a source5. However, seafloor temperatures before the events were at least 4–7 °C higher than today1, which would have limited the area of sea floor suitable for hosting gas hydrates6,7. Palaeogene gas hydrate reservoirs may therefore not have been sufficient to provide a significant fraction of the carbon released. Here we use numerical simulations of gas hydrate accumulation8 at Palaeogene seafloor temperatures to show that near-present-day values of gas hydrates could have been hosted in the Palaeogene. Our simulations show that warmer temperatures during the Palaeogene would have enhanced the amount of organic carbon reaching the sea floor as well as the rate of methanogenesis. We find that under plausible temperature and pressure conditions, the abundance of gas hydrates would be similar or higher in the Palaeogene than at present. We conclude that methane hydrates could have been an important source of carbon during the Palaeogene hyperthermal events.

Asphaltenes often tend to deposit in reservoirs, flow lines, separators, and other systems along production lines causing significant production loss due to restricted oil flow or damages caused to the units and instruments used along the flow lines. Asphaltenes are typically stable in the oil; however, changes in conditions such as temperature or pressure or compositional changes can trigger the phase separation and resultant deposition of these asphaltenes on the surfaces encountered along the flow. Hence, it is required to be able to forecast the possibility of precipitation of asphaltenes for given operating conditions and quantify the amount of deposition. In this work, the development of an asphaltene deposition tool (ADEPT) that can predict the occurrence and calculate the magnitude and profile of asphaltene deposition in a well bore is discussed. The simulator consists of a thermodynamic module and a deposition module. The thermodynamic module uses the Perturbed Chain Statistical Associating Fluid...

Although polyethylene oxide (PEO) is soluble in water, polymethylene oxide (PMO) is not, even though PMO has more association sites. Some suggest this is due to orientation effects in the water hydrogen-bond network. A simulation and theory study of the effect of bonding site density on thermodynamic properties and extent of bonding of a linear flexible chain in a hydrogen-bonding solvent is performed. Predictions from Wertheim's theory are compared against simulation results. Thermodynamic properties and extent of bonding were obtained. The solvent molecules are modeled as hard spheres with four association sites in a tetrahedral arrangement. The chains are flexible and consist of six tangent segments of hard spheres with bonding sites that interact with the solvent molecules. A solvent molecule can also form a bond with a second solvent molecule. The association interaction is modeled with an orientation-dependent square-well. The total number of bonding sites on each chain is varied and the effects stu...

A general theory for modelling intramolecular association within the SAFT framework is proposed. Sear and Jackson [Phys. Rev. E. 50 (1), 386 (1994)] and Ghonasgi and Chapman [J. Chem. Phys. 102 (6), 2585 (1995)] have previously extended SAFT to include intramolecular association for chains with two sites. We show that the resulting equations from the two approaches are equivalent, and use their work as a basis for developing a new general theory. The approach used by Ghonasgi and Chapman is based on mass balances and an infinite dilution result and provides the equations needed to determine the contribution to the Helmholtz free energy from association (inter- as well as intramolecularly) at equilibrium. Sear and Jackson rederived the contribution to the Helmholtz free energy from association from the theory by Wertheim [J. Stat. Phys. 42 (3–4), 459 (1986)] with inclusion of intramolecular association, and using this approach we obtain an expression for the Helmholtz free energy that is valid also at non-...

A successful equation of state for ring molecules [J. Chem. Phys. 101, 6880 (1994); Phys. Rev. E 50, 386 (1994)] is extended using Wertheim's first-order thermodynamic perturbation theory to predict the degree of bonding and thermodynamic properties of associating cyclic molecules. This new theory is tested against molecular simulation results for pure fluids of cyclic trimers with either one or three association sites. The trimers’ association potential of interaction is modeled by an orientation-dependent square-well, and the segment–segment interactions are of the hard-sphere type. A wide range of densities and temperatures is studied. Values for fraction of molecules not bonded, configurational internal energy and compressibility factor were obtained using Metropolis Monte Carlo simulations in the canonical and isothermal–isobaric ensembles. The theory predictions are in excellent agreement with these simulation results.

Self-assembly, important in industrial and biological processes, is governed not only by molecular size and shape, but also by hydrogen bonding forces and hydrophobicities, both of which have important temperature-dependent consequences. Recent advances in statistical mechanics enable the study of these processes at the molecular level. We discuss one such approach based on classical density functional theory. The theory, based on extensions and simplifications of Wertheim’s theory for associating molecules, is applicable to study the behavior of water at hydrophobic and hydrophilic surfaces and supramolecular assembly based on intermolecular association of polyatomic molecules. Insights into the theory and into the physics of associating molecules are discussed.

The function and stability of macromolecular systems, important in industrial and biological processes, are governed not only by molecular size and shape, but also by temperature-dependent hydrogen bonding forces and hydrophobicity of the system. Such processes can be found in many facets of chemistry and biology, most notably in self-assembly processes such as the formation of membranes and micelles in liquid or surfactant solutions, structuring in polymer/nanoparticle systems and the folding of proteins into stable, functional complexes. We explore the range of capabilities to model such systems through classical density functional theory. Examples demonstrate capabilities of providing molecular insight into properties and structure of branched copolymer/nanoparticle systems, surfactant systems, and grafted polymers. Further opportunities to extend the theories to supramolecular assemblies and to create hybrid approaches with molecular simulation are discussed.

The goal of this paper is to show that the wetting behavior of simple fluids on a repulsive solid surface – especially the “drying” phenomenon – is closely related to the proximity of the supercritical state of the bulk fluids to their vapor–liquid coexistence region. We propose here a new DFT (the star-function based density functional theory, s-DFT) that is based on the functional Taylor expansion of the intrinsic free energy F[ρ] and the singlet direct correlation C w ( 1 ) ( r ) to arrive at closed-form expressions for both quantities without truncations or approximations. The two formulas are mutually consistent because of the introduction of a star function S w that has been shown to be the functional primitive of the bridge function B w , i.e. δ S w * / δ ρ = B w (L.L. Lee, J. Chem. Phys. 97 (1992) 8606 [34] ). The new formulation is applied to the Lennard-Jones molecules adsorbed on a planar hard wall (LJ/HW). We carried out new Monte Carlo simulations for this system. Since the s-DFT uses a bridge function B w , we demonstrate (i) the existence of a set of data (inverted from the MC information) that can perform as the bridge function and reproduce accurately the density profiles ρ w ( 1 ) ( r ) ; (ii) this set of data can be “fitted” by a function-form with acronym ZSEP; finally (iii) ZSEP expresses the bridge function B w in terms of a new renormalized basis function γH, i.e. B w ( γ H ) . The existence of a bridge function dispels some of the misconceptions that the bridge-function based formulations did not describe the “drying” behavior. We also show that for the high density case ZSEP equation can qualify as a “closure relation”, but seems to deteriorate for the two low-density supercritical states that are close to the bulk saturated liquid phase boundaries.

Modified inhomogeneous statistical associating fluid theory (iSAFT) density functional theory is extended to dendrimer molecules in solvents of varying quality. The detailed structures of isolated dendrimers in implicit solvent are calculated and have a semi-quantitative agreement with simulation results available in the literature. The dendrimers form dense-core structures under all conditions, while their radius of gyration follows different scaling laws. Factors that affect the quality of the solvent are systematically studied in the explicit solvent case. It is found that the solvent size, density, chemical affinity and temperature all play a role in determining a solvent to be good or poor. New molecular dynamics simulations are performed to validate the iSAFT results. Our results provide insight into the phase behavior of dendrimer solutions as well as guidance in practical applications.

Thermodynamic and transport properties of hydrocarbon systems are determined, in part, by the van der Waals attractions between molecules. These van der Waals attractions (or London dispersion interactions) are related to the electronic polarizability and molar refractivity of the molecules. Molar refractivity is related to refractive index and molar volume through the Lorentz–Lorenz equation. In this work we present a method that relates the refractive index of a hydrocarbon substance with its mass density. This is based on the observation that the molar refractivity is approximately proportional to the molecular weight of a hydrocarbon molecule. The proportionality constant is approximately equal to one-third for hydrocarbons and crude oil systems. Although both the refractive index and the mass density are functions of temperature, it has been found that the proportionality constant of this model is nearly invariable over a wide range of pressures and temperatures. Therefore, given the temperature dependence of one of the variables, it is possible, by applying the “One-Third” rule, to estimate the temperature dependence of the other variable. This correlation has been validated with over 200 crude oil samples, in a wide range of densities (10–55°API) and temperatures (10–70 °C), and the results obtained are remarkable. The applications of the One-Third rule in the calculation of properties such as solubility parameter, viscosity, thermal conductivity, diffusivity and heat capacity of hydrocarbons and crude oil systems, as a function of the mass density, are presented and discussed. This approach also provides an alternative to calculate the refractive index based on densities obtained from an equation of state. The method presented in this work is promising in offering a new, simple and reliable method for estimating hydrocarbon and crude oil properties as a function of their mass density.

The potential problem of asphaltene deposition during oil production has motivated the development of several experimental techniques and theoretical models to understand and predict asphaltene behavior. Although important progress has been made in this area, prediction of the rate of asphaltene deposition remains an unsolved problem. We have previously reported the successful application of the perturbed chain version of the statistical associating fluid theory equation of state (PC-SAFT EOS) in modeling asphaltene phase behavior under both ambient and reservoir conditions. In this work, we present the development of a simulation tool that simultaneously accounts for asphaltene precipitation, aggregation, and deposition. The thermodynamic modeling using the PC-SAFT EOS is coupled with kinetic models and transport equations. The mechanism for asphaltene precipitation and deposition, proposed in this work, has been found to be consistent with various experiments and field observations. Furthermore, it also...

This work analyzes water-in-oil emulsion formation in a Taylor−Couette shear cell. ASTM certified brine is the dispersed phase, and an assortment of crude and model oils are used as continuous phas...

A study of the effect of a short-chain branching distribution (SCBD) on the phase behavior of linear low density polyethylene (LLDPE) was conducted. The perturbed chain-SAFT equation of state was the underlying thermodynamic model chosen for the study; the branched polyolefins were described using a simple modeling concept previously proposed by Dominik and Chapman.(1) To isolate the effect of the SCBD on phase behavior, the copolymer systems were considered monodisperse in molecular weight. The study revealed that, in the case of low to moderate values of the polydispersity index of the SCBD, the compositional polydispersity affected the phase behavior at low polymer concentrations only. When the polydispersity index of the SCBD is high, or, in other words, when the difference in branch content between the components of the distribution is significant, additional phases appear. The formation of multiple phases results from the incompatibility of branched and linear polymers. Similar observations were mad...

Water-in-oil emulsions are of particular interest concerning methane hydrate formation during crude oil production. The objective of this work is to understand the morphology of the hydrate/water d...

Asphaltene precipitation and deposition can occur at different stages during petroleum production, causing reservoir formation damage and plugging of pipeline and production equipment. Predicting asphaltene flow assurance issues requires the ability to model the phase behavior of asphaltenes as a function of the temperature, pressure, and composition. In this paper, we briefly review some recent approaches to model asphaltene phase behavior. We also present a method to characterize crude oil, including asphaltenes using the perturbed chain form of the statistical associating fluid theory (PC-SAFT). The theory accurately predicts the crude oil bubble point and density as well as asphaltene precipitation conditions. The theory is used to examine the effects of gas injection, oil-based mud contamination, and asphaltene polydispersity on the phase behavior of asphaltenes. The analysis produces some interesting insights into field and laboratory observations of asphaltene phase behavior.

Diblock copolymers consist of two chemically distinct polymer chains (blocks) covalently bonded together. Due to the differences in the physiochemical properties of these blocks, the copolymers can undergo order–disorder transitions (ODT) at the molecular level, resulting in microphase-separated states. Control of these microstructures has significant impact on the technological applications of diblock copolymers. Diblock copolymers exhibit a variety of microstructures, which depend on the relative compositions of the two distinct blocks. In the case of symmetric diblock copolymers, the equilibrium structure is the lamellar structure where each lamellar layer is an A–B or B–A repeat unit. We use interfacial statistical associating fluid theory (iSAFT) density functional theory to analyse the morphologies of symmetric diblock copolymers in bulk melts and ultra-thin films confined between two planar surfaces. For bulk melts, the effect of the length of the copolymer and incompatibility between the blocks of...

Asphaltenes constitute a potential problem in deep-water production because of their tendency to precipitate and deposit. A universal model to predict the stability of these species, under differen...

Molecular dynamics (MD) simulations were performed to investigate the influence of molecular structure on the ability of n-alkanethiolate self-assembled monolayers (SAMs) on gold and copper to act as barrier films against through-film oxygen transport as relevant to the uses of these films in corrosion inhibition. Specific explorations focused on the effects of the packing density of the adsorbates, their chain length, and the size of the diffusing species. The MD simulations showed that the resistances offered by these monolayers against transport of small molecules strongly depend on the penetrant size and that the free volume alignment within these films has a large impact on the diffusivities of oxygen through them. The MD simulations revealed that the barrier for transport through these films increase by approximately 0.3 to 0.6 kJ/mol for increases in their thickness of only roughly an angstrom. MD simulations show the existence of a middle section in these SAMs for nor= 12 that was more crystalline and dense than the rest of the monolayer. The barrier resistances offered by these films toward oxygen transport, as calculated by the MD simulations, were a function of the crystallinity, density, and thickness of the middle section of the SAMs.

A perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state has been extended to include a crossover correction, based on White’s work with renormalization-group (RG) theory, that accounts for the contributions of long-wavelength density fluctuations in the critical region. In the critical region, the improved crossover equation of state provides the correct nonclassical critical exponents. Away from the critical region, the crossover equation reduces to the original PC-SAFT equation, therefore maintaining the accuracy of PC-SAFT in this region. No modifications to the original PC-SAFT molecular parameters are necessary, therefore allowing for an accurate description of thermodynamic properties from the triple point to the critical point. Excellent agreement with vapor−liquid equilibrium experimental data for the n-alkane family is obtained inside and outside the critical region, and all critical constants Tc, Pc, and ρc are calculated within their respective experimental errors. Th...

The insolubility of asphaltenes in light paraffin liquids and other incompatible fluids, such as carbon dioxide (CO2), is a source of problems during crude oil production operations. Asphaltene precipitation can cause formation damage and wellbore plugging, requiring expensive treatment and cleanup procedures. In the presence of light gases, such methane, ethane, propane, and nitrogen (N2), asphaltenes are usually more stable as pressure and temperature increase; however, experimental measurements indicate that, in the presence of CO2, asphaltenes become more stable as the temperature decreases. In this work, the asphaltene phase behavior in a live reservoir fluid and a dead oil from South America is investigated in a range of pressures and temperatures in the presence of CO2 using the perturbed chain–statistical associating fluid theory (PC–SAFT) equation of state (EoS). This thermodynamic model has been applied to asphaltene precipitation with different crude oil systems, such as those that involve meth...

This paper describes a proton nuclear magnetic resonance (NMR) technique, pulsed field gradient with diffusion editing (PFG-DE), to quantify drop size distributions of brine/crude oil emulsions. The drop size distributions obtained from this technique were compared to results from the traditional pulsed field gradient (PFG) technique. The PFG-DE technique provides both transverse relaxation (T2) and drop size distributions simultaneously. In addition, the PFG-DE technique does not assume a form of the drop size distribution. An algorithm for the selection of the optimal parameters to use in a PFG-DE measurement is described in this paper. The PFG-DE technique is shown to have the ability to resolve drop size distributions when the T2 distribution of the emulsified brine overlaps either the crude oil or the bulk brine T2 distribution. Finally, the PFG-DE technique is shown to have the ability to resolve a bimodal drop size distribution.

Using a density functional theory (DFT) based on Rosenfeld's formalism for hard spheres, we investigate the influence of model solutes of different sizes on the structure and interfacial properties...

The dimer-chain term proposed by Ghonasgi and Chapman [J. Chem. Phys. 1994, 100, 6633−6639], which accounts for the chain connectivity, is used in conjunction with the perturbed-chain dispersion term proposed by Gross and Sadowski [Ind. Eng. Chem. Res. 2001, 40, 1244−1260] to form a new equation of state (EOS) for polyatomic fluids. The new EOS is applied to the homologous series of n-alkanes, mixtures of long- and short-chain alkanes, and polymers. The predictive and extrapolative capabilities of the model are demonstrated through comparisons with experimental data for pure components and mixtures. The new EOS provides an accurate description of liquid densities over the entire range of temperature, including the critical region. The predictions for mixtures of long- and short-chain alkanes are in excellent agreement with experimental data without using a binary interaction parameter. The description of the phase behavior of polymer solutions is improved, in comparison to perturbed-chain−statistical asso...

Gas hydrates dominated by methane naturally occur in deep marine sediment along continental margins. These compounds form in pore space between the seafloor and a sub-bottom depth where appropriate stability conditions prevail. However, the amount and distribution of gas hydrate within this zone, and free gas below, can vary significantly at different locations. To understand this variability, we develop a one-dimensional numerical model that simulates the accumulation of gas hydrates in marine sediments due to upward and downward fluxes of methane over time. The model contains rigorous thermodynamic and component mass balance equations that are solved using expressions for fluid flow in compacting sediments. The effect of salinity on gas hydrate distribution is also included. The simulations delineate basic modes of gas hydrate distribution in marine sediment, including systems with no gas hydrate, gas hydrate without underlying free gas, and gas hydrate with underlying free gas below the gas hydrate stability zone, for various methane sources. The results are scaled using combinations of dimensionless variables, particularly the Peclet number and Damkohler number, such that the dependence of average hydrate saturation on numerous parameters can be summa- rized using two contour maps, one for a biogenic source and one for upward flux from a deeper source. Simulations also predict that for systems at steady state, large differences in parameters like seafloor depth, seafloor temperature and geothermal gradient cause only small differences in average hydrate saturation when examined with scaled variables, although important caveats exist. Our model presents a unified picture of hydrate accumulations that can be used to understand well-characterized gas hydrate systems or to predict steady-state average hydrate saturation and distribution at locations for which seismic or core data are not available.

Asphaltene precipitation and deposition adversely affect production assurance and are key risk factors in assessing difficult environments such as deepwater. Deposition in the near well bore regions and production tubulars implies high intervention and remediation costs. Prediction of asphaltene precipitation onset represents a challenge for the flow assurance area. The applicability of the PC-SAFT equation of state model to predict asphaltene onset of the precipitation in live oils is demonstrated by studying representative examples from field experiences with asphaltene problems during production. These examples not only validate the proposed model but also confirm the theory that asphaltene phase behavior can be explained based only on molecular size and van der Waals interactions. The PC-SAFT equation of state estimates also properties such as densities and bubble points of live oil systems using a minimum number of real components and “realistic” pseudocomponents. The amount and composition of the as...

Fluid properties and phase behavior of systems such as glycol ethers, carboxylic acids, and proteins are affected by the competition between intra- and intermolecular hydrogen bonding. Here we study this competition by extending Wertheim’s first-order thermodynamic perturbation theory to include intramolecular hydrogen bonding in chain molecules in the presence of an explicit water-like solvent. The theory derived here is found to be in good agreement with molecular simulation. It is shown that intramolecular association is most important for shorter chains at low temperature, low density, and high chain concentration. The theory is also extended into a density functional theory formalism to study the effect of intramolecular association on the structuring of the different segments of the molecules close to a hydrophobic surface. Intramolecular association is found to be enhanced close to the surface, with the total density of the system having the most effect on structuring close to the surface.

We describe a new flow cell apparatus for measuring the water content of gases up to 120 MPa at 490 K that uses a combination of gravimetric and electrical resistance techniques to determine the solubility of water in the gaseous phase. The new experimental data for the solubility of water in supercritical methane and ethane were obtained with our apparatus spanning pressures from 3.4 to 110 MPa covering a temperature range from 310 to 477 K. We model the experimental results by combining two equations of state; vapor-phase fugacities and fugacity coefficients are calculated with a modified Peng−Robinson equation of state, and aqueous-phase fugacities are calculated using an equation by Wagner and Pruss (J. Phys. Chem. Ref. Data 2002, 31, 387−535) or by a modification of a correlation developed by Saul and Wagner (J. Phys. Chem. Ref. Data 1987, 16, 893−901). We compare the model results with new and existing experimental data and with commercially available simulators. Our model reproduces the experimenta...

Bulk and interfacial properties of n-alkanes and polymers are modeled in the framework of the interfacial statistical associating fluid theory (iSAFT) density functional theory (DFT). The theory reduces to an equivalent of the SAFT equation of state in the bulk, thus making possible the modeling of both bulk and interfacial properties with a single set of parameters. The performance of iSAFT is illustrated through comparisons of the theory predictions with molecular simulation as well as experimental data for vapor−liquid surface tension. The formulation of iSAFT DFT preserves the computational efficiency of an atomic DFT, thus enabling the theory to deal with molecules comprising hundreds or even thousands of segments, such as the polystyrene and polyethylene included in the present work.

Molecular dynamics (MD) simulations were performed to investigate odd-even chain length dependencies in the wetting properties of self-assembled monolayers (SAMs) of n-alkanethiols [CH3(CH2)n-1SH] on gold by water and hexadecane. Experimentally, the contact angle of hexadecane on the SAMs depends on whether n is odd or even, while contact angles for water show no odd-even dependence. Our MD simulations of this system included a microscopic droplet of either 256 water molecules or 60 hexadecane molecules localized on an n-alkanethiolate SAM on gold with either an even or odd chain length. Contact angles calculated for these nanoscopic droplets were consistent with experimentally observed macroscopic trends in wettability, namely, that hexadecane is sensitive to structural differences between odd- and even-chained SAMs while water is not. Structural properties for the SAMs (including features such as chain tilt, chain twist, and terminal methyl group tilt) were calculated during the MD simulations and used to generate IR spectra of these films that compared favorably with experimental spectra. MD simulations of SAMs in contact with slabs of water and hexadecane revealed that the effects of these solvents on the structure of the SAM was restricted to the chain terminus and had no effect on the inner structure of the SAM. The density profiles for water and hexadecane on the SAMs were different in that water displayed a significant depletion in its density at the liquid/SAM interface from its bulk value, while no such depletion occurred for hexadecane. This difference in contact may explain the lack of an odd-even variation in the wetting characteristics of water on these surfaces, because the water molecules are positioned further away from the surface and, therefore, are not sensitive to the structural differences in the average orientations for the terminal methyl groups in odd- and even-chained SAMs. In contrast, the differences in the wetting properties of hexadecane on the odd- and even-chained SAMs may reflect the closer proximity of these molecules to the SAM surface and a resulting greater sensitivity to the differences in the terminal methyl group orientations in the SAMs. SAM-solvent interaction energies were calculated during the MD simulations, yielding interaction energies that differed on the even- and odd-chained surfaces by approximately 10% for hexadecane and negligibly for water, in accord with estimates using experimental wetting results.

This article presents a simple thermodynamic model for branched polyolefin systems in the framework of the PC-SAFT equation of state. The physical properties of the polymeric material can be tailored to a customer's specific needs by the choice of the α-olefin comonomer and the adjustment of its content in the chain. The model efficiently captures the effect of chain branching, comonomer type, and branching density on the phase behavior of polyolefin solutions. Since few adjustable parameters are required, it can be applied to systems for which little experimental data is available. The model parameters follow sensible trends, demonstrating its robustness for extrapolations and parameter prediction. Finally, the simple formulation facilitates its incorporation into commercial software for phase equilibria calculations.

To better understand clathrate hydrate mechanisms, nuclear magnetic resonance (NMR) and viscosity measurements were employed to investigate tetrahydrofuran (THF) hydrate formation and dissociation processes. In NMR experiments, the proton spin lattice relaxation time (T1) of THF in deuterium oxide (D2O) was measured as the sample was cooled from room temperature down to the hydrate formation region. The D2O structural change around THF during this process was examined by monitoring the rotational activation energy of THF, which can be obtained from the slope of ln(1/T1) vs 1/T. No evidence of hydrate precursor formation in the hydrate region was found. T1 measurements of THF under constant subcooling temperature indicate that THF hydration shells do not undergo much structural rearrangement during induction. The T1 of THF was also measured as the sample was warmed back to room temperature after hydrate dissociation. T1 values of THF after hydrate dissociation were consistently smaller than those before hy...

The Perturbed-Chain SAFT (PC-SAFT) equation of state is applied to model phase equilibria and the thermodynamic properties of ethers and esters. A systematic study of these two homologous series is conducted, and the performance of two different approaches for including the dipolar interactions in the equation of state is evaluated. Although both Polar PC−SAFT [dipolar contribution due to Jog and Chapman, Mol. Phys. 1999, 97, 307−319] and PC−SAFT + Fischer [dipolar contribution by Saager and Fischer, Mol. Simul. 1991, 6, 27−49] yield similar results for the considered systems, the parameters of Polar PC−SAFT are more physically meaningful than those of PC−SAFT + Fischer. Consequently, Polar PC−SAFT is considered to be more suitable for extrapolations, and for application to components that have multiple polar groups in the chain.

Wertheim's thermodynamic perturbation theory of first order (TPT1) is based on the approximation that the monomer–monomer distribution functions can be approximated by the reference fluid distribution functions regardless of the amount of bonding. This is remarkably accurate for chains formed by tangent spheres, but no longer valid for chains of fused spheres. This constitutes the reason for the inadequacy of TPT1 for fused sphere chains. We present a systematic modification of TPT1, the path integral perturbation method, that takes into account the variations of the distribution functions with extent of bonding. We demonstrate the accuracy of the theory for mixtures of hard spheres and diatomics over a range of extent of bonding (pure monomers to pure dimers) and degree of fusion (bond length 0–1). We found that the choice of reference fluid was decisive for the accuracy of the model's predictions. The proposed theory can accurately predict the properties of mixtures of hard spheres and diatomics, and of...

Proper prediction of the potential for asphaltene precipitation in deepwater reservoirs represents a challenge for the flow assurance area due to both high intervention costs in case of asphaltene problems and extreme conditions of pressure, temperature, and composition. The probability of asphaltene precipitation increases when light-oil gases are considered for miscible gas injection processes. In this work, the applicability of simple and recombined live oil models using the PC-SAFT equation of state to predict the onset of asphaltene precipitation is demonstrated by studying pressure depletion and gas injection (carbon dioxide (CO2), nitrogen (N2), methane, and ethane) in oil reservoirs. The recombined model input includes the SARA characterization, the oil compositional analysis, and the gas−oil ratio. The pure component PC-SAFT parameters are fitted to the saturated liquid density and vapor pressure data, and the oil/gas pseudocomponent parameters are estimated from molecular-weight-based correlatio...

In this work, we present a systematic study of the ability of the SAFT equation of state with the dispersion term of Gross and Sadowski (PC-SAFT) and the dispersion term of Chen and Kreglewski (CK-SAFT) to model vapor–liquid (VLE) and liquid–liquid equilibria (LLE) for methanol + n-alkane systems. In addition to using pure component SAFT parameters derived from the standard method of fitting to liquid density and saturated vapor pressure, CK-SAFT with associating parameters determined from two different methods of molecular orbital quantum mechanic (MO) calculation, the Hartree–Fock (HF) and Becke-3LYP (B3), are also used to reproduce experimental data. One adjustable temperature-dependent binary interaction parameter, kij, is used to fit the calculated results to the experimental data. Both CK- and PC-SAFT reproduce experimental VLE data for methanol + alkane systems quantitatively when kij is fit to experimental data and both qualitatively predict experimental results when kij = 0. For these systems, widely used cubic equations of state, like the Peng–Robinson or Soave, Redlch and Kwong, are incapable of quantitatively reproducing VLE for these highly non-ideal systems. MO-based CK-SAFT parameters show agreement with the experimental results at approximately 298 K, but at higher temperatures, no kij value can reproduce the experimental data without introducing liquid–liquid splitting. For VLE calculations, a correlation for predicting kij based on molecular weight and temperature is presented, which is also shown to provide reasonable estimates for the kij need to reproduce LLE data. LLE calculations show good agreement with experimental data for pentane through heptane. Calculations for alkanes longer than heptane show only qualitative agreement with experimental data.

In low temperature gas processing, the presence of water can result in the formation of gas hydrate plugs. To avoid this problem, it is important to know the water solubility in natural gas components in equilibrium with gas hydrate. In this study experimental measurements of water content in gaseous methane in equilibrium with hydrate at 3.45 MPa (500 psia) and 6.90 MPa (1000 psia) and temperatures ranging from −3.2 °C (26.2 °F) to −80 °C (−112 °F) are presented. Similar measurements are presented for liquid ethane at 3.45 MPa (500 psia) and temperatures from −2.2 °C (28.0 °F) to −70 °C (−94 °F), and for liquid propane at 0.86 MPa (125 psia) and temperatures down to −60 °C (−76 °F), respectively. In measuring the water content, a Panametrics moisture sensor (calibrated to 1 ppb water content in nitrogen) has been used in flowing streams of the hydrocarbon-rich phases that are saturated with water. The results obtained with the Panametrics hygrometer show good agreement (normally better than ±4%) with previous measurements, which were obtained by a gas chromatographic technique for methane, ethane, and propane at temperatures ranging from −2.0 °C (28.4 °F) to −30 °C (−22 °F), which are within the hydrate region.

Thermodynamic phase equilibrium calculations for systems containing polymers and/or many molecular species are computationally expensive and difficult because the number of equations that need to b...

The statistical associating fluid theory (SAFT) equation of state has been shown to accurately predict the effects of molecular weight, intermolecular association, and compressibility on the phase behavior of mixtures containing solvents, monomers, and polymers. In this paper, we briefly review the physical basis for the SAFT approach and present recent extensions of the equation of state. An advantage of theoretically based equations of state is that the theory can be systematically improved to address weaknesses or to extend the theory to more complex fluid systems. Recent developments beyond SAFT allow us to predict the effects of multiple polar groups on the phase behavior of polar copolymers, model the effects of intramolecular association, and efficiently calculate phase equilibrium for polydisperse polymer solutions. These points are illustrated through comparisons with phase equilibria data for monomers, solvents, and copolymer systems.

We present a novel perturbation density-functional theory (DFT) to describe adsorption of associating fluids on surfaces activated with polar sites to which fluid molecules can bond or associate, such as water adsorbing on activated carbon, silica, clay minerals, etc. Association is modeled within the framework of first order thermodynamic perturbation theory (TPT1). In this first of two papers, we explore in detail the changes brought about in a system due to fluid–wall (FW) association. Hence fluid–fluid association is not considered here. However, the theory can be coupled with existing DF theories of associating fluids to study the interplay between the wall–fluid and fluid–fluid association as is shown in a future paper by S. Tripathi. The proposed theory, in excellent agreement with simulations, shows that FW association significantly changes the fluid structure and adsorption behavior. The theory accurately predicts the distribution of bonded and nonbonded species away from the surface, adsorption ...