Imperial College London

Professor Erich A. Muller

Faculty of EngineeringDepartment of Chemical Engineering

Professor of Thermodynamics
 
 
 
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Contact

 

+44 (0)20 7594 1569e.muller Website

 
 
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Assistant

 

Miss Raluca Leonte +44 (0)20 7594 5557

 
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Location

 

409ACE ExtensionSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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172 results found

Zheng L, Trusler JPM, Bresme F, Muller Eet al., 2019, Predicting the pressure dependence of the viscosity of 2,2,4-trimethylhexane using the SAFT coarse-grained force field, Fluid Phase Equilibria, Vol: 496, Pages: 1-6, ISSN: 0378-3812

This work is framed within AIChE's 10th Industrial Fluid Properties Simulation Challenge, with the aim of assessing the capability of molecular simulation methods and force fields to accurately predict the pressure dependence of the shear viscosity of 2,2,4-trimethylhexane at 293.15 K (20 °C) at pressures up to 1 GPa. In our entry for the challenge, we employ coarse-grained intermolecular models parametrized via a top-down technique where an accurate equation of state is used to link the experimentally-observed macroscopic volumetric properties of fluids to the force-field parameters. The state-of-the-art version of the statistical associating fluid theory (SAFT) for potentials of variable range as reformulated in the Mie incarnation is employed here. The potentials are used as predicted by the theory, with no fitting to viscosity data. Viscosities are calculated by molecular dynamics (MD) employing two independent methods; an equilibrium-based procedure based on the analysis of the pressure fluctuations through a Green-Kubo formulation and a non-equilibrium method where periodic perturbations of the boundary conditions are employed to simulate experimental shear stress conditions. There is an indication that, at higher pressures, the model predicts a solid phase (freezing) which we believe to be an artefact of the simplified molecular geometry used in the modelling. A comparison (made after disclosure of the experimental data) show that the model consistently underpredicts the viscosity by about 30%, but follows the pressure dependency accurately.

Journal article

Muller E, Trusler J, Bresme F, Zheng Let al., Employing SAFT coarse grained force fields for the molecular simulation of thermophysical and transport properties of CO2 – n-alkane mixtures, Journal of Chemical and Engineering Data, ISSN: 0021-9568

Journal article

Muller E, Aasen A, Ervik Å, Hammer M, Wilhelmsen Øet al., Equation of state and force fields for Feynman–Hibbs-corrected Mie fluids. I. Application to pure helium, neon, hydrogen and deuterium, Journal of Chemical Physics, ISSN: 0021-9606

We present a perturbation theory that combines the use of a third-order Barker–Henderson expansion of theHelmholtz energy with Mie-potentials that include first (Mie-FH1) and second-order (Mie-FH2) Feynman–Hibbs corrections. The resulting equation of state (SAFT-VRQ Mie) is compared to molecular simulations,and is seen to reproduce the thermodynamic properties of generic Mie-FH1 and Mie-FH2 fluids accurately.SAFT-VRQ Mie is exploited to obtain optimal parameters for the potentials of neon, helium, deuterium,ortho-, para- and normal-hydrogen for the Mie-FH1 and Mie-FH2 formulations. For helium, hydrogen anddeuterium, the use of either the first or second-order corrections yields significantly higher accuracy in therepresentation of supercritical densities, heat capacities and speed of sounds when compared to classical Miefluids, although the Mie-FH2 is slightly more accurate than Mie-FH1 for supercritical properties. The MieFH1 potential is recommended for most of the fluids since it yields a more accurate representation of thepure-component phase equilibria and extrapolates better to low temperatures. Notwithstanding, for helium,where the quantum effects are largest, we find that none of the potentials give an accurate representation ofthe entire phase envelope, and its thermodynamic properties are represented accurately only at temperaturesabove 20 K. Overall, supercritical heat capacities are well represented, with some deviations from experimentsseen in the liquid phase region for helium and hydrogen.

Journal article

Kaimaki D-M, Haire B, Ryan H, Jiménez-Serratos G, Alloway RM, Little M, Morrison J, Salama I, Tillotson M, Smith BE, Moorhouse S, Totton TS, Hodges MG, Yeates SG, Quayle P, Clarke S, Muller EA, Durkan Cet al., 2019, A multi-scale approach linking self-aggregation & surface interactions of synthesised foulants to fouling mitigation strategies, Energy & Fuels, ISSN: 0887-0624

Fouling of oil-exposed surfaces remains a crucial issue as a result of the continued importance of oil as the world’s primary energy source. The key perpetrators in crude oil fouling have been identified as asphaltenes, a poorly described mixture of diverse polyfunctional molecules that form part of the heaviest fractions of oil. Asphaltenes are responsible for a decrease in oil production and energy efficiency and an increase in the risk of environmental hazards. Hence, understanding and managing systems that are prone to fouling is of great value but constitutes a challenge as a result of their complexity. In an effort to reduce that complexity, a study of a synthesized foulant of archipelago structure is presented. A critical perspective on previously described solubility and aggregation mechanisms (e.g., critical nanoaggrerate concentration and critical clustering concentration) is offered because the characterized system favors a continuous distribution of n-mers instead. A battery of experimental and modeling techniques have been employed to link the bulk and interfacial behavior of a representative foulant monomer to effective fouling mitigation strategies. This systematic approach defines a new multiscale methodology in the investigation of fouling systems.

Journal article

Wand CR, Fayaz-Torshizi M, Jimenez-Serratos G, Müller EA, Frenkel Det al., 2019, Solubilities of pyrene in organic solvents: Comparison between chemical potential calculations using a cavity-based method and direct coexistence simulations, The Journal of Chemical Thermodynamics, Vol: 131, Pages: 620-629, ISSN: 0021-9614

In this paper, we benchmark a cavity-based simulation method for calculating the relative solubility of large molecules in explicit solvents. The essence of the procedure is the accounting of the Gibbs energy change associated with an alchemical thermodynamic cycle where, in sequence, a cavity is created in a solvent, a solute is inserted in the cavity and the cavity is annihilated. The Gibbs energy change is equated to the excess chemical potential allowing the comparison of solubilities in different solvents. The results obtained using the cavity-based method are compared to direct large-scale molecular dynamics simulations performed using coarse-grained models for calculating the partition coefficient of pyrene between heptane and toluene. We demonstrate the applicability of this cavity-based technique under high pressure/temperature conditions.

Journal article

Cardenas H, Muller E, 2019, Molecular simulation of the adsorption and diffusion in cylindrical nanopores – effect of shape and fluid-solid interactions, Molecules, Vol: 24, ISSN: 1420-3049

We report on molecular simulations of model fluids composed of three tangentially bonded Lennard-Jones interaction sites with three distinct morphologies: a flexible “pearl-necklace” chain, a rigid “stiff” linear configuration, and an equilateral rigid triangular ring. The adsorption of these three models in cylindrical pores of diameters 1, 2, and 3 nm and with varying solid–fluid strength was determined by direct molecular dynamics simulations, where a sample pore was placed in contact with a bulk fluid. Adsorption isotherms of Type I, V, and H1 were obtained depending on the choice of pore size and solid–fluid strength. Additionally, the bulk-phase equilibria, the nematic order parameter of the adsorbed phase, and the self-diffusion coefficient in the direction of the pore axis were examined. It was found that both the molecular shape and the surface attractions play a decisive role in the shape of the adsorption isotherm. In general, the ring molecules showed a larger adsorption, while the fully flexible model showed the smallest adsorption. Morphology and surface strength were found to have a lesser effect on the diffusion of the molecules. An exceptional high adsorption and diffusion, suggesting an enhanced permeability, was observed for the linear stiff molecules in ultraconfinement, which was ascribed to a phase transition of the adsorbed fluid into a nematic liquid crystal.

Journal article

Jiménez-Serratos G, Totton TS, Jackson G, Muller EAet al., 2019, Aggregation behavior of model asphaltenes revealed from large-scale coarse-grained molecular simulations, Journal of Physical Chemistry B, ISSN: 1520-5207

Fully atomistic simulations of models of asphaltenes in simple solvents have allowed the study of trends in aggregation phenomena and the understanding of the role that molecular structure plays therein. However, the detail included at this scale of molecular modeling is at odds with the required spatial and temporal resolution needed to fully understand the asphaltene aggregation. The computational cost required to explore the relevant scales can be reduced by employing coarse-grained (CG) models, which consist of lumping a few atoms into a single segment that is characterised by effective interac- tions. In this work CG force fields developed via the SAFT-γ [Müller, E.A., Jackson, G. (2014) Annu. Rev. Chem. Biomolec. Eng., 5, 405–427] equation of state (EoS) provide a reliable pathway to link the molecular description with macroscopic thermophysical data. A recent modification of the SAFT-VR EoS [Müller, E.A. and Mejía, A. (2017) Langmuir, 33, 11518–11529], that allows parametrizing homonuclear rings, is selected as the starting point to propose CG models for polycyclic aromatic hydrocarbons (PAHs). The new aromatic-core parameters, along with others published for simpler organic molecules, are adopted for the construction of asphaltene models by combining different chemical moieties in a group-contribution fashion. We apply the procedure to two previously reported asphaltene models and perform Molecular Dynamics simulations to validate the coarse-grained representation against benchmark systems of 27 asphaltenes in pure solvent (toluene or heptane) described in a fully atomistic fashion. An excellent match between both levels of description is observed for cluster size, radii of gyration, and relative-shape-anisotropy-factor distributions. We exploit the advantages of the CG representation by simulating systems containing up to 2000 asphaltene molecules in explicit solvent investigating the effect of asphaltene concentration, so

Journal article

, 2019, Chemical Engineering Research: Reports of the 4th year research projects in the Department of Chemical Engineering at Imperial College London, London, Publisher: Department of Chemical Engineering, Imperial College London, ISBN: 9781916005006

This book is a compilation of manuscripts created as part of a teaching assignment on the 4th year’s Chemical Engineering CE4-01-1 course.

Book

Shahruddin S, Jimenez-Serratos G, Britovsek G, Matar O, Muller Eet al., 2019, Fluid-solid phase transition of n-alkane mixtures: Coarse-grained molecular dynamics simulations and diffusion-ordered spectroscopy nuclear magnetic resonance, Scientific Reports, Vol: 9, ISSN: 2045-2322

Wax appearance temperature (WAT), defined as the temperature at which the first solid paraffin crystal appears in a crude oil, is one of the key flow assurance indicators in the oil industry. Although there are several commonly-used experimental techniques to determine WAT, none provides unambiguous molecular-level information to characterize the phase transition between the homogeneous fluid and the underlying solid phase. Molecular Dynamics (MD) simulations employing the statistical associating fluid theory (SAFT) force field are used to interrogate the incipient solidification states of models for long-chain alkanes cooled from a melt to an arrested state. We monitor the phase change of pure long chain n-alkanes: tetracosane (C24H50) and triacontane (C30H62), and an 8-component surrogate n-alkane mixture (C12-C33) built upon the compositional information of a waxy crude. Comparison to Diffusion Ordered Spectroscopy Nuclear Magnetic Resonance (DOSY NMR) results allows the assessment of the limitations of the coarse-grained models proposed. We show that upon approach to freezing, the heavier components restrict their motion first while the lighter ones retain their mobility and help fluidize the mixture. We further demonstrate that upon sub-cooling of long n-alkane fluids and mixtures, a discontinuity arises in the slope of the self-diffusion coefficient with decreasing temperature, which can be employed as a marker for the appearance of an arrested state commensurate with conventional WAT measurements.

Journal article

Jaeger F, Matar OK, Müller EA, 2019, Transport properties of water confined in a graphene nanochannel

Equilibrium molecular dynamics simulations are used to investigate the effectof phase transitions on the transport properties of highly-confined waterbetween parallel graphene sheets. An abrupt reduction by several orders ofmagnitude in the mobility of water is observed in strong confinement, asindicated by reduced diffusivity and increased shear viscosity values. The bulkviscosity, which is related to the resistance to expansion and compression of asubstance, is also calculated, showing an enhancement compared to the bulkvalue for all levels of confinement. An investigation into the phase behaviourof confined water reveals a transition from a liquid monolayer to a rhombicfrozen monolayer at nanochannel heights between 6.8-7.8 \r{A}; for largerseparations, multilayer liquid water is recovered. It is shown how this phasetransition is at the root of the impeded transport.

Working paper

Joss L, Muller E, 2019, Machine learning for fluid property correlations: Classroom examples with MATLAB, Journal of Chemical Education, ISSN: 0021-9584

Recent advances in computer hardware and algorithms are spawning an explosive growth in the use of computer-based systems aimed at analyzing and ultimately correlating large amounts of experimental and synthetic data. As these machine learning tools become more widespread, it is becoming imperative that scientists and researchers become familiar with them, both in terms of understanding the tools and the current limitations of artificial intelligence, and more importantly being able to critically separate the hype from the real potential. This article presents a classroom exercise aimed at first-year science and engineering college students, where a task is set to produce a correlation to predict the normal boiling point of organic compounds from an unabridged data set of >6000 compounds. The exercise, which is fully documented in terms of the problem statement and the solution, guides the students to initially perform a linear correlation of the boiling point data with a plausible relevant variable (the molecular weight) and to further refine it using multivariate linear fitting employing a second descriptor (the acentric factor). Finally, the data are processed through an artificial neural network to eventually provide an engineering-quality correlation. The problem statements, data files for the development of the exercise, and solutions are provided within a MATLAB environment but are general in nature.

Journal article

Herdes Moreno C, Ervik A, Mejia A, Muller EAet al., 2018, Prediction of the water/oil interfacial tension from molecular simulations using the coarse-grained SAFT-γ Mie force field, Fluid Phase Equilibria, Vol: 476, Pages: 9-15, ISSN: 0378-3812

This work reports the award-winning entry for the Ninth Industrial Fluid Properties Simulation Challenge. This worldwide competition was set with the aim of assessing the capability of molecular simulation methods and force fields to accurately predict the interfacial tension of oil + water mixtures at high temperatures and pressures. The challenge focused on predicting the liquid-liquid interfacial tension of binary mixtures of dodecane + water, toluene + water and a 50:50 (wt%) mixture of dodecane:toluene + water at 1.825 MPa (250 psig) and temperatures from 110 to 170 °C. In our entry for the challenge, we employed coarse-grained intermolecular models parametrized via a top-down technique in which an accurate equation of state is used to link experimentally observed macroscopic properties of fluids with the force-field parameters. The state-of-the-art version of the statistical associating fluid theory (SAFT) for potentials of variable range as reformulated in terms of the Mie potential is employed here. Interfacial tensions are calculated through a direct method, where an elongated simulation cell is sampled through molecular dynamics in the isobaric-isothermal constant area ensemble (NPzzAT). The coarse-grained nature of the force field allows for the accelerated calculation of relatively large systems. The binary interaction parameters that describe the cross-interactions have been obtained in previous works by fitting to interfacial tensions of the constituent binaries at lower pressures and temperatures; these are taken as constant for all conditions and mixtures studied. After disclosure of the challenge results, we observe that the interfacial properties of the mixtures are described with an error of less than 5 mN/m over the whole range of conditions, demonstrating the accuracy and transferability of the top-down SAFT-γ Mie force field approach.

Journal article

cumicheo C, cartes M, Muller EA, Mejia Aet al., 2018, Experimental measurements and theoretical modeling of high-pressure mass densities and interfacial tensions of carbon dioxide + n-heptane + toluene and its carbon dioxide binary systems, Fuel, Vol: 228, Pages: 92-102, ISSN: 0016-2361

Experimental determination and theoretical predictions of the isothermal (344.15 K) mass densities and interfacial tensions for the system carbon dioxide (CO2) with heptol (n-heptane + toluene) mixtures varying liquid volume fraction compositions of toluene (0, 25, 50, 75, 100 % v/v) and over the pressure range 0.1 to 8 MPa are reported. Measurements are carried out on a high-pressure device that includes a vibrating tube densimeter and a pendant drop tensiometer. Theoretical modeling of mass densities phase equilibria and interfacial properties (i.e., interfacial tension and interfacial concentration profiles) are performed by employing the Square Gradient Theory using an extension of the Statistical Associating Fluid Theory equation of state that accounts for ring fluids. The experimental bulk phase equilibrium densities and interfacial tensions obtained are in very good agreement with the theoretical predictions. Although there are no previous experimental data of these mixtures at the conditions explored herein, the results follow the same trends observed from experimental data at other conditions. The combination of experimental and modeling approaches provides a route to simultaneously predict phase equilibrium and interfacial properties within acceptable statistical deviations. For the systems and conditions studied here, we observe that the phase equilibrium of the mixtures display zeotropic vapor-liquid equilibria with positive deviations from ideal behavior. The mass bulk densities behave ordinarily whereas the interfacial tensions decrease as the pressure or liquid mole fraction of CO2 increases and/or the ratio toluene/heptane decreases. The interfacial concentration along the interfacial region exhibits a remarkable high excess adsorption of CO2, which increases with pressure and it is larger in n-heptane than in toluene. Toluene does not exhibit any special ads

Journal article

Galindo A, Rahman S, Lobanova O, Jimenez-Serratos G, Braga C, Raptis V, Muller E, Jackson G, Avendano Cet al., 2018, SAFT‑γ force field for the simulation of molecular fluids. 5. Hetero Group coarse-grained models of linear alkanes and the importance of intramolecular interactions, Journal of Physical Chemistry B, Vol: 122, Pages: 9161-9177, ISSN: 1520-5207

The SAFT-γ Mie group-contribution equation of state [Papaioannou J. Chem. Phys. 2014, 140, 054107] is used to develop a transferable coarse-grained (CG) force-field suitable for the molecular simulation of linear alkanes. A heterogroup model is fashioned at the resolution of three carbon atoms per bead in which different Mie (generalized Lennard-Jones) interactions are used to characterize the terminal (CH3–CH2–CH2−) and middle (−CH2–CH2–CH2−) beads. The force field is developed by combining the SAFT-γ CG top-down approach [Avendaño J. Phys. Chem. B 2011, 115, 11154], using experimental phase-equilibrium data for n-alkanes ranging from n-nonane to n-pentadecane to parametrize the intermolecular (nonbonded) bead–bead interactions, with a bottom-up approach relying on simulations based on the higher resolution TraPPE united-atom (UA) model [Martin; , Siepmann J. Phys. Chem. B 1998, 102, 2569] to establish the intramolecular (bonded) interactions. The transferability of the SAFT-γ CG model is assessed from a detailed examination of the properties of linear alkanes ranging from n-hexane (n-C6H14) to n-octadecane (n-C18H38), including an additional evaluation of the reliability of the description for longer chains such as n-hexacontane (n-C60H122) and a prototypical linear polyethylene of moderate molecular weight (n-C900H1802). A variety of structural, thermodynamic, and transport properties are examined, including the pair distribution functions, vapor–liquid equilibria, interfacial tension, viscosity, and diffusivity. Particular focus is placed on the impact of incorporating intramolecular interactions on the accuracy, transferability, and representability of the CG model. The novel SAFT-γ CG force field is shown to provide a reliable description of the thermophysical properties of the n-alkanes, in most cases at a level comparable to the that obtained with higher resolution models.

Journal article

Herdes C, Petit C, Mejia A, Muller EAet al., 2018, Combined experimental, theoretical and molecular simulation approach for the description of the fluid phase behavior of hydrocarbon mixtures within shale rocks, Energy and Fuels, Vol: 32, Pages: 5750-5762, ISSN: 0887-0624

An experimental, theoretical and molecular simulation consolidated framework for the efficient characterization of the adsorption and fluid phase behaviorof multicomponent hydrocarbon mixtures within tight shale rocks is presented.Fluid molecules are described by means of a top-down coarse-grained modelwheresimple Mie intermolecular potentials areparametrizedby means of the statisticalassociatingfluid theory (SAFT). A four component (methane, pentane, decane, naphthalene) mixture is used a surrogate model with a composition representative of commonly encountered shale oils. Shales are modelledas a hierarchical network of nanoporous slits in contact with a mesoporous region. The rock model is informed by the characterization of four distinctand representative shale core samples through nitrogen adsorption, thermogravimetric analysis and contact angle measurements. Experimental results suggest the consideration of two types of pore surfaces; a carbonaceous wall representing the kerogen regions of a shale rock and an oxygenated wall representing the clay-based porosity.Molecular dynamics (MD)simulations are performed at constant overall compositionsat a temperature of 398.15 K(257 °F) and explorepressures from6.9 MPa up to 68.95 MPa (1000to 10000 psi).Simulationsrevealthat it is the organic nanopores of 1 nm and 2 nm that preferentially adsorb the heavier components, while the oxygenated counterparts show little selectivitybetween the adsorbed and free fluid.Upon desorption, this trend is intensified, as the gas phase in equilibrium with a carbon nanopore becomes increasing leaner (richerin light components)and almost completely depleted of the heavy components which remain trapped in the nanopores and surfaces of the mesopores.Oxygenated pores do not contribute tothis unusualbehavior, even for the very tight pores considered. The results presentedelucidate

Journal article

Jaeger F, Matar OK, Muller EA, 2018, Bulk viscosity of molecular fluids, Journal of Chemical Physics, Vol: 148, ISSN: 0021-9606

The bulk viscosity of molecular models of gases and liquids is determined by molecular simulations as acombination of a dilute gas contribution, arising due to the relaxation of internal degrees of freedom, and aconfigurational contribution, due to the presence of intermolecular interactions. The dilute gas contributionis evaluated using experimental data for the relaxation times of vibrational and rotational degrees of freedom.The configurational part is calculated using Green-Kubo relations for the fluctuations of the pressure tensorobtained from equilibrium microcanonical molecular dynamics simulations. As a benchmark, the Lennard-Jones fluid is studied. Both atomistic and coarse-grained force fields for water, CO2and n-decane areconsidered and tested for their accuracy. Comparison to experimental data, where present, demonstratesthat the tested models show various degrees of success in predicting bulk viscosity values, although atomisticforce fields in general seem to perform more consistently than the corresponding coarse-grained counterparts.The dilute gas contribution to the bulk viscosity is seen to be significant only in the cases when intramolecularrelaxation times are in theμs range, and for low vibrational wave numbers (<1000 cm−1); This explainsthe abnormally high values of bulk viscosity reported for CO2. In all other cases studied, the dilute gascontribution is negligible, and the configurational contribution dominates the overall behaviour. In particular,the configurational term is responsible for the enhancement of the bulk viscosity near the critical point.

Journal article

Wu L, Malijevsky A, Avendano C, Muller E, Jackson Get al., 2018, Demixing, surface nematization, and competing adsorption in binarymixtures of hard rods and hard spheres under confinement, Journal of Chemical Physics, Vol: 148, ISSN: 0021-9606

A molecular simulation study of binary mixtures of hard spherocylinders (HSCs) and hard spheres (HSs)confined between two structureless hard walls is presented. The principal aim of the work is to understandthe effect of the presence of hard spheres on the entropically-driven surface nematization of the hard rod-likeparticles at the walls. The mixtures are studied using a constant normal-pressure Monte Carlo algorithm.The surface adsorption at different compositions of hard spheres is examined in detail. At moderate hard-sphere concentrations preferential adsorption of the spheres at the wall is found. However, at moderate tohigh pressure (density), we observe a crossover in the adsorption behaviour with nematic layers of the rodsforming at the walls leading to a local demixing of the system. The presence of the spherical particles is seento destabilize the surface nematization of the rods, and the degree of demixing increases on increasing the HSconcentration.

Journal article

Britovsek G, Tomov A, Muller E, Matar O, Shahruddin S, Young Cet al., 2018, Ethylene oligomerisation beyond Schulz-Flory distributions, 255th National Meeting and Exposition of the American-Chemical-Society (ACS) - Nexus of Food, Energy, and Water, Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727

Conference paper

Shahruddin S, Jimenez-Serratos G, Britovsek G, Matar O, Muller Eet al., 2018, Exploring wax precipitation in crude oils through diffusion ordered spectroscopy nuclear magnetic resonance combined with molecular dynamics simulations, 255th National Meeting and Exposition of the American-Chemical-Society (ACS) - Nexus of Food, Energy, and Water, Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727

Conference paper

Muller EA, Mejia A, 2017, Extension of the SAFT-VR Mie EoS to model homonuclear rings and its parameterization based on the principle of corresponding states, Langmuir, Vol: 33, Pages: 11518-11529, ISSN: 1520-5827

The statistical associating fluid theory of variable range employing a Mie potential (SAFT-VR-Mie) proposed by Lafitte et al. (J. Chem Phys. 2013, 139, 154504) is one of the latest versions of the SAFT family. This particular version has been shown to have a remarkable capability to connect experimental determinations, theoretical calculations, and molecular simulations results. However, the theoretical development restricts the model to chains of beads connected in a linear fashion. In this work, the capabilities of the SAFT-VR Mie equation of state for modeling phase equilibria are extended for the case of planar ring compounds. This modification proposed replaces the Helmholtz energy of chain formation by an empirical contribution based on a parallelism to the second-order thermodynamic perturbation theory for hard sphere trimers. The proposed expression is given in terms of an extra parameter, χ, that depends on the number of beads, ms, and the geometry of the ring. The model is used to describe the phase equilibrium for planar ring compounds formed of Mie isotropic segments for the cases of ms equals to 3, 4, 5 (two configurations), and 7 (two configurations). The resulting molecular model is further parametrized, invoking a corresponding states principle resulting in sets of parameters that can be used indistinctively in theoretical calculations or in molecular simulations without any further refinements. The extent and performance of the methodology has been exemplified by predicting the phase equilibria and vapor pressure curves for aromatic hydrocarbons (benzene, hexafluorobenzene, toluene), heterocyclic molecules (2,5-dimethylfuran, sulfolane, tetrahydro-2H-pyran, tetrahydrofuran), and polycyclic aromatic hydrocarbons (naphthalene, pyrene, anthracene, pentacene, and coronene). An important aspect of the theory is that the parameters of the model can be used directly in molecular dynamics (MD) simulations to calculate equilibrium phase properties and in

Journal article

Theodorakis PE, Muller EA, Craster RV, Matar OKet al., 2017, Physical insights into the blood-brain barrier translocation mechanisms, Physical Biology, Vol: 14, ISSN: 1478-3975

The number of individuals suffering from diseases of the central nervous system (CNS) is growing with an aging population. While candidate drugs for many of these diseases are available, most of these pharmaceutical agents cannot reach the brain rendering most of the drug therapies that target the CNS inefficient. The reason is the blood–brain barrier (BBB), a complex and dynamic interface that controls the influx and efflux of substances through a number of different translocation mechanisms. Here, we present these mechanisms providing, also, the necessary background related to the morphology and various characteristics of the BBB. Moreover, we discuss various numerical and simulation approaches used to study the BBB, and possible future directions based on multi-scale methods. We anticipate that this review will motivate multi-disciplinary research on the BBB aiming at the design of effective drug therapies.

Journal article

Jimenez-serratos M, Herdes C, Haslam A, Jackson G, Muller EAet al., 2017, Group-contribution coarse-grained molecular simulations of polystyrene melts and polystyrene solutions in alkanes using the SAFT-γ force field, Macromolecules, Vol: 50, Pages: 4840-4853, ISSN: 0024-9297

A coarse-grained (CG) model for atactic polystyrene is presented and studied with classical molecular-dynamics simulations. The interactions between the CG segments are described by Mie potentials, with parameters obtained from a top-down approach using the SAFT-γ methodology. The model is developed by taking a CG model for linear-chain-like backbones with parameters corresponding to those of an alkane and decorating it with side branches with parameters from a force field of toluene, which incorporate an “aromatic-like” nature. The model is validated by comparison with the properties of monodisperse melts, including the effect of temperature and pressure on density, as well as structural properties (the radius of gyration and end-to-end distance as functions of chain length). The model is employed within large-scale simulations that describe the temperature–composition fluid-phase behavior of binary mixtures of polystyrene in n-hexane and n-heptane. A single temperature-independent unlike interaction energy parameter is employed for each solvent to reproduce experimental solubility behavior; this is sufficient for the quantitative prediction of both upper and lower critical solution points and the transition to the characteristic “hourglass” phase behavior for these systems.

Journal article

Berreda D, Pérez-Mas AM, Casco ME, Rudić S, Herdes C, Muller EA, Blanco C, Santamaria R, Silvestre-Albero J, Rodriguez-Reinoso Fet al., 2017, Unusual flexibility of mesophase pitch-derived carbon materials: An approach to the synthesis of graphene, Carbon, Vol: 115, Pages: 539-545, ISSN: 0008-6223

Structural flexibility in a petroleum pitch-derived carbon material has been indirectly evaluated using X-ray diffraction (XRD), immersion calorimetry and inelastic neutron scattering (INS) measurements. Exposure of the carbon material to an organic solvent (e.g., n-nonane) gives rise to a large internal rearrangement, associated with a drastic re-ordering of the graphitic microdomains. These structural changes are also associated with a high flexibility of the internal porous network, as observed by inelastic neutron scattering measurements. The internal rearrangement and the structural flexibility could be responsible for the excellent performance of this kind of activated carbons in a wide variety of adsorption processes. Last but not least, the structural characteristics of these carbon materials composed of graphitic microdomains has been used to synthesize graphene “egg-like” flakes following a simple procedure based on exfoliation with organic solvents.

Journal article

Headen T, Boek E, Jackson G, Totton T, Muller EAet al., 2017, Simulation of asphaltene aggregation through molecular dynamics: insights and limitations, Energy & Fuels, Vol: 31, Pages: 1108-1125, ISSN: 1520-5029

We report classical atomistic molecular dynamics simulations of four structurally diverse model asphaltenes, a model resin,and their respective mixtures in toluene or heptane at ambient conditions. Relatively large systems (~50,000 atoms) and long timescales(> 80 ns)are explored. Whereever possible,comparisons are madeto available experimental observations asserting the validity of the models. When the asphaltenes are dissolved in toluene, a continuous distribution of cluster sizesis observed with average aggregation number ranging between 3.6and 5.6,monomers and dimers being thepredominantspecies. As expected for mixtures in heptane the asphaltene molecules tend to aggregate to form a segregated phase. There is no evidence of a distinct formation of nanoaggregates, the distributions of clusters is found to becontinuous in character.The analysis of the shape of the clusters of asphaltenes suggests that they are generally spherical incharacter, with the archipelago models favouring longer prolate structuresand the continental modeltending towards oblate structures. The aggregates areseen to bediffuse in nature, containing at least 50% solventon average, being denser in heptane than in toluene. Mixtures of asphaltenes with different architectureare found to have cluster properties that are intermediate between those of the individual components. The presence of resins in the mixture does not appear to alter the shape of the asphaltene aggregates, their size or density when toluene is the solvent; on the otherhand theresins lead to an increase in the density of the resulting aggregatesin heptane. Quantification of these observations is made from the histograms of cluster distributions, the potential of mean force calculations,and an analysis of the shape factors. We illustrate howthe time scales for complete aggregationof molecules in heptanearelarger t

Journal article

Smith ER, Müller EA, Craster RV, Matar OKet al., 2016, A Langevin model for fluctuating contact angle behaviour parametrised using molecular dynamics, Soft Matter, Vol: 12, Pages: 9604-9615, ISSN: 1744-6848

Molecular dynamics simulations are employed to develop a theoretical model to predict the fluid-solid contact angle as a function of wall-sliding speed incorporating thermal fluctuations. A liquid bridge between counter-sliding walls is studied, with liquid-vapour interface-tracking, to explore the impact of wall-sliding speed on contact angle. The behaviour of the macroscopic contact angle varies linearly over a range of capillary numbers beyond which the liquid bridge pinches off, a behaviour supported by experimental results. Nonetheless, the liquid bridge provides an ideal test case to study molecular scale thermal fluctuations, which are shown to be well described by Gaussian distributions. A Langevin model for contact angle is parametrised to incorporate the mean, fluctuation and auto-correlations over a range of sliding speeds and temperatures. The resulting equations can be used as a proxy for the fully-detailed molecular dynamics simulation allowing them to be integrated within a continuum-scale solver.

Journal article

Muscatello J, Muller EA, Mostofi AA, Sutton Aet al., 2016, Multiscale molecular simulations of the formation and structure of polyamide membranes created by interfacial polymerization, Journal of Membrane Science, Vol: 527, Pages: 180-190, ISSN: 0376-7388

Large scale molecular simulations to model the formation of polyamide membranes have been carried out using a procedure that mimics experimental interfacial polymerization of trimesoyl chloride (TMC) and metaphenylene diamine (MPD) monomers. A coarse-grained representation of the monomers has been developed to facilitate these simulations, which captures essential features of the stereochemistry of the monomers and of amide bonding between them. Atomic models of the membranes are recreated from the final coarse-grained representations. Consistent with earlier treatments, membranes are formed through the growth and aggregation of oligomer clusters. The membranes are inhomogeneous, displaying opposing gradients of trapped carboxyl and amine side groups, local density variations, and regions where the density of amide bonding is reduced as a result of the aggregation process. We observe the interfacial polymerization reaction is self-limiting and the simulated membranes display a thickness of 5–10 nm. They also display a surface roughness of 1–4 nm. Comparisons are made with recently published experimental results on the structure and chemistry of these membranes and some interesting similarities and differences are found.

Journal article

Ervik A, Lysgaard MO, Herdes C, Jimenez-Serratos G, Mueller EA, Munkejord ST, Mueller Bet al., 2016, A multiscale method for simulating fluid interfaces covered with large molecules such as asphaltenes, Journal of Computational Physics, Vol: 327, Pages: 576-611, ISSN: 0021-9991

The interface between two liquids is fully described by the interfacial tension only for very pure liquids. In most cases the system also contains surfactant molecules which modify the interfacial tension according to their concentration at the interface. This has been widely studied over the years, and interesting phenomena arise, e.g. the Marangoni effect. An even more complicated situation arises for complex fluids like crude oil, where large molecules such as asphaltenes migrate to the interface and give rise to further phenomena not seen in surfactant-contaminated systems. An example of this is the “crumpling drop” experiments, where the interface of a drop being deflated becomes non-smooth at some point. In this paper we report on the development of a multiscale method for simulating such complex liquid–liquid systems. We consider simulations where water drops covered with asphaltenes are deflated, and reproduce the crumpling observed in experiments. The method on the nanoscale is based on using coarse-grained molecular dynamics simulations of the interface, with an accurate model for the asphaltene molecules. This enables the calculation of interfacial properties. These properties are then used in the macroscale simulation, which is performed with a two-phase incompressible flow solver using a novel hybrid level-set/ghost-fluid/immersed-boundary method for taking the complex interface behaviour into account. We validate both the nano- and macroscale methods. Results are presented from nano- and macroscale simulations which showcase some of the interesting behaviour caused by asphaltenes affecting the interface. The molecular simulations presented here are the first in the literature to obtain the correct interfacial orientation of asphaltenes. Results from the macroscale simulations present a new physical explanation of the crumpled drop phenomenon, while highlighting shortcomings in previous hypotheses.

Journal article

Ervik AS, Serratos GJ, Müller EA, 2016, raaSAFT: A framework enabling coarse-grained molecular dynamics simulations based on the SAFT-γ Mie force field, Computer Physics Communications, Vol: 212, Pages: 161-179, ISSN: 0010-4655

We describe here raaSAFT, a Python code that enables the setup and running of coarse-grained molecular dynamics simulations in a systematic and efficient manner. The code is built on top of the popular HOOMD-blue code, and as such harnesses the computational power of GPUs. The methodology makes use of the SAFT-γ Mie force field, so the resulting coarse grained pair potentials are both closely linked to and consistent with the macroscopic thermodynamic properties of the simulated fluid. In raaSAFT both homonuclear and heteronuclear models are implemented for a wide range of compounds spanning from linear alkanes, to more complicated fluids such as water and alcohols, all the way up to nonionic surfactants and models of asphaltenes and resins. Adding new compounds as well as new features is made straightforward by the modularity of the code. To demonstrate the ease-of-use of raaSAFT, we give a detailed walkthrough of how to simulate liquid–liquid equilibrium of a hydrocarbon with water. We describe in detail how both homonuclear and heteronuclear compounds are implemented. To demonstrate the performance and versatility of raaSAFT, we simulate a large polymer-solvent mixture with 300 polystyrene molecules dissolved in 42 700 molecules of heptane, reproducing the experimentally observed temperature-dependent solubility of polystyrene. For this case we obtain a speedup of more than three orders of magnitude as compared to atomistically-detailed simulations.

Journal article

Morgado P, Lobanova O, Almedia M, Muller EA, Jackson G, Filipe Eet al., 2016, SAFT-γ force field for the simulation of molecular fluids: 8. hetero-group coarse-grained models of perfluoroalkylalkanes assessed with new vapour-liquid interfacial tension data, Molecular Physics, Vol: 114, Pages: 2597-2614, ISSN: 1362-3028

The air-liquid interfacial behaviour of linear perfluoroalkylalkanes (PFAAs) is reportedthrough a combined experimental and computer simulation study. The surfacetensions of seven liquid PFAAs (perfluorobutylethane, F4H2; perfluorobutylpentane,F4H5; perfluorobutylhexane, F4H6, perfluorobutyloctane, F4H8; perfluorohexylethane,F6H2; perfluorohexylhexane, F6H6; and perfluorohexyloctane, F6H8) are experimentallydetermined over a wide temperature range (276 to 350 K). The corresponding surfacethermodynamic properties and the critical temperatures of the studied compounds areestimated from the temperature dependence of the surface tension. Experimentaldensity and vapour pressure data are employed to parameterize a genericheteronuclear coarse-grained intermolecular potential of the SAFT- γ family for PFAAs.The resulting force field is used in direct molecular dynamics simulations to predictwith quantitative agreement the experimental tensions and to explore theconformations of the molecules in the interfacial region revealing a preferentialalignment of the PFAA molecules towards the interface and an enrichment of theperfluoro-groups at the outer interface region.

Journal article

Ervik A, Mejia A, Muller EA, 2016, Bottled SAFT: a web app providing SAFT-γ Mie force field parameters for thousands of molecular fluids, Journal of Chemical Information and Modeling, Vol: 56, Pages: 1609-1614, ISSN: 1549-960X

Coarse-grained molecular simulation has become a popular tool for modelling simpleand complex fluids alike. The defining aspects of a coarse grained model are theforce field parameters, which must be determined for each particular fluid. Since thenumber of molecular fluids of interest in nature and in engineering processes is immense,constructing force field parameter tables by individually fitting to experimental data isa futile task. A step towards solving this challenge was taken recently by Mejia et al.,who proposed a correlation that provides SAFT-γ Mie force field parameters for a fluidprovided one knows the critical temperature, the acentric factor and a liquid density,all relatively accesible properties. Building on this, we have applied the correlationto more than 6000 fluids, and constructed a web application, called “Bottled SAFT”which makes this data set easily searchable by CAS number, name or chemical formula. Alternatively, the application allows the user to calculate parameters for componentsnot present in the database. Once the intermolecular potential has been found throughBottled SAFT, code snippets are provided for simulating the desired substance usingthe “raaSAFT” framework, which leverages established molecular dynamics codes torun the simulations. The code underlying the web application is written in Pythonusing the Flask microframework; this allows us to provide a modern high-performanceweb app while also making use of the scientific libraries available in Python. BottledSAFT aims at taking the complexity out of obtaining force field parameters for a widerange of molecular fluids, and facilitates setting up and running coarse-grained molecularsimulations. The web application is freely available at http://www.bottledsaft.org.The underlying source code is available on Bitbucket under a permissive license.

Journal article

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