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Gadolinium-based contrast agents are key in clinical MRI for enhancing the longitudinal NMR relativity (r1) of hydrogen nuclei (1H) in water and improving the contrast among different tissues.

Deviation from ideal solution behavior is due to differences in intermolecular interactions, e.g., molecular size, shape, dispersion, multipolar, and hydrogen bonding interactions. Activity coeffic...

The hydration free energy of a macromolecule is the central property of interest for understanding its distribution over conformations and its state of aggregation. Calculating the hydration free energy of a macromolecule in all-atom simulations has long remained a challenge, necessitating the use of models wherein the effect of the solvent is captured without explicit account of solvent degrees of freedom. This situation has changed with developments in the molecular quasi-chemical theory (QCT), an approach that enables calculation of the hydration free energy of macromolecules within all-atom simulations at the same resolution as is possible for small molecule solutes. The theory also provides a rigorous and physically transparent framework to conceptualize and model interactions in molecular solutions, and thus provides a convenient framework to investigate the assumptions in implicit-solvent models. In this study, we compare the results using molecular QCT versus predictions from EEF1, ABSINTH, and GB/SA implicit-solvent models for poly-glycine and poly-alanine solutes covering a range of chain lengths and conformations. Among the three models, GB/SA does best in capturing the broad trends in hydration free energy. We trace the deficiencies of the group-additive EEF1 and ABSINTH models to their under-appreciation of the cooperativity of hydration between solute groups; seen in this light, the better performance of GB/SA can be attributed to its treatment of the collective properties of hydration, albeit within a continuum dielectric framework. We highlight the importance of validating the individual physical components that enter implicit solvent models for protein solution thermodynamics.

Atomistic molecular dynamics simulations are used to predict 1H NMR T1 relaxation of water from paramagnetic Gd3+ ions in solution at 25 °C. Simulations of the T1 relaxivity dispersion function r1 computed from the Gd3+–1H dipole–dipole autocorrelation function agree within ≃8% of measurements in the range f0 ≃ 5 ↔ 500 MHz, without any adjustable parameters in the interpretation of the simulations, and without any relaxation models. The simulation results are discussed in the context of the Solomon-Bloembergen-Morgan inner-sphere relaxation model, and the Hwang-Freed outer-sphere relaxation model. Below f0 ≲ 5 MHz, the simulation overestimates r1 compared to measurements, which is used to estimate the zero-field electron-spin relaxation time. The simulations show potential for predicting r1 at high frequencies in chelated Gd3+ contrast-agents used for clinical MRI.

The viscosity of crude oils and their blends is a key parameter for studying hydrocarbon flow in reservoirs with gas injection and well analyzing the performance during gas lift. Even though severa...

The intramolecular ¹H NMR dipole–dipole relaxation of molecular fluids has traditionally been interpreted within the Bloembergen–Purcell–Pound (BPP) theory of NMR intramolecular relaxation. The BPP theory draws upon Debye’s theory for describing the rotational diffusion of the ¹H–¹H pair and predicts a monoexponential decay of the ¹H–¹H dipole–dipole autocorrelation function between distinct spin pairs. Using molecular dynamics (MD) simulations, we show that for both n-heptane and water this is not the case. In particular, the autocorrelation function of individual ¹H–¹H intramolecular pairs itself evinces a rich stretched-exponential behavior, implying a distribution in rotational correlation times. However, for the high-symmetry molecule neopentane, the individual ¹H–¹H intramolecular pairs do conform to the BPP description, suggesting an important role of molecular symmetry in aiding agreement with the BPP model. The intermolecular autocorrelation functions for n-heptane, water, and neopentane also do not admit a monoexponential behavior of individual ¹H–¹H intermolecular pairs at distinct initial separations. We suggest expanding the autocorrelation function in terms of modes, provisionally termed molecular modes, that do have an exponential relaxation behavior. With care, the resulting Fredholm integral equation of the first kind can be inverted to recover the probability distribution of the molecular modes. The advantages and limitations of this approach are noted.

While the SAFT equation of state, based on extensions and simplifications of Wertheim’s multi-density perturbation theory for associating molecules, has found wide application in industry and acade...

The cubic-plus-chain (CPC) equation of state (Sisco & Abutaqiya et al., Industrial & Engineering Chemistry Research, 2019) is a recent development in the literature in which the classical cubic equ...

The mechanism behind the NMR surface-relaxation times (T1S,2S) and the large T1S/T2S ratio of light hydrocarbons confined in the nanopores of kerogen remains poorly understood and consequently has ...

In this work we present an efficient numerical algorithm for the solution of molecular density functional theory (DFT) in cylindrical geometry to facilitate the study of how curvature affects the m...

Patchy colloids and associating fluids have attracted continued interest due to the interesting phase behavior and self-assembly in solution. The ability to fabricate patchy colloids with multiple attractive surface patches of different number, size, shape, and relative location makes patchy colloids a good candidate as building blocks to form complex advanced materials. However, a theory that clearly relates the self-assembled structures that form based on the anisotropic interactions has been missing. Although Wertheim's theory in the form of the SAFT model is widely used to predict self-assembly and phase behavior in solution, SAFT does not include multibody correlations necessary to model any shape of association site or sites that can form multiple bonds. We have recently developed a new theory for associating colloids that naturally incorporates multibody correlations based on a cluster distribution approach due to Bansal, Asthagiri, Marshall, and Chapman (BAMC). In this paper, we extended the cluster distribution theory to predict the thermodynamic properties and phase behavior of binary systems consisting of anisotropic particles with any geometry of bonding site. In particular, we consider self-assembly of Janus particles, Saturn particles, and ternary particles mixed with solvent colloids that have two directional patchy sites. Good agreement between theoretical predictions and molecular simulation is shown for self-assembly, thermodynamic properties in this system. Re-entrant phase behavior has been investigated and low density gels is predicted.

Atomistic molecular dynamics simulations are used to predict

Adsorption is an important process used in many industrial unit operations. However, due to the heterogeneity of surfaces studied experimentally, it is not a simple task to evaluate the validity of theories for adsorption. Here, we present a thorough investigation of density functional theory and density gradient theory based on molecular simulation. While comparisons between these theories have been made in the literature, only our recently developed equation of state (EOS), the Perturbed Truncated and Shifted (PeTS) equation of state and the according functional version, allow a thorough validation of both theories, because the EOS separates repulsive and attractive free energy contributions consistently with the inhomogeneous theory. The PeTS EOS represents the thermodynamics of the Lennard-Jones truncated and shifted fluid with a cutoff radius of 2.5 times the fluid diameter very accurately and is valid in the metastable range, too. To check the validity of both density gradient and density functional...

Dissipative particle dynamic (DPD) simulations were performed to study the interfacial properties such as interfacial tension, area compressibility, stress profile, and conformation of surfactant a...

Water molecules bind strongly to the polar calcite surface and form a surface-adsorbed layer that has properties akin to an apolar surface. This has important implications for understanding the thermodynamic driving forces underlying the adsorption of acid groups from crude oil, in particular, naphthenic acid, onto calcite. Free energy calculations show that naphthenic acid binds favorably to the water monolayer adsorbed on the calcite surface. However, to bond directly to calcite, a free energy barrier has to be overcome to expel the intervening layer of water. Further, naphthenic acids with longer alkyl chains bind with lower free energy to the calcite surface than those with shorter alkyl chains, and, for the same chain length, branching enhances adsorption. To better understand this behavior, for a specified alkyl chain length, we study adsorption at different temperatures. Consistent with experiments, we find that adsorption is enhanced at higher temperatures. Examination of the enthalpic and entropi...

The cubic-plus-chain (CPC) equation of state framework for nonpolar chain molecules hybridizes the classical cubic equation of state with the chain equation of state from SAFT (Sisco and Abutaqiya ...

CO2 competitive sorption with shale gas under various conditions from simple to complex pore characteristics is studied using a molecular density functional theory (DFT) that reduces to perturbed chain-statistical associating fluid theory in the bulk fluid region. The DFT model is first verified by grand canonical Monte Carlo simulation in graphite slit pores for pure and binary component systems at different temperatures, pressures, pore sizes, and bulk gas compositions for methane/ethane with CO2. Then, the model is utilized in multicomponent systems that include CH4, C2H6, and C3+ components of different compositions. It is shown that the selectivity of CO2 decreases with increases in temperature, pressure, nanopore size, and average molecular weight of shale gas. Extending the model to more realistic situations, we consider the impact of water present in the pore and consider the effect of permeation of fluid molecules into the kerogen that forms the pore walls. The water-graphite interaction is calibrated with contact angle from molecular simulation data from the literature. The kerogen pore model prediction of gas absolute sorption is compared with experimental and molecular simulation values in the literature. It is shown that the presence of water reduces the CO2 adsorption but improves the CO2 selectivity. The dissolution of gases into the kerogen matrix also leads to the increase in CO2 selectivity. The effect of kerogen type and maturity on the gas sorption amount and CO2 selectivity is also studied. The associated mechanisms are discussed to provide fundamental understanding for gas recovery by CO2.

An equation of state framework for nonpolar chain molecules is proposed that hybridizes the classical cubic equation of state with the chain equation of state from SAFT (Chapman et al., Fluid Phase Equilibria, 1989). The cubic equation of state serves as the physical description of the monomer beads, providing repulsive and attractive character to the segments, and the chain term bonds these monomer segments to form chains of homogeneous beads. Whereas the model molecule of the standard cubic equation of state is a sphere with variable attraction energy and volume but whose fundamental shape remains roughly spherical, the model molecule of the proposed cubic-plus-chain (CPC) equation of state consists of beads bonded covalently to form linear chains. The CPC model molecule is a better representation of chain-like molecules, such as n-alkanes and polymers, and phase behavior modeling with the proposed CPC equation of state shows considerable improvement over the reference cubic equation of state, mainly du...

The intramolecular

International audience; The mechanism behind the 1H nuclear magnetic resonance (NMR) frequency dependence of T1 and the viscosity dependence of T2 for polydisperse polymers and bitumen remains elusive. We elucidate the matter through NMR relaxation measurements of polydisperse polymers over an extended range of frequencies (f0 = 0.01–400 MHz) and viscosities (η = 385–102 000 cP) using T1 and T2 in static fields, T1 field-cycling relaxometry, and T1ρ in the rotating frame. We account for the anomalous behavior of the log-mean relaxation times T1LM ∝ f0 and T2LM ∝ (η/T)−1/2 with a phenomenological model of 1H–1H dipole–dipole relaxation, which includes a distribution in molecular correlation times and internal motions of the nonrigid polymer branches. We show that the model also accounts for the anomalous T1LM and T2LM in previously reported bitumen measurements. We find that molecular dynamics (MD) simulations of the T1 ∝ f0 dispersion and T2 of similar polymers simulated over a range of viscosities (η = 1–1000 cP) are in good agreement with measurements and the model. The T1 ∝ f0 dispersion at high viscosities agrees with previously reported MD simulations of heptane confined in a polymer matrix, which suggests a common NMR relaxation mechanism between viscous polydisperse fluids and fluids under nanoconfinement, without the need to invoke paramagnetism.

The mechanism behind the

The mechanism behind the NMR surface-relaxation times (

The self-assembly of block copolymer melts and solutions with two-dimensional density inhomogeneity is studied using modified inhomogeneous statistical associating fluid theory (iSAFT). A real-space combinatorial screening method under density functional theory formalism is proposed and used to map out the phase diagram of block copolymer melts including order-disorder transitions and order-order transitions. The predicted phase diagram agrees well with molecular dynamics simulation and self-consistent field theory. The compressibility effect on order-disorder transition temperature for block copolymer melts is modeled using iSAFT. The pressure induced temperature change by theory has a similar trend to experimental studies. Then, the lyotropic and thermotropic self-assembly phase behavior of block copolymer solutions is investigated. Detailed density distributions by iSAFT provide insight into the lyotropic properties of the block copolymer solutions at the molecular level. The effect of the block copolymer molecular architecture is studied by comparing block copolymers with different molecular packing parameters. Block copolymer solutions in the inverted hexagonal phase are predicted by theory for the block copolymer having a large molecular packing parameter. Finally, solvent selectivity is studied by modeling the block copolymers in a neutral good solvent. The enhanced local solvent concentration predicted by theory explains the reason for fewer ordered phases found in experiments.

The vapor-liquid interface of the Lennard-Jones truncated and shifted (LTJS) fluid with a cut-off radius of 2.5 σ is investigated for temperatures covering the range between the triple point and the critical point. Three different approaches to model the vapor-liquid interface are used: molecular dynamics (MD) simulations, density gradient theory (DGT) and density functional theory (DFT). The surface tension, pressure and density profiles, including the oscillatory layering structure of the fluid at the interface, are investigated. The PeTS (Perturbed truncated and shifted) equation of state and PeTS-i functional, based on perturbation theory, are used to calculate the Helmholtz free energy in the DGT and DFT approach. They are consistent with the LJTS force field model. Overall, both DGT and DFT describe the results from computer experiments well. An oscillatory layering structure is found in MD and DFT.

Asphaltene has been recognized as the cholesterol of petroleum for decades due to its precipitation and deposition in oil production, transportation and processing facilities, causing tremendous losses to the oil industry each year. This work presents a numerical model to predict asphaltene deposition in wellbores/pipelines. A Thermodynamic Module is developed to model asphaltene precipitation, based on the sequential stability-testing-and-phase-split-calculation method using Peng-Robinson equation of state with Peneloux volume correction. A Transport Module is developed to model fluid transport, asphaltene particle transport and asphaltene deposition, according to basic conservation laws. Using a thermodynamic properties look-up table, these two modules are linked to each other to account for the effects of a finite deposit layer thickness on the coupled flow fields and deposition process. In this article, verification and validation of the Thermodynamic Module are first carried out. Then, the integrated model is utilized to study asphaltene deposition problems in an actual oilfield where the asphaltene deposit layer profile is reasonably accurately predicted. This case shows that the presented model has great potential as a predicting tool to assist reservoir engineers in assessing asphaltene deposition risks in wellbores/pipelines.

Low salinity water flooding has shown great promise due to its cost-effectiveness and low environmental impact for improving and sustaining oil production. It is believed that injecting water with ionic strength lower than that of the reservoir changes the reservoir from less to more water-wet and enhances oil recovery. This alteration phenomenon is not well understood, due to complex interactions between oil, water, and rock. Here we use molecular simulations to characterize the wettability of the 10.4-face of calcite in a calcite/brine/oil system, and address how wettability is altered by changing ionic strength and salt type (NaCl vs. CaCl2). Using the test area method we calculate the superficial tension of the fluids against the solid and the surface tension between the two fluid phases. As the salinity is decreased, the wetting of calcite by brine is progressively less favored, contrary to what might be expected based on low salinity flooding. However, as salinity is decreased, forming the oil-brine interface is more favored. On balance, it is the latter effect that leads to the enhanced wetting of calcite by brine in the oil-brine-calcite system, and it is suggested as an important element in the physics underlying low-salinity flooding.

Asphaltene deposition in wellbores has been recognized as the cholesterol of petroleum for decades causing billions of dollars in losses to the oil and gas industry every year. This necessitates great efforts in precise and fast forecasting of the production problems induced by asphaltene deposition. From perspective of the large length-to-diameter ratios of wellbores and the unidirectional nature of the crude oil flow, this work presents a numerical procedure to predict the coupled velocity, pressure and concentration distribution in a transient one-dimensional one-way framework. This procedure is general-purpose for flow passages of large aspect ratios with the precipitation rate, aggregation rate and deposition rate embedded in known forms. In this numerical procedure, the governing equations are solved using the finite volume method on a regular mesh arrangement with fully implicit spatial and temporal schemes. For verification purpose, a few cases having exact solutions are studied. Then, application of the presented procedure to capillary asphaltene deposition is illustrated where good agreement is achieved between the simulation results and the experimental measurements. This case demonstrates that the proposed procedure can be used to investigate oilfield asphaltene problems and assist reservoir engineers in assessing the potential asphaltene deposition risk in wellbores.

The PC-SAFT Equation of State is applied here to study polymer – blowing agent phase equilibrium in various systems of industrial interest. Where required, PC-SAFT parameters of blowing agents and polymers are obtained by regression to experimental data. Gas solubility and swelling ratio of polymer material predicted by PC-SAFT is seen to agree well with experiments. The PC-SAFT model is then extended to predict gas solubility and swelling ratio in a broad range of operating conditions, well beyond those reported by corresponding experiments. PC-SAFT has also been robust in predicting gas solubility in random copolymer or gas blend foaming systems showing additional degrees of freedom in foam formulation design, and requiring only inputs from the individual homopolymer or single blowing agent systems. From this work, PC-SAFT offers a reliable method to extrapolate existing measured data and provide guidelines for screening and design of new products and processes.

For more than a century, density gradient theory (DGT) has been developed and applied for interfacial property calculations of pure and mixed fluid systems. However, due to the local density approximation, DGT has not been applicable to amphiphilic molecules. By developing a modified DGT model with the chain contribution to the free energy, this paper extends the application of the DGT model to heteronuclear chain molecules, such as surfactants. The chain contribution term is derived based on the work from the iSAFT model. With the help of the Stabilized Density Gradient Theory (SDGT) algorithm developed in our previous work and the PC-SAFT equation of state (EoS), the modified DGT model is tested in water/oil/surfactant mixture systems, the results of which have been qualitatively verified with other theories and experimental data.

In this paper we present an extension of the statistical associating fluid theory (SAFT) for branched molecules with a Lennard-Jones dimer reference fluid (SAFTD-LJ-Branch). The theory successfully predicts how branched architecture affects the attraction and repulsion between molecules. SAFTD-LJ-Branch takes a form similar to SAFTD-LJ with an additional parameter NB introduced to account for the branching effect. We propose an approach relating NB to the number of different types of articulation segments. The theory is used to study the effect of chain architecture on the thermodynamic properties of isomeric alkanes. SAFTD-LJ-Branch accurately predicts the phase diagram of pure butane, pentane or hexane isomers. Further, vapor pressures of n-triacontane and squalane are predicted without further fitting and shown to be in semiquantitative agreement with experimental data. Finally, SAFTD-LJ-Branch is demonstrated to be well applicable to mixtures as we model the vapor–liquid coexistence of binary alkane m...

We study binary mixtures of multi-bonding single site solute particles in a solvent comprising patchy colloid particles. The particles in the mixture interact by very short-ranged attraction and hard-sphere repulsion. The attractive patch on the solute can bond with multiple solvent particles, whereas the patch on the solvent is restricted to bond only once. From a quasi-chemical analysis of association, in the hard-sphere reference we develop an accurate multi-body correlation information for the distribution of solvent particles over the patch region of the solute. We use this information within Wertheim's multi-density formalism to develop a cluster size distribution theory that is capable of capturing the physics of multi-body association for any geometry of association sites on the solute. We use this general framework to study a mixture containing Janus solutes and one- or two-patch solvent particles over a range of concentration of the solute and association strengths. We find that a mixture of two-patch solvent (with both patches of the same kind) and multi-bonding solutes with different patch geometries can have a vapor-liquid equilibrium, although the pure components themselves cannot phase separate. The liquid state occurs at very low densities, forming a so-called empty liquid. For the relative association strengths studied in this work, we observe that the vapor-liquid coexistence curve broadens as the concentration of the patchy solvent particles in the liquid phase is increased. The pressure-composition phase equilibrium curves show negative azeotropes for these mixtures. We also observe that, for these mixtures, as the size of the patch on the solute particles is decreased, the critical temperature and the critical packing fraction decreases.

This study presents experiment and surface complexation modeling (SCM) of synthetic calcite zeta potential in brine with mixed potential determining ions (PDI) under various CO2 partial pressures. Such SCM, based on systematic zeta potential measurement in mixed brines (Mg2+, SO42-, Ca2+ and CO32-), is currently not available in the literature and is expected to facilitate understanding of the role of electrostatic forces in calcite wettability alteration. We first use a double layer SCM to model experimental zeta potential measurements and then systematically analyze the contribution of charged surface species. Calcite surface charge is investigated as a function of four PDIs and CO2 partial pressure. We show that our model can accurately predict calcite zeta potential in brine containing a combination of four PDIs and apply it to predict zeta potential in ultra-low and pressurized CO2 environments for potential application in enhanced oil recovery in carbonate reservoirs. Model prediction reveals that calcite surface will be positively charged in all considered brines in pressurized CO2 environment (>1atm). The calcite zeta potential is sensitive to CO2 partial pressure in the various brine in the order of Na2CO3>Na2SO4>NaCl>MgCl2>CaCl2 (Ionic strength=0.1M).

Interfacial tension (IFT) between water and alkane liquid phases has been studied by dissipative particle dynamics (DPD) simulations. The DPD interaction parameters as a function of temperature are estimated using the COSMO-RS model (Conductor-like Screening Model for Real Solvents) through the Flory-Huggins interaction parameter matching the infinite dilution activity coefficient. The computed interfacial tension agrees well with the experimental values for all temperatures. The proposed method for determining DPD interactions provides the basis to estimate interactions for complex systems such as nonionic and ionic surfactant systems.

The prediction of fluid phase behavior in nanoscale pores is critical for shale gas/oil development. In this work, we use a molecular density functional theory (DFT) to study the effect of molecular size and shape on partitioning to graphite nanopores as a model of shale. Here, interfacial statistical associating fluid theory (iSAFT) is applied to model alkane (C1 – C8) adsorption/desorption/phase behavior in graphite slit pores for both pure fluids and mixtures. The pure component parameters were fit to the bulk saturated liquid density and vapor pressure data in selected temperature ranges. The potential of interaction between the fluid and graphite is modeled with a Steele 10-4-3 potential that is fit to the potential of mean force from single-molecule simulations. Good agreement is found between theory and molecular simulation for the density distributions of pure components in slit pores. The critical properties of methane, ethane, and their mixtures as well as the shift in bubble point and dew point...

With the shale gas boom, natural gas has become one of the main energy sources in the United States. But in the cryogenic processes of this natural gas, the formation of solid carbon dioxide remains a concern because it can cause the blockage of equipment. Accurately modeling the rich phase behavior of carbon dioxide in methane is helpful for optimizing the industrial processes, especially where experimental data are lacking at the operation conditions. In this work, the Perturbed Chain-SAFT (PC-SAFT) equation of state is applied to model the phase behavior of hydrocarbon (i.e., methane, ethane, butane) + carbon dioxide systems over a wide range of temperatures and pressures. It is observed that the PC-SAFT equation of state can accurately predict the vapor-liquid equilibria (VLE), solid-vapor equilibria (SVE), and solid-liquid equilibria (SLE) of hydrocarbon + carbon dioxide systems using a single set of temperature and pressure independent binary interaction parameters. In the end, the effect of addition of ethane and butane on SLE of methane + carbon dioxide is also investigated which has applications in reducing the carbon dioxide solidification conditions.

Typically, the addition of a non-adsorbing polymer to a system containing colloidal particles causes a phase separation in a mechanism known as depletion flocculation. Recent experiments have shown that the addition of polystyrene to a mixture of asphaltenes and toluene causes phase separation into two liquids.1 In this paper, this effect is modeled using the perturbed-chain form of statistical associating fluid theory (PC-SAFT) equation of state, demonstrating agreement with the experimental data. The effects of the temperature, pressure, and polystyrene mean molecular weight on the mixture phase behavior are investigated. The phase behavior of the system was not sensitive to pressures up to 1500 atm; however, increasing the temperature or reducing the mean molecular weight of polystyrene caused the one-phase region to expand. The paper demonstrates that a solution model with rigorous physics is able to capture the phase behavior that is typically described as a depletion flocculation mechanism in the co...

Solid sodium chloride solubility is measured at temperatures from 297 to 467 K at pressure of 6.45 MPa in ethane-1,2-diol/water mixed solvent systems and at a concentration of cosolvent up to 75% w/w. The corresponding solubility is also predicted from an extension of the unified theory of electrolytes to mixed solvents (J. Phys. Chem. B 2012, 116, 9033−9042) for sodium and potassium chloride up to 473.15 K and over the whole composition range of the cosolvent. A comparison of the predicted solubility with the corresponding experimental values from this study and the available literature data indicates good agreement in all cases to well within the uncertainties of the experimental data. Also, for the saturated solution of sodium and potassium chloride in ethane-1,2-diol/water mixed solvent systems, the stoichiometric activity coefficient values are estimated up to 473.15 K. These stoichiometric activity coefficients, over the complete range of mole fraction of the cosolvent, are then extended to all conc...

Arterial deposition of asphaltene is a major flow assurance issue in pipelines and wellbores. The numerous compounds constituting crude oils are mutually soluble at reservoir conditions, but precipitation can occur with changes in the pressure, temperature, or composition. As the pressure and temperature change through the wellbore, asphaltenes can precipitate and potentially deposit. Unfortunately, remediation by solvent soaks is expensive; hence, the need to forecast the potential risk of asphaltene deposition. In this paper, a previously reported simulation tool, asphaltene deposition tool (ADEPT), is used to predict the magnitude and location of asphaltene deposits in flow lines and wellbores. ADEPT is to be used to gauge the frequency and location of deposits and how often intervention will be needed. The phase behavior of asphaltene is described by the perturbed-chain statistical associating fluid theory equation of state, while the transport equations are coupled with kinetic rates of precipitation...

Density gradient theory (DGT) allows fast and accurate determination of surface tension and density profile through a phase interface. Several algorithms have been developed to apply this theory in practical calculations. While the conventional algorithm requires a reference substance of the system, a modified "stabilized density gradient theory" (SDGT) algorithm is introduced in our work to solve DGT equations for multiphase pure and mixed systems. This algorithm makes it possible to calculate interfacial properties accurately at any domain size larger than the interface thickness without choosing a reference substance or assuming the functional form of the density profile. As part of DGT inputs, the perturbed chain statistical associating fluid theory (PC-SAFT) equation of state (EoS) was employed for the first time with the SDGT algorithm. PC-SAFT has excellent performance in predicting liquid phase properties as well as phase behaviors. The SDGT algorithm with the PC-SAFT EoS was tested and compared with experimental data for several systems. Numerical stability analyses were also included in each calculation to verify the reliability of this approach for future applications.

The stability of foam in the presence of oil is of fundamental interest in enhanced oil recovery processes using foam for mobility control. Experimentally, it is known that lauryl betaine (LB) increases foam stability of certain anionic surfactants, and LB is referred to as a foam booster. However, the molecular basis for this effect is not well understood. Using molecular dynamics simulations, here we study a system of LB and alpha olefin sulfonate (AOS-14), an anionic surfactant that is used as a foam stabilizer. We monitor the area per molecule, a quantity that correlates with the surface shear viscosity, and the surface dilatational modulus to infer the stability of the various mixtures. We find that the influence of LB is nonmonotonic: the area per molecule and surface dilatational modulus have a minimum and maximum, respectively, around 30% LB in the monolayer. We show that this net effect has its basis in two competing effects: the favorable interaction between LB and AOS-14 that tends to shrink th...

Glass transition in a condensed core asphaltene model was investigated using molecular dynamics simulations performed in the isobaric–isothermal ensemble. Glass transition temperature obtained from the discontinuities in the slope of specific volume versus temperature plots was in close agreement with experimental results reported in the literature. These discontinuities also correspond to those in isothermal compressibility versus temperature plots. In this paper, we separate the contributions of aliphatic and aromatic regions of the asphaltene molecule to the glass transition behavior. We demonstrate that the aliphatic chains contribute significantly to volumetric changes and impose restrictions to the molecular orientations. Glass transition is accompanied by breaking of π–π stacking of the asphaltene molecule. Therefore, the size of the fused aromatic region in the condensed core determines the strength of intermolecular interactions and the glass transition temperature Tg.

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