12,854 research outputs found

    CO2-C4H10 Mixtures Simulated in Silica Slit Pores: Relation between Structure and Dynamics

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    Equilibrium molecular dynamics simulations were conducted for pure n-butane and for mixtures containing n-butane and carbon dioxide confined in 2 nm wide slit-shaped pores carved out of cristobalite silica. A range of thermodynamic conditions was explored, including temperatures ranging from subcritical to supercritical, and various densities. Preferential adsorption of carbon dioxide near the -OH groups on the surface was observed, where the adsorbed CO2 molecules tend to interact simultaneously with more than one -OH group. Analysis of the simulation results suggests that the preferential CO2 adsorption to the pore walls weakens the adsorption of n-butane, lowers the activation energy for n-butane diffusivity, and consequently enhances n-butane mobility. The diffusion results obtained for pure CO2 are consistent with strong adsorption on the pore walls, as the CO2 self-diffusion coefficient is low at low densities, increases with loading, and exhibits a maximum as the density is increased further because of hindrance effects. As the temperature increases, the maximum in self-diffusion coefficient is narrower, steeper, and shifted to lower loading. The simulation results are also quantified in terms of molecular density profiles for both butane and CO2 and in terms of residence time of the various molecules near the solid substrate. Our results could be useful for designing separation devices and also for better understanding the behavior of fluids in subsurface environments

    Structural and dynamical properties predicted by reactive force fields simulations for four common pure fluids at liquid and gaseous non-reactive conditions

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    Four common pure fluids were chosen to elucidate the reliability of reactive force fields in estimating bulk properties of selected molecular systems: CH 4 , H 2 O, CO 2 and H 2 . The pure fluids are not expected to undergo chemical reactions at the conditions chosen for these simulations. The ‘combustion’ ReaxFF was chosen as reactive force field. In the case of water, we also considered the ‘aqueous’ ReaxFF model. The results were compared to data obtained implementing popular classic force fields. In the gas phase, it was found that simulations conducted using the ‘combustion’ ReaxFF formalism yield structural properties in reasonable good agreement with classic simulations for CO 2 and H 2 , but not for CH 4 and H 2 O. In the liquid phase, ‘combustion’ ReaxFF simulations reproduce reasonably well the structure obtained from classic simulations for CH 4 , degrade for CO 2 and H 2 , and are rather poor for H 2 O. In the gas phase, the simulation results are compared to experimental second virial coefficient data. The ‘combustion’ ReaxFF simulations yield second virial coefficients that are not sufficiently negative for CH 4 and CO 2 , and slightly too negative for H 2 . The ‘combustion’ ReaxFF parameterisation induces too strong an effective attraction between water molecules, while the ‘aqueous’ ReaxFF yields a second virial coefficient that is in reasonable agreement with experiments. The ‘combustion’ ReaxFF parameterisation yields acceptable self-diffusion coefficients for gas-phase properties of CH 4 , CO 2 and H 2 . In the liquid phase, the results are good for CO 2 , while the self-diffusion coefficient predicted for liquid CH 4 is slower, and that predicted for liquid H 2 is about nine times faster than those expected based on classic simulations. The ‘aqueous’ ReaxFF parameterisation yields good results for both the structure and the diffusion of both liquid and vapour water

    Partial CO₂ Reduction in Amorphous Cylindrical Silica Nanopores Studied with Reactive Molecular Dynamics Simulations

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    It is known that pore confinement affects the structure and transport properties of fluids. It has also been shown that confinement can affect the equilibrium composition of a reactive system. Such effects could be related to the possible abiotic hydrocarbon synthesis in deep-sea hydrothermal vents, especially when the CO2 methanation reaction occurs within nanopores. In an attempt to identify possible rate-limiting steps of such a reaction, we report here molecular dynamics simulations conducted implementing the reactive force field. The reaction is considered within a cylindrical nanopore carved out of amorphous silica. Within the constraints of our simulations, which were conducted for 5 ns, no CH4 molecules were detected in the temperature range of 400–1000 K, suggesting that the silica pore hinders the complete CO2 reduction. This is consistent with the fact that silica is not an effective catalyst for CO2 methanation. Our simulations, in agreement with literature reports, suggest that the silica pore surface facilitates the partial reduction of CO2 to CO, which, within the conditions of our study, is found to be a stable product within the silica nanopores simulated. Analysis of the reaction products suggests that, although C–C bonds did not form, fragments reminiscent of carboxylic acids and formate were observed. Because these compounds are part of the biological Krebs cycle, our results suggest that confinement could provide prebiotic precursors of core metabolic pathways. Our results could be useful for further developing applications in which catalysts are designed to promote CO2 activation, for example, the one-step thermolysis of CO2

    On the free volume in nuclear multifragmentation

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    In many statistical multifragmentation models the volume available to the NN nonoverlapping fragments forming a given partition is a basic ingredient serving to the simplification of the density of states formula. One therefore needs accurate techniques for calculating this quantity. While the direct Monte-Carlo procedure consisting of randomly generating the fragments into the freeze-out volume and counting the events with no overlapped fragments is numerically affordable only for partitions with small NN, the present paper proposes a Metropolis - type simulation which allows accurate evaluations of the free volume even for cases with large NN. This procedure is used for calculating the available volume for various situations. Though globally this quantity has an exponential dependence on NN, variations of orders of magnitude for partitions with the same NN may be identified. A parametrization based on the virial approximation adjusted with a calibration function, describing very well the variations of the free volume for different partitions having the same NN is proposed. This parametrization was successfully tested within the microcanonical multifragmentation model from [Al. H. Raduta and Ad. R. Raduta, Phys. Rev. C {\bf 55}, 1344 (1997); {\it ibid.}, {\bf 56}, 2059 (1997)]. Finally, it is proven that parametrizations of the free volume solely dependent on NN are rather inadequate for multifragmentation studies producing important deviations from the exact results.Comment: 20 pages, 9 figures, Nucl. Phys. A (in press

    N-octane diffusivity enhancement via carbon dioxide in silica slit-shaped nanopores – a molecular dynamics simulation

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    Equilibrium molecular dynamics simulations were conducted to study the competitive adsorption and diffusion of mixtures containing n-octane and carbon dioxide confined in slit-shaped silica pores of width 1.9 nm. Atomic density profiles substantiate strong interactions between CO2 molecules and the protonated pore walls. Non-monotonic change in n-octane self-diffusion coefficients as a function of CO2 loading was observed. CO2 preferential adsorption to the pore surface is likely to attenuate the surface adsorption of n-octane, lower the activation energy for n-octane diffusivity, and consequently enhance n-octane mobility at low CO2 loading. This observation was confirmed by conducting test simulations for pure n-octane confined in narrower pores. At high CO2 loading, n-octane diffusivity is hindered by molecular crowding. Thus, n-octane diffusivity displays a maximum. In contrast, within the concentration range considered here, the self-diffusion coefficient predicted for CO2 exhibits a monotonic increase with loading, which is attributed to a combination of effects including the saturation of the adsorption capacity of the silica surface. Test simulations suggest that the results are strongly dependent on the pore morphology, and in particular on the presence of edges that can preferentially adsorb CO2 molecules and therefore affect the distribution of these molecules equally on the pore surface, which appears to be required to provide the effective enhancement of n-octane diffusivity

    Confinement Effects on Carbon Dioxide Methanation: A Novel Mechanism for Abiotic Methane Formation

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    An important scientific debate focuses on the possibility of abiotic synthesis of hydrocarbons during oceanic crust-seawater interactions. While on-site measurements near hydrothermal vents support this possibility, laboratory studies have provided data that are in some cases contradictory. At conditions relevant for sub-surface environments it has been shown that classic thermodynamics favour the production of CO2 from CH4, while abiotic methane synthesis would require the opposite. However, confinement effects are known to alter reaction equilibria. This report shows that indeed thermodynamic equilibrium can be shifted towards methane production, suggesting that thermal hydrocarbon synthesis near hydrothermal vents and deeper in the magma-hydrothermal system is possible. We report reactive ensemble Monte Carlo simulations for the CO2 methanation reaction. We compare the predicted equilibrium composition in the bulk gaseous phase to that expected in the presence of confinement. In the bulk phase we obtain excellent agreement with classic thermodynamic expectations. When the reactants can exchange between bulk and a confined phase our results show strong dependency of the reaction equilibrium conversions, [Formula: see text], on nanopore size, nanopore chemistry, and nanopore morphology. Some physical conditions that could shift significantly the equilibrium composition of the reactive system with respect to bulk observations are discussed

    A scalable parallel finite element framework for growing geometries. Application to metal additive manufacturing

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    This work introduces an innovative parallel, fully-distributed finite element framework for growing geometries and its application to metal additive manufacturing. It is well-known that virtual part design and qualification in additive manufacturing requires highly-accurate multiscale and multiphysics analyses. Only high performance computing tools are able to handle such complexity in time frames compatible with time-to-market. However, efficiency, without loss of accuracy, has rarely held the centre stage in the numerical community. Here, in contrast, the framework is designed to adequately exploit the resources of high-end distributed-memory machines. It is grounded on three building blocks: (1) Hierarchical adaptive mesh refinement with octree-based meshes; (2) a parallel strategy to model the growth of the geometry; (3) state-of-the-art parallel iterative linear solvers. Computational experiments consider the heat transfer analysis at the part scale of the printing process by powder-bed technologies. After verification against a 3D benchmark, a strong-scaling analysis assesses performance and identifies major sources of parallel overhead. A third numerical example examines the efficiency and robustness of (2) in a curved 3D shape. Unprecedented parallelism and scalability were achieved in this work. Hence, this framework contributes to take on higher complexity and/or accuracy, not only of part-scale simulations of metal or polymer additive manufacturing, but also in welding, sedimentation, atherosclerosis, or any other physical problem where the physical domain of interest grows in time

    Comparing semi-analytic particle tagging and hydrodynamical simulations of the Milky Way's stellar halo

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    Particle tagging is an efficient, but approximate, technique for using cosmological N-body simulations to model the phase-space evolution of the stellar populations predicted, for example, by a semi-analytic model of galaxy formation. We test the technique developed by Cooper et al. (which we call STINGS here) by comparing particle tags with stars in a smooth particle hydrodynamic (SPH) simulation. We focus on the spherically averaged density profile of stars accreted from satellite galaxies in a Milky Way (MW)-like system. The stellar profile in the SPH simulation can be recovered accurately by tagging dark matter (DM) particles in the same simulation according to a prescription based on the rank order of particle binding energy. Applying the same prescription to an N-body version of this simulation produces a density profile differing from that of the SPH simulation by ≲10 per cent on average between 1 and 200 kpc. This confirms that particle tagging can provide a faithful and robust approximation to a self-consistent hydrodynamical simulation in this regime (in contradiction to previous claims in the literature). We find only one systematic effect, likely due to the collisionless approximation, namely that massive satellites in the SPH simulation are disrupted somewhat earlier than their collisionless counterparts. In most cases, this makes remarkably little difference to the spherically averaged distribution of their stellar debris. We conclude that, for galaxy formation models that do not predict strong baryonic effects on the present-day DM distribution of MW-like galaxies or their satellites, differences in stellar halo predictions associated with the treatment of star formation and feedback are much more important than those associated with the dynamical limitations of collisionless particle tagging

    Cardiovascular risk profile and frailty in a population-based study of older British men.

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    BACKGROUND: Frailty in older age is known to be associated with cardiovascular disease (CVD) risk. However, the extent to which frailty is associated with the CVD risk profile has been little studied. Our aim was to examine the associations of a range of cardiovascular risk factors with frailty and to assess whether these are independent of established CVD. METHODS: Cross-sectional study of a socially representative sample of 1622 surviving men aged 71-92 examined in 2010-2012 across 24 British towns, from a prospective study initiated in 1978-1980. Frailty was defined using the Fried phenotype, including weight loss, grip strength, exhaustion, slowness and low physical activity. RESULTS: Among 1622 men, 303 (19%) were frail and 876 (54%) were pre-frail. Compared with non-frail, those with frailty had a higher odds of obesity (OR 2.03, 95% CI 1.38 to 2.99), high waist circumference (OR 2.30, 95% CI 1.67 to 3.17), low high-density lipoprotein-cholesterol (HDL-C) (OR 2.28, 95% CI 1.47 to 3.54) and hypertension (OR 1.79, 95% CI 1.27 to 2.54). Prevalence of these factors was also higher in those with frailty (prevalence in frail vs non-frail groups was 46% vs 31% for high waist circumference, 20% vs 11% for low HDL and 78% vs 65% for hypertension). Frail individuals had a worse cardiovascular risk profile with an increased risk of high heart rate, poor lung function (forced expiratory volume in 1 s (FEV1)), raised white cell count (WCC), poor renal function (low estimated glomerular filtration rate), low alanine transaminase and low serum sodium. Some risk factors (HDL-C, hypertension, WCC, FEV1, renal function and albumin) were also associated with being pre-frail. These associations remained when men with prevalent CVD were excluded. CONCLUSIONS: Frailty was associated with increased risk of a range of cardiovascular factors (including obesity, HDL-C, hypertension, heart rate, lung function, renal function) in older people; these associations were independent of established CVD
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