277 results found
Jiménez-Serratos G, Totton TS, Jackson G, et 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
Skutnik RA, Lehmann L, Puschel-Schlotthauer S, et al., The formation of biaxial nematic phases in binary mixtures ofthermotropic liquid-crystals composed of uniaxial molecules, Molecular Physics, ISSN: 0026-8976
By means of Monte Carlo simulations in the isothermal-isobaric ensemble we inves-tigate the formation of an ordered, biaxial nematic phase in a binary mixture ofthermotropic liquid crystals. The orientation dependence of the interaction betweenmolecules of each pure component is the same as in the well-known Maier-Saupemodel. Thus, each pure component of the mixture is capable of forming a uniaxialnematic phase. For the interaction between molecules of different components weuse the same Maier-Saupe model but change the sign of the coupling constant. Asa consequence a T-shaped arrangement of these molecules is energetically favored.The formation of the biaxial phase occurs in two steps. At higher temperatures T ,one of the two mixture components turns out to form a uniaxial nematic phasewhereas the other one is in a quasi two-dimensional restricted isotropic liquid state.We develop a simple theoretical model to understand the surprisingly high degreeof (ostensible) nematic order in the latter. At lower T , the second component of themixture becomes nematic and then the entire mixture of the two ordered compoundshas biaxial symmetry. The biaxial nematic phase does not decompose into domainsrich in molecules of one or the other species under conditions chosen here. We ana-lyze the changes in orientational structure in terms of a two-dimensional orientationdistribution function which exhibits a substantial system size effect.
Lee YS, Graham E, Jackson G, et al., 2019, A comparison of the performance of multi-objective optimization methodologies for solvent design, Computer Aided Chemical Engineering, Pages: 37-42
© 2019 Elsevier B.V. In this work, we present a systematic comparison of the performance of five mixed-integer non-linear programming (MINLP) multiobjective optimisation algorithms on a computer-aided solvent design problem. The five methods are designed to address the nonconvexity of the problem, with the aim of generating an accurate and complete approximation of the Pareto front. The approaches includes: a weighted sum approach with simulated annealing (SA), a weighted sum approach with multi level single linkage (MLSL), the sandwich algorithm with SA, the sandwich algorithm with MLSL and the non dominated sorting genetic algorithm-II. These five combinations of optimisation techniques are applied to the design of a solvent for chemical absorption of carbon dioxide (CO2). The results shows that the sandwich algorithm with MLSL can efficiently generate diverse Pareto points leading to a construction of more complete Pareto front.
Watson OL, Galindo A, Jackson G, et al., 2019, Computer-aided Design of Solvent Blends for the Cooling and Anti-solvent Crystallisation of Ibuprofen, Computer Aided Chemical Engineering, Pages: 949-954
© 2019 Elsevier B.V. We present a general computer-aided mixture/blend design (CAMbD) formulation for the design of optimal solvent mixtures for the crystallisation of pharmaceutical products. The proposed methodology enables the simultaneous identification of the optimal process temperature, solvent and anti-solvent molecules, and solvent mixture composition. The SAFT-γ Mie equation of state is used for the first time in the design of crystallisation solvents; based on an equilibrium model, the formulation considers both the crystal yield and solvent consumption. This design formulation is implemented in gPROMS and successfully applied to the crystallisation of ibuprofen, showing that this more general approach to crystallisation design can be used effectively to optimise the desired metrics.
Kazepidis P, Papadopoulos AI, Seferlis P, et al., 2019, Optimal design of post combustion CO<inf>2</inf> capture processes based on phase-change solvents, Computer Aided Chemical Engineering, Pages: 463-468
© 2019 Elsevier B.V. The current work addresses the investigation of phase-change solvents behaviour during the design of post-combustion CO2 capture processes. The use of phase-change solvents leads to energetic gains due to their lower regeneration energy demands. The latter are enhanced in this work by the consideration of systematic structural and operating modifications imposed on a reference absorption/desorption flowsheet. Such modifications are realized with the help of a rigorous and flexible model that can represent the phase-change behaviour and includes stream redistribution options that aim to enhance the main process driving forces. An aqueous N-methylcyclohexylamine (MCA) solution is employed in an effort to exploit the solvent's phase separation behaviour towards the reduction of the total process cost and energy requirements.
Galindo A, Rahman S, Lobanova O, et 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.
Avendano C, Jackson G, Wensink HH, 2018, Nanorings in planar confinement: the role of repulsive surfaces on the formation of lacuna smectics, Molecular Physics, Vol: 116, Pages: 2901-2910, ISSN: 0026-8976
We study the structure and liquid-crystalline phase behaviour of a model of nonconvexcircular soft-repulsive nanorings con ned in a planar slit geometry usingmolecular-dynamics simulation. The separation distance between the structurelessparallel soft-repulsive walls is large enough to allow for the formation of a distinctbulk phase in the central region of the box which is in coexistence with the adsorbeduid thus allowing the analysis of single wall e ects. As the concentrationof the particles is increased, the uid adsorbs (wets) onto the planar surfaces leadingto the formation of well-de ned smectic-A layers with a spacing proportional tothe diameter of the rings. An analysis of the nematic order parameter at distancesperpendicular to the surface reveals that the particles in each layer exhibit antinematicbehaviour and planar (edge-on) anchoring relative to the short symmetryaxis of the rings. This behaviour is in stark contrast to the behaviour observed inconvex disc-like particles that have the tendency to form nematic (discotic) structureswith hometropic (face-on) anchoring. The smectic phases formed by nanoringsin the bulk and under con nement are characterized by the formation of low-densitylayered liquid-crystalline states with large voids, referred to here as lacuna smecticphases. In contrast to what is typically found for con ned liquid-crystalline systemsinvolving convex particles, no apparent biaxiality is found for the nanorings in planarcon nement. We argue that formation of the low-density lacuna smectic layers withplanar anchoring is a consequence of the non-convex shape of the circular rings thatallow for interpenetration between the particles as observed for nanorings underbulk conditions [Avenda~no et al., Proc. Natl. Acad. Sci. U. S. A. 113, 9699 (2016);H. H. Wensink and C. Avenda~no, Phys. Rev. E 94 062704 (2016)].
Sarkisov L, Sweatman MB, Jackson G, 2018, Thermodynamics 2017 Conference Edinburgh, Scotland, 5-8 September 2017, Molecular Physics, Vol: 116, Pages: 1909-1914, ISSN: 0026-8976
Wu L, Malijevsky A, Avendano C, et 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.
Carbon capture and storage (CCS) is broadly recognised as having the potential to play a key role in meeting climate change targets, delivering low carbon heat and power, decarbonising industry and, more recently, its ability to facilitate the net removal of CO2 from the atmosphere. However, despite this broad consensus and its technical maturity, CCS has not yet been deployed on a scale commensurate with the ambitions articulated a decade ago. Thus, in this paper we review the current state-of-the-art of CO2 capture, transport, utilisation and storage from a multi-scale perspective, moving from the global to molecular scales. In light of the COP21 commitments to limit warming to less than 2 °C, we extend the remit of this study to include the key negative emissions technologies (NETs) of bioenergy with CCS (BECCS), and direct air capture (DAC). Cognisant of the non-technical barriers to deploying CCS, we reflect on recent experience from the UK's CCS commercialisation programme and consider the commercial and political barriers to the large-scale deployment of CCS. In all areas, we focus on identifying and clearly articulating the key research challenges that could usefully be addressed in the coming decade.
Jackson G, dufal S, Lafitte T, et al., 2018, Corrigendum: The A in SAFT: developing the contribution of association to the Helmholtz free energy within a Wertheim TPT1 treatment of generic Mie fluids, Molecular Physics, Vol: 116, Pages: 283-285, ISSN: 0026-8976
brumby PE, wensink HH, haslam AJ, et al., 2017, Structure and interfacial tension of a hard-rod fluid in planar confinement., Langmuir, Vol: 33, Pages: 11754-11770, ISSN: 0743-7463
The structural properties and interfacial tension of a fluid of hard-spherocylinder rod-like particles in contact with hard structureless flat walls are studied by means of Monte Carlo simulation. The calculated surface tension between the rod fluid and the substrate is characterized by a non-monotonic trend as a function of bulk concentration (density) over the range of isotropic bulk concentrations. As suggested by earlier theoretical studies, a surface-ordering scenario can be confirmed from our simulations: the local orientational order close to the wall changes from uniaxial to biaxial nematic when the bulk concentration reaches about 85% of the value at the onset of the isotropic-nematic phase transition. The surface ordering coincides with a wetting transition whereby the hard wall is wetted by a nematic film. Accurate values of the fluid-solid surface tension, the adsorption, and the average particle-wall contact distance are reported (over a broad range of densities into the dense nematic region for the first time), which may serve as a useful benchmark for future theoretical and experimental studies on confined rod fluids. The simulation data are supplemented with predictions from a second-virial density functional theory, which are in good qualitative agreement with the simulation results.
Zhao B, Lindeboom T, Benner S, et al., 2017, Predicting the Fluid-Phase Behavior of Aqueous Solutions of ELP (VPGVG) Sequences Using SAFT-VR., Langmuir, Vol: 33, Pages: 11733-11745, ISSN: 0743-7463
The statistical associating fluid theory for potentials of variable range (SAFT-VR) is used to predict the fluid phase behavior of elastin-like polypeptide (ELP) sequences in aqueous solution with special focus on the loci of lower critical solution temperatures (LCSTs). A SAFT-VR model for these solutions is developed following a coarse-graining approach combining information from atomistic simulations and from previous SAFT models for previously reported relevant systems. Constant-pressure temperature-composition phase diagrams are determined for solutions of (VPGVG)n sequences + water with n = 1 to 300. The SAFT-VR equation of state lends itself to the straightforward calculation of phase boundaries so that complete fluid-phase equilibria can be calculated efficiently. A broad range of thermodynamic conditions of temperature and pressure are considered, and regions of vapor-liquid and liquid-liquid coexistence, including LCSTs, are found. The calculated phase boundaries at low concentrations match those measured experimentally. The temperature-composition phase diagrams of the aqueous ELP solutions at low pressure (0.1 MPa) are similar to those of types V and VI phase behavior in the classification of Scott and van Konynenburg. An analysis of the high-pressure phase behavior confirms, however, that a closed-loop liquid-liquid immiscibility region, separate from the gas-liquid envelope, is present for aqueous solutions of (VPGVG)30; such a phase diagram is typical of type VI phase behavior. ELPs with shorter lengths exhibit both liquid-liquid and gas-liquid regions, both of which become less extensive as the chain length of the ELP is decreased. The strength of the hydrogen-bonding interaction is also found to affect the phase diagram of the (VPGVG)30 system in that the liquid-liquid and gas-liquid regions expand as the hydrogen-bonding strength is decreased and shrink as it is increased. The LCSTs of the mixtures are seen to decrease as the ELP chain length is in
Schoen M, Haslam AJ, Jackson G, 2017, Perturbation Theory versus Thermodynamic Integration. Beyond a Mean-Field Treatment of Pair Correlations in a Nematic Model Liquid Crystal., Langmuir, Vol: 33, Pages: 11345-11365, ISSN: 0743-7463
The phase behavior and structure of a simple square-well bulk fluid with anisotropic interactions is described in detail. The orientation dependence of the intermolecular interactions allows for the formation of a nematic liquid-crystalline phase in addition to the more conventional isotropic gas and liquid phases. A version of classical density functional theory (DFT) is employed to determine the properties of the model, and comparisons are made with the corresponding data from Monte Carlo (MC) computer simulations in both the grand canonical and canonical ensembles, providing a benchmark to assess the adequacy of the DFT results. A novel element of the DFT approach is the assumption that the structure of the fluid is dominated by intermolecular interactions in the isotropic fluid. A so-called augmented modified mean-field (AMMF) approximation is employed accounting for the influence of anisotropic interactions. The AMMF approximation becomes exact in the limit of vanishing density. We discuss advantages and disadvantages of the AMMF approximation with respect to an accurate description of isotropic and nematic branches of the phase diagram, the degree of orientational order, and orientation-dependent pair correlations. The performance of the AMMF approximations is found to be good in comparison with the MC data; the AMMF approximation has clear advantages with respect to an accurate and more detailed description of the fluid structure. Possible strategies to improve the DFT are discussed.
Hutacharoen P, Dufal S, Papaioannou V, et al., 2017, Predicting the solvation of organic compounds in aqueous environments: from alkanes and alcohols to pharmaceuticals, Industrial and Engineering Chemistry Research, Vol: 56, Pages: 10856-0876, ISSN: 0888-5885
The development of accurate models to predict the solvation, solubility, and partitioning of nonpolar and amphiphilic compounds in aqueous environments remains an important challenge. We develop state-of-the-art group-interaction models that deliver an accurate description of the thermodynamic properties of alkanes and alcohols in aqueous solution. The group-contribution formulation of the statistical associating fluid theory based on potentials with a variable Mie form (SAFT-γ Mie) is shown to provide accurate predictions of the phase equilibria, including liquid–liquid equilibria, solubility, free energies of solvation, and other infinite-dilution properties. The transferability of the model is further exemplified with predictions of octanol–water partitioning and solubility for a range of organic and pharmaceutically relevant compounds. Our SAFT-γ Mie platform is reliable for the prediction of challenging properties such as mutual solubilities of water and organic compounds which can span over 10 orders of magnitude, while remaining generic in its applicability to a wide range of compounds and thermodynamic conditions. Our work sheds light on contradictory findings related to alkane–water solubility data and the suitability of models that do not account explicitly for polarity.
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.
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
Jackson G, 2016, Editors of Molecular Physics 1958–2016, Molecular Physics, Vol: 114, Pages: 3420-3425, ISSN: 1362-3028
Eriksen DK, Lazarou G, Galindo A, et al., 2016, Development of intermolecular potential models for electrolyte solutions using an electrolyte SAFT-VR Mie equation of state, Molecular Physics, Vol: 114, Pages: 2724-2749, ISSN: 1362-3028
We present a theoretical framework and parameterisation of intermolecular potentials for aqueous electrolyte solutions using the statistical associating fluid theory based on the Mie interaction potential (SAFT-VR Mie), coupled with the primitive, non-restricted mean-spherical approximation (MSA) for electrolytes. In common with other SAFT approaches, water is modelled as a spherical molecule with four off-centre association sites to represent the hydrogen-bonding interactions; the repulsive and dispersive interactions between the molecular cores are represented with a potential of the Mie (generalised Lennard-Jones) form. The ionic species are modelled as fully dissociated, and each ion is treated as spherical: Coulombic ion–ion interactions are included at the centre of a Mie core; the ion–water interactions are also modelled with a Mie potential without an explicit treatment of ion–dipole interaction. A Born contribution to the Helmholtz free energy of the system is included to account for the process of charging the ions in the aqueous dielectric medium. The parameterisation of the ion potential models is simplified by representing the ion–ion dispersive interaction energies with a modified version of the London theory for the unlike attractions. By combining the Shannon estimates of the size of the ionic species with the Born cavity size reported by Rashin and Honig, the parameterisation of the model is reduced to the determination of a single ion–solvent attractive interaction parameter. The resulting SAFT-VRE Mie parameter sets allow one to accurately reproduce the densities, vapour pressures, and osmotic coefficients for a broad variety of aqueous electrolyte solutions; the activity coefficients of the ions, which are not used in the parameterisation of the models, are also found to be in good agreement with the experimental data. The models are shown to be reliable beyond the molality range considered during parameter estimatio
Smit B, Styring P, Wilson G, et al., 2016, Modelling - from molecules to megascale: general discussion, Faraday Discussions, Vol: 192, Pages: 493-509, ISSN: 1359-6640
Brand CV, Graham E, Rodriguez J, et al., 2016, On the use of molecular-based thermodynamic models to assess theperformance of solvents for CO₂capture processes:monoethanolamine solutions, Faraday Discussions, Vol: 192, Pages: 337-390, ISSN: 1364-5498
Predictive models play an important role in the design of post-combustion processes for the capture of carbon dioxide (CO2) emitted from power plants. A rate-based absorber model is presented to investigate the reactive capture of CO2 using aqueous monoethanolamine (MEA) as a solvent, integrating a predictive molecular-based equation of state: SAFT-VR SW (Statistical Associating Fluid Theory-Variable Range, Square Well). A distinctive physical approach is adopted to model the chemical equilibria inherent in the process. This eliminates the need to consider reaction products explicitly and greatly reduces the amount of experimental data required to model the absorber compared to the more commonly employed chemical approaches. The predictive capabilities of the absorber model are analyzed for profiles from 10 pilot plant runs by considering two scenarios: (i) no pilot-plant data are used in the model development; (ii) only a limited set of pilot-plant data are used. Within the first scenario, the mass fraction of CO2 in the clean gas is underestimated in all but one of the cases, indicating that a best-case performance of the solvent can be obtained with this predictive approach. Within the second scenario a single parameter is estimated based on data from a single pilot plant run to correct for the dramatic changes in the diffusivity of CO2 in the reactive solvent. This parameter is found to be transferable for a broad range of operating conditions. A sensitivity analysis is then conducted, and the liquid viscosity and diffusivity are found to be key properties for the prediction of the composition profiles. The temperature and composition profiles are sensitive to thermodynamic properties that correspond to major sources of heat generation or dissipation. The proposed modelling framework can be used as an early assessment of solvents to aid in narrowing the search space, and can help in determining target solvents for experiments and more detailed modelling.
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: 1091-6490
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.
Papadopoulos AI, Badr S, Chremos A, et al., 2016, Computer-aided molecular design and selection of CO2 capture solvents based on thermodynamics, reactivity and sustainability, Molecular Systems Design & Engineering, Vol: 1, Pages: 313-334, ISSN: 2058-9689
The identification of improved carbon dioxide (CO2) capture solvents remains a challenge due to the vast number of potentially-suitable molecules. We propose an optimization-based computer-aided molecular design (CAMD) method to identify and select, from hundreds of thousands of possibilities, a few solvents of optimum performance for CO2 chemisorption processes, as measured by a comprehensive set of criteria. The first stage of the approach involves a fast screening stage where solvent structures are evaluated based on the simultaneous consideration of important pure component properties reflecting thermodynamic, kinetic, and sustainability behaviour. The impact of model uncertainty is considered through a systematic method that employs multiple models for the prediction of performance indices. In the second stage, high-performance solvents are further selected and evaluated using a more detailed thermodynamic model, i.e. the group-contribution statistical associating fluid theory for square well potentials (SAFT-γ SW), to predict accurately the highly non-ideal chemical and phase equilibrium of the solvent–water–CO2 mixtures. The proposed CAMD method is applied to the design of novel molecular structures and to the screening of a data set of commercially available amines. New molecular structures and commercially-available compounds that have received little attention as CO2 capture solvents are successfully identified and assessed using the proposed approach. We recommend that these solvents should be given priority in experimental studies to identify new compounds.
Gopinath S, Jackson G, Galindo A, et al., 2016, Outer approximation algorithm with physical domain reduction for computer-aided molecular and separation process design, AICHE Journal, Vol: 62, Pages: 3484-3504, ISSN: 0001-1541
Integrated approaches to the design of separation systems based on computer-aided molecular and process design (CAMPD) can yield an optimal solvent structure and process conditions. The underlying design problem, however, is a challenging mixed integer nonlinear problem, prone to convergence failure as a result of the strong and nonlinear interactions between solvent and process. To facilitate the solution of this problem, a modified outer-approximation (OA) algorithm is proposed. Tests that remove infeasible regions from both the process and molecular domains are embedded within the OA framework. Four tests are developed to remove subdomains where constraints on phase behavior that are implicit in process models or explicit process (design) constraints are violated. The algorithm is applied to three case studies relating to the separation of methane and carbon dioxide at high pressure. The process model is highly nonlinear, and includes mass and energy balances as well as phase equilibrium relations and physical property models based on a group-contribution version of the statistical associating fluid theory (SAFT-γ Mie) and on the GC+ group contribution method for some pure component properties. A fully automated implementation of the proposed approach is found to converge successfully to a local solution in 30 problem instances. The results highlight the extent to which optimal solvent and process conditions are interrelated and dependent on process specifications and constraints. The robustness of the CAMPD algorithm makes it possible to adopt higher-fidelity nonlinear models in molecular and process design.
Gopinath S, Galindo A, Jackson G, et al., 2016, A feasibility-based algorithm for Computer Aided Molecular and Process Design of solvent-based separation systems, 26th European Symposium on Computer Aided Process Engineering (ESCAPE 26), Publisher: Elsevier, Pages: 73-78, ISSN: 1570-7946
Computer-aided molecular and product design (CAMPD) can in principle be used to find simultaneouslythe optimal conditions in separation processes and the structure of the optimal solvents.In many cases, however, the solution of CAMPD problems is challenging. In this paper, we proposea solution approach for the CAMPD of solvent-based separation systems in which implicitconstraints on phase behaviour in process models are used to test the feasibility of the processand solvent domains. The tests not only eliminate infeasible molecules from the search space butalso infeasible combinations of solvent molecules and process conditions. The tests also providebounds for the optimization of the process model (primal problem) for each solvent, facilitatingnumerical solution. This is demonstrated on a prototypical natural gas purification process.
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.
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