Publications
216 results found
Britovsek G, Tomov A, Muller E, et 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
Shahruddin S, Jimenez-Serratos G, Britovsek G, et 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
Jimenez-serratos M, Herdes C, Haslam A, et 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.
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
Theodorakis PE, Muller EA, Craster RV, et 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.
Berreda D, Pérez-Mas AM, Casco ME, et 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.
Headen T, Boek E, Jackson G, et 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
Smith ER, Müller EA, Craster RV, et 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.
Muscatello J, Muller EA, Mostofi AA, et 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.
Ervik A, Lysgaard MO, Herdes C, et 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.
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.
Morgado P, Lobanova O, Almedia M, et 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.
Muller EA, Jackson G, Avendaño C, et al., 2016, Assembly of porous smectic structures formed from interlocking high-symmetry planar nanorings, Proceedings of the National Academy of Sciences of the United States of America, Vol: 113, Pages: 9699-9703, ISSN: 0027-8424
Materials comprising porous structures, often in the form of interconnected concave cavities, are typically assembled from convex molecular building blocks. The use of nanoparticles with a characteristic non-convex shape provide a promising strategy to create new porous materials, an approach that has been recently employed with cage-like molecules to form remarkable liquids with “scrabbled” porous cavities [Giri, N. et al. (2015) Nature 527:216]. Nonconvex mesogenic building blocks can be engineered to form unique self-assembled open structures with tunable porosity and long-range order that is intermediate between that of isotropic liquids and of crystalline solids. Here we propose the design of highly open liquid-crystalline structures from rigid nanorings with unique classes of geometry. By exploiting the entropic ordering characteristics of athermal colloidal particles [Allen, M. P., Evans, G. T., Frenkel, D., Mulder, B. (1993) Adv. Chem. Phys.86:1], we demonstrate that high-symmetry nonconvex rings with large internal cavities interlock within a two-dimensional layered structure leading to the formation of distinctive liquid-crystalline smectic phases. We show that these novel smectic phases possess uniquely high free volumes of up to∼95%, a value significantly larger than the 50% that is typically achievable with smectic phases formed by more conventional convex rodor disc-like mesogenic particles.
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.
Oyewunmi OA, Kirmse CJW, Haslam AJ, et al., 2016, Working-fluid selection and performance investigation of a two-phase single-reciprocating-piston heat-conversion engine, Applied Energy, Vol: 186, Pages: 376-395, ISSN: 0306-2619
We employ a validated first-order lumped dynamic model of the Up-THERM converter, a two-phase unsteadyheat-engine that belongs to a class of innovative devices known as thermofluidic oscillators, which containfewer moving parts than conventional engines and represent an attractive alternative for remote or off-gridpower generation as well as waste-heat recovery. We investigate the performance the Up-THERM withrespect to working-fluid selection for its prospective applications. An examination of relevant working-fluidthermodynamic properties reveals that the saturation pressure and vapour-phase density of the fluid play importantroles in determining the performance of the Up-THERM – the device delivers a higher power outputat high saturation pressures and has higher exergy efficiencies at low vapour-phase densities. Furthermore,working fluids with low critical temperatures, high critical pressures and exhibiting high values of reducedpressures and temperatures result in designs with high power outputs. For a nominal Up-THERM designcorresponding to a target application with a heat-source temperature of 360 ◦C, water is compared withforty-five other pure working fluids. When maximizing the power output, R113 is identified as the optimalfluid, followed by i-hexane. Fluids such as siloxanes and heavier hydrocarbons are found to maximize theexergy and thermal efficiencies. The ability of the Up-THERM to convert heat over a range of heat-sourcetemperatures is also investigated, and it is found that the device can deliver in excess of 10 kW when utilizingthermal energy at temperatures above 200 ◦C. Of all the working fluids considered here, ammonia, R245ca,R32, propene and butane feature prominently as optimal and versatile fluids delivering high power over awide range of heat-source temperatures.
Muller EA, Matar OK, Jaeger F, et al., 2016, Optimising water transport through graphene-based membranes: Insights from non-equilibrium molecular dynamics, ACS Applied Materials & Interfaces, Vol: 8, Pages: 12330-12336, ISSN: 1944-8244
Recent experimental results suggest that stacked layers of graphene oxide exhibitstrong selective permeability to water. To construe this observation the transportmechanism of water permeating through a membrane consisting of layered graphenesheets is investigated via non-equilibrium and equilibrium molecular dynamics simulations.The effect of sheet geometry is studied by changing the offset between theentrance and exit slits of the membrane. The simulation results reveal that the permeabilityis not solely dominated by entrance effects; the path traversed by watermolecules has a considerable impact on the permeability. We show that contrary tospeculation in the literature, water molecules do not pass through the membrane as ahydrogen-bonded chain; instead, they form well-mixed fluid regions confined betweenthe graphene sheets. The results of the present work are used to provide guidelinesfor the development of graphene and graphene oxide membranes for desalination andsolvent separation.
Mejia A, Garrido JM, Piñeiro MM, et al., 2016, Interfacial tensions of industrial fluids from a molecular-based square gradient theory, AICHE Journal, Vol: 62, Pages: 1781-1794, ISSN: 0001-1541
This work reports a procedure for predicting the interfacial tension of pure fluids. It is basedon scaling arguments applied to the influence parameter of the van der Waals theory of inhomogeneousfluids. The molecular model stems from the application of the Square Gradient Theory tothe SAFT-VR Mie equation of state. The theory is validated against computer simulation resultsfor homonuclear pearl-necklace linear chains made up to six Mie (λ−6) beads with repulsive exponentsspanning from λ = 8 to 44 by combining the theory with a corresponding states correlationto determine the intermolecular potential parameters. We provide a predictive tool to determineinterfacial tensions for a wide range of molecules including hydrocarbons, fluorocarbons, polarmolecules, among others. The proposed methodology is tested against comparable existing correlationsin the literature, proving to be vastly superior, exhibiting an average absolute deviation of2.2 %.
Muller EA, jackson G, Muscatello J, et al., 2016, Nonequilibrium study of the intrinsic free-energy profile across a liquid-vapour interface, Journal of Chemical Physics, Vol: 144, ISSN: 1089-7690
We calculate an atomistically detailed free-energy profile across a heterogeneous system using anonequilibrium approach. The path-integral formulation of Crooks fluctuation theorem is used inconjunction with the intrinsic sampling method (ISM) to calculate the free-energy profile for theliquid-vapour interface of the Lennard-Jones fluid. Free-energy barriers are found correspondingto the atomic layering in the liquid phase as well as a barrier associated with the presence of anadsorbed layer as revealed by the intrinsic density profile. Our findings are in agreement withprofiles calculated using Widom’s potential distribution theorem applied to both the average andthe intrinsic profiles as well as literature values for the excess chemical potential.
Muller EA, jackson G, Forte E, et al., 2016, Predicting the adsorption of n-perfluorohexane in BAM P109 standard activated carbon by molecular simulation using SAFT-c Mie coarse-grained force fields, Adsorption Science & Technology, Vol: 34, Pages: 64-78, ISSN: 0263-6174
This work is framed within the 8th International Fluid Properties Simulation Challenge(IFPSC), with the aim of assessing the capability of molecular simulation methodsand force fields to accurately predict adsorption in porous media for systems of relevantpractical interest. The current challenge focuses on predicting adsorption isotherms ofn-perfluorohexane in the certified reference material BAM P109 standard activated carbon.A temperature of T = 273 K and pressures of p/p0 = 0.1, 0.3, and 0.6 relative tothe bulk saturation pressure p0 (as predicted by the model) are the conditions selectedin this challenge. In our methodology we use coarse-grained (CG) intermolecular modelsand a top-down technique where an accurate equation of state (EoS) is used to link theexperimental macroscopic properties of a fluid to the force-field parameters. The stateof-the-artversion of the statistical associating fluid theory for potentials of variable rangeas reformulated in the Mie group contribution incarnation (SAFT-γ Mie) is employedhere. The parameters of the SAFT-γ Mie force field are estimated directly from thevapour pressure and saturated liquid density data of the pure fluids using the EoS, andfurther validated by molecular dynamic simulations. The coarse-grained intermolecularpotential models are then used to obtain the adsorption isotherm kernels for argon, carbondioxide, and n-perfluorohexane in graphite slit pores of various widths using GrandCanonical Monte Carlo (GCMC) simulations. A unique and fluid-independent pore sizedistribution (PSD) curve with total micropore volume of 0.5802 cm3/g is proposed for theBAM P109. The PSD is obtained by applying a non-linear regression procedure over theadsorption integral equation to minimise the quadratic error between the available ex-perimental adsorption isotherms for argon and carbon dioxide and purpose-built GCMCkernels.The predicted adsorption levels of n-perfluorohexane at 273K in BAM P109 are72.75±0.01, 7
Jackson G, Lau GV, Muller EA, et al., 2015, Water droplet excess free energy determined by cluster mitosis using guidedmolecular dynamics, Journal of Chemical Physics, Vol: 143, ISSN: 1089-7690
Atmospheric aerosols play a vital role in affecting climate by influencing the properties and lifetimes of clouds and precipitation. Understanding the underlying microscopic mechanisms involved in the nucleation of aerosol droplets from the vapour phase is therefore of great interest. One key thermodynamic quantity in nucleation is the excess free energy of cluster formation relative to that of the saturated vapour. In our current study, the excess free energy is extracted for clusters of pure water modelled with the TIP4P/2005 intermolecular potential using a method based on nonequilibrium molecular dynamics and the Jarzynski relation. The change in free energy associated with the “mitosis” or division of a cluster of N water molecules into two N/2 sub-clusters is evaluated. This methodology is an extension of the disassembly procedure used recently to calculate the excess free energy of argon clusters [H. Y. Tang and I. J. Ford, Phys. Rev. E 91, 023308 (2015)]. Our findings are compared to the corresponding excess free energies obtained from classical nucleation theory (CNT) as well as internally consistent classical theory (ICCT). The values of the excess free energy that we obtain with the mitosis method are consistent with CNT for large cluster sizes but for the smallest clusters, the results tend towards ICCT; for intermediate sized clusters, we obtain values between the ICCT and CNT predictions. Furthermore, the curvature-dependent surface tension which can be obtained by regarding the clusters as spherical droplets of bulk density is found to be a monotonically increasing function of cluster size for the studied range. The data are compared to other values reported in the literature, agreeing qualitatively with some but disagreeing with the values determined by Joswiak et al. [J. Phys. Chem. Lett. 4, 4267 (2013)] using a biased mitosis approach; an assessment of the differences is the main motivation for our current study.
Theodorakis PE, Müller EA, Craster RV, et al., 2015, Modelling the superspreading of surfactant-laden droplets with computer simulation, Soft Matter, Vol: 11, Pages: 9254-9261, ISSN: 1744-683X
The surfactant-driven superspreading of droplets on hydrophobic substrates is considered. A key element of the superspreading mechanism is the adsorption of surfactant molecules from the liquid-vapour interface onto the substrate through the contact line, which must be coordinated with the replenishment of interfaces with surfactant from the interior of the droplet. We use molecular dynamics simulations with coarse-grained force fields to provide a detailed structural description of the droplet shape and surfactant dynamics during the superspreading process. We also provide a simple method for accurate estimation of the contact angle subtended by the droplets at the contact line.
Wu L, Malijevsky A, Jackson G, et al., 2015, Orientational ordering and phase behaviour of binary mixtures of hard spheres and hard spherocylinders (vol 143, 044906, 2015), Journal of Chemical Physics, Vol: 143, ISSN: 1089-7690
Muller EA, Jackson G, Lobanova O, et al., 2015, SAFT-γ Force Field for the Simulation of Molecular Fluids 6. Binary and ternary mixtures comprising water, carbon dioxide, and n-alkanes, The Journal of Chemical Thermodynamics, Vol: 93, Pages: 320-336, ISSN: 0021-9614
The SAFT-γ coarse graining methodology [E. A. Muller and G. Jackson, Ann. Rev. Chem. Biomol. Eng. ¨ 5, 405(2014)] is used to develop force fields for the fluid-phase behaviour of binary and ternary mixtures comprising water,carbon dioxide, and n-alkanes. The effective intermolecular interactions between the coarse grained (CG) segmentsare directly related to macroscopic thermodynamic properties by means of the SAFT-γ equation of state for molecularsegments represented with the Mie (generalized Lennard-Jones) intermolecular potential [V. Papaioannou, T. Lafitte,C. Avendano, C. S. Adjiman, G. Jackson, E. A. M ˜ uller, and A. Galindo, J. Chem. Phys. ¨ 140, 054107 (2014)].The unlike attractive interactions between the components of the mixtures are represented with a single adjustableparameter, which is shown to be transferable over a wide range of conditions. The SAFT-γ Mie CG force fieldsare used in molecular-dynamics simulations to predict the challenging vapour-liquid and liquid-liquid fluid-phaseequilibria characterising these mixtures, and to study properties that are not accessible directly from the equation ofstate, such as the interfacial properties. The description of the fluid-phase equilibria and interfacial properties predictedwith the SAFT-γ Mie force fields is in excellent with the corresponding experimental data, and of comparable if notsuperior quality to that reported for the more sophisticated atomistic or united-atom models.
Jover J, Galindo A, Jackson G, et al., 2015, Fluid-fluid coexistence in an athermal colloid-polymer mixture: thermodynamic perturbation theory and continuum molecular-dynamics simulation, MOLECULAR PHYSICS, Vol: 113, Pages: 2608-2628, ISSN: 0026-8976
Jover J, Galindo A, Jackson G, et al., 2015, Fluid-fluid coexistence in an athermal colloid-polymer mixture: thermodynamic perturbation theory and continuum molecular-dynamics simulation (vol 113, pg 2608, 2015), MOLECULAR PHYSICS, Vol: 113, ISSN: 0026-8976
Jackson G, Wu L, Avendano C, et al., 2015, Orientational ordering and phase behaviour of binary mixtures of hardspheres and hard spherocylinders, The Journal of Chemical Physics, Vol: 143, ISSN: 0021-9606
We study structure and fluid-phase behaviour of a binary mixture of hard spheres(HSs) and hard spherocylinders (HSCs) in isotropic and nematic states using theNPnAT ensemble Monte Carlo (MC) method in which a normal pressure tensorcomponent is fixed in a system confined between two hard walls. The method allowsone to estimate the location of the isotropic-nematic phase transition and to observethe asymmetry in the composition between the coexisting phases, with the expectedincrease of the HSC concentration in the nematic phase. This is in stark contrastwith the previously reported MC simulations where a conventional isotropic NP Tensemble was used. We further compare the simulation results with the theoreticalpredictions of two analytic theories that extend the original Parsons-Lee theory usingthe one-fluid and the many-fluid approximation [Malijevsk´y at al J. Chem. Phys.129, 144504 (2008)]. In the one-fluid version of the theory the properties of themixture are mapped on an effective one-component HS system while in the many-fluid theory the components of the mixtures are represented as separate effective HSparticles. The comparison reveals that both the one- and the many-fluid approachesprovide a reasonably accurate quantitative description of the mixture including thepredictions of the isotropic-nematic phase boundary and degree of orientational orderof the HSC-HS mixtures.
Muller EA, totton TS, Herdes C, 2015, Coarse grained force field for the molecular simulation of natural gases and condensates, Fluid Phase Equilibria, Vol: 406, Pages: 91-100, ISSN: 0378-3812
The atomistically-detailed molecular modelling of petroleum fluids is challenging, amongst other aspects, due to the very diverse multicomponent and asymmetric nature of the mixtures in question. Complicating matters further, the time scales for many important processes can be much larger than the current and foreseeable capacity of modern computers running fully-atomistic models. To overcome these limitations, a coarse grained (CG) model is proposed where some of the less-important degrees of freedom are safely integrated out, leaving as key parameters the average energy levels, the molecular conformations and the range of the Mie intermolecular potentials employed as the basis of the model. The parametrization is performed by using an analytical equation of state of the statistical associating fluid theory (SAFT) family to link the potential parameters to macroscopically observed thermophysical properties. The parameters found through this top-down approach are used directly in molecular dynamics simulations of multi-component multi-phase systems. The procedure is exemplified by calculating the phase envelope of the methane–decane binary and of two synthetic light condensate mixtures. A methodology based on the discrete expansion of a mixture is used to determine the bubble points of these latter mixtures, with an excellent agreement to experimental data. The model presented is entirely predictive and an abridged table of parameters for some fluids of interest is provided.
Theodorakis P, Kovalchuk NM, Starov VM, et al., 2015, Superspreading: Mechanisms and Molecular Design, Mainz Material Simulation Days 2015, Pages: 29-29
Ramrattan NS, Avendaño C, Mueller EA, et al., 2015, A corresponding-states framework for the description of the Mie family of intermolecular potentials, Molecular Physics: An International Journal at the Interface Between Chemistry and Physics, Vol: 113, Pages: 932-947, ISSN: 0026-8976
The Mie (λr, λa) intermolecular pair potential has been suggested as an alternative to the traditional Lennard–Jones (12–6) potential for modelling real systems both via simulation and theory as its implementation leads to an accuracy and flexibility in the determination of thermophysical properties that cannot be obtained when potentials of fixed range are considered. An additional advantage of using variable-range potentials is noted in the development of coarse-grained models where, as the superatoms become larger, the effective potentials are seen to become softer. However, the larger number of parameters that characterise the Mie potential (λr, λa, σ, ϵ) can hinder a rational study of the particular effects that each individual parameter have on the observed thermodynamic properties and phase equilibria, and higher degeneracy of models is observed. Here a three-parameter corresponding states model is presented in which a cohesive third parameter α is proposed following a perturbation expansion and assuming a mean-field limit. It is shown that in this approximation the free energy of any two Mie systems sharing the same value of α will be the same. The parameter α is an explicit function of the repulsive and attractive exponents and consequently dictates the form of the intermolecular pair potential. Molecular dynamics simulations of a variety of Mie systems over a range of values of α are carried out and the solid–liquid, liquid–vapour and vapour–solid phase boundaries for the systems considered are presented. Using the simulation data, we confirm that systems of the same α exhibit conformal phase behaviour for the fluid-phase properties as well as for the solid–fluid boundary, although larger differences are noted in the solid region; these can be related to the approximations in the definition of the parameter. Furthermore, it is found that the temperature rang
Lobanova O, Avendaño C, Avendaño C, et al., 2015, SAFT-γ force field for the simulation of molecular fluids: 4. A single-site coarse-grained model of water applicable over a wide temperature range, Molecular Physics, Vol: 113, Pages: 1228-1249, ISSN: 1362-3028
In this work, we develop coarse-grained (CG) force fields for water, where the effective CG intermolecular interactions between particles are estimated from an accurate description of the macroscopic experimental vapour-liquid equilibria data by means of a molecular-based equation of state. The statistical associating fluid theory for Mie (generalised Lennard-Jones) potentials of variable range (SAFT-VR Mie) is used to parameterise spherically symmetrical (isotropic) force fields for water. The resulting SAFT-γ CG models are based on the Mie (8-6) form with size and energy parameters that are temperature dependent; the latter dependence is a consequence of the angle averaging of the directional polar interactions present in water. At the simplest level of CG where a water molecule is represented as a single bead, it is well known that an isotropic potential cannot be used to accurately reproduce all of the thermodynamic properties of water simultaneously. In order to address this deficiency, we propose two CG potential models of water based on a faithful description of different target properties over a wide range of temperatures: our CGW1-vle model is parameterised to match the saturated-liquid density and vapour pressure; our other CGW1-ift model is parameterised to match the saturated-liquid density and vapour-liquid interfacial tension. A higher level of CG corresponding to two water molecules per CG bead is also considered: the corresponding CGW2-bio model is developed to reproduce the saturated-liquid density and vapour-liquid interfacial tension in the physiological temperature range, and is particularly suitable for the large-scale simulation of bio-molecular systems. A critical comparison of the phase equilibrium and transport properties of the proposed force fields is made with the more traditional atomistic models.
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