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

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

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

Propane-Water Mixtures Confined within Cylindrical Silica Nano-Pores: Structural and Dynamical Properties probed by Molecular Dynamics

Despite the multiple length and time scales over which fluid-mineral interactions occur, interfacial phenomena control the exchange of matter and impact the nature of multiphase flow, as well as the reactivity of C-O-H fluids in geologic systems. In general, the properties of confined fluids, and their influence on porous geologic phenomena are much less well understood compared to those of bulk fluids. We used equilibrium molecular dynamics simulations to study fluid systems composed of propane and water, at different compositions, confined within cylindrical pores of diameter ~16 Å carved out of amorphous silica. The simulations are conduced within a single cylindrical pore. In the simulated system all the dangling silicon and oxygen atoms were saturated with hydroxyl groups and hydrogen atoms, respectively, yielding a total surface density of 3.8 -OH/nm2. Simulations were performed at 300 K, at different bulk propane pressures, and varying the composition of the system. The structure of the confined fluids was quantified in terms of the molecular distribution of the various molecules within the pore as well as their orientation. This allowed us to quantify the hydrogen bond network and to observe the segregation of propane near the pore center. Transport properties were quantified in terms of the mean square displacement in the direction parallel to the pore axis, which allows us to extract self-diffusion coefficients. The diffusivity of propane in the cylindrical pore was found to depend on pressure, as well as on the amount of water present. It was found that the propane self-diffusion coefficient decreases with increasing water loading because of the formation of water bridges across the silica pores, at sufficiently high water content, which hinder propane transport. The rotational diffusion, the lifespan of hydrogen bonds, and the residence time of water molecules at contact with the silica substrate were quantified from the simulated trajectories using the appropriate auto-correlation functions. The simulations contribute to a better understanding of the molecular phenomena relevant to the behavior of fluids in the sub-surface

Effects of water on the stochastic motions of propane confined in MCM-41-S pores

Hydrocarbons confined in porous media find applications in a wide variety of industries and therefore their diffusive behavior is widely studied. Most of the porous media found in natural environments are laden with water, which might affect the confined hydrocarbons. To quantify the effect of hydration, we report here a combined quasielastic neutron scattering (QENS) and molecular dynamics (MD) simulation study on the dynamics of propane confined in the 1.5 nm-wide micropores of MCM-41-S in the presence of water at 230 and 250 K. To eliminate the strong incoherent signal from water and emphasize the propane signal we have used heavy water (D2O). QENS data show two dynamically different populations of propane in MCM-41-S and suggest that the presence of water hinders the diffusion of propane. Weak elastic contributions to the QENS spectra suggest that only long-range translational motion of propane molecules contributes to the quasielastic broadening. MD simulations carried out using a model cylindrical silica pore of 1.6 nm diameter filled with water and propane agree with the experimental finding of water hindering the diffusion of propane. Further, the simulation results suggest that the slowing down of propane motions is a function of the water content within the pore and is stronger at higher water contents. At high water content, the structure and the dynamics, both translational and rotational, of propane are severely impacted. Simulation data suggest that the rotational motion of the propane molecule occurs on time scales much faster than those accessible with the QENS instrument used, and thus explain the weak elastic contribution to the QENS spectra measured in the experiments. This study shows the effects of hydration on the structure and dynamics of volatiles in porous media, which are of interest for fundamental understanding and applied studies of confined fluids

Supercritical CO2 Effects on Calcite Wettability: A Molecular Perspective

The wettability behavior of reservoir rocks plays a vital role in determining CO2 storage capacity and containment security. Several experimental studies characterized the wettability of CO2/brine/rock systems for a wide range of realistic conditions. To develop a fundamental understanding of the molecular mechanisms responsible for such observations, the results of molecular dynamics simulations, conducted at atomistic resolution, are reported here for representative systems in a wide range of pressure and temperature conditions. Several force fields are considered, achieving good agreement with experimental data for the structure of interfacial water but only partial agreement in terms of contact angles. In general, the results suggest that, at the conditions chosen, water strongly wet calcite, resulting in water contact angles either too low to be determined accurately with the algorithms implemented here or up to ∼46°, depending on the force field implemented. These values are in agreement with some, but not all experimental data available in the literature, some of which report contact angles as high as 90°. One supercritical CO2 droplet was simulated in proximity of the wet calcite surface. The results show pronounced effects due to salinity, which are also dependent on the force field implemented to describe the solid substrate. When the force field predicts complete water wettability, increasing NaCl salinity seems to slightly increase the calcite affinity for CO2, monotonically as the NaCl concentration increases, because of the preferential adsorption of salt ions at the water–rock interface. When the other force field was implemented, it was not possible to quantify salt effects, but the simulations suggested strong interactions between the supercritical CO2 droplet and the second hydration layer on calcite. The results presented could be relevant for predicting the longevity of CO2 sequestration in geological repositories

Salt Effects on the Structure and Dynamics of Interfacial Water on Calcite Probed by Equilibrium Molecular Dynamics Simulations

It is important to understand the properties of interfacial water at mineral surfaces. Since calcite is one of the most common minerals found in rocks and sedimentary deposits, and since it represents a likely phase encountered in reservoirs dedicated to carbon sequestration, it is crucial to understand the behaviour of fluids on its surface. In this study, the impacts of sodium chloride (NaCl), potassium chloride (KCl) and magnesium chloride (MgCl2) on the structure and dynamics of water on the calcite interface were investigated using equilibrium molecular dynamics simulations. Two force fields were compared to model calcite. The resultant properties of interfacial water were quantified and compared in terms of atomic density profiles, surface density distributions, radial distribution functions, hydrogen bond density profiles, angular distributions, and residence times. Our results show the formation of distinct interfacial molecular layers, with water molecules in each layer having slightly different orientations, depending on the force field implemented. The fluid behaviour within the first interfacial layers differs from that observed in bulk water. There was a tendency for water molecules in adjacent layers to form hydrogen bonds between each other or the surface, as opposed to the formation of hydrogen bonds within each hydration layer. The addition of ions disrupts the well-organized structure of oxygen atoms in the first and second hydration layers, with KCl having the biggest effect. Conversely, far from the interface, MgCl2 leads to the lowest number of hydrogen bonds per water, out of the salts considered. The residence time of water within the second hydration layer follows a bi-exponential decay, suggesting the simultaneous presence of two dynamic mechanisms, one characterized by shorter time scales than the other. The time scale associated with the former mechanism decreases as the salt concentration is increased, whereas the opposite is observed for the slower mechanism. In general, the results obtained with the two force fields used to simulate calcite are similar in terms of the features of the hydration layers and hydrogen bond network but differ significantly in their predictions for the residence times. Although experimental results are not available to identify which of the two force fields yields predictions that more closely resemble reality, the results highlight the contributions of surface-water, water-water, and ion-water interactions on the wetting properties of calcite, which are especially important for calcite-water-electrolyte interactions commonly observed in nature

Safety and efficacy of fluoxetine on functional outcome after acute stroke (AFFINITY): a randomised, double-blind, placebo-controlled trial

Background: Trials of fluoxetine for recovery after stroke report conflicting results. The Assessment oF FluoxetINe In sTroke recoverY (AFFINITY) trial aimed to show if daily oral fluoxetine for 6 months after stroke improves functional outcome in an ethnically diverse population. Methods: AFFINITY was a randomised, parallel-group, double-blind, placebo-controlled trial done in 43 hospital stroke units in Australia (n=29), New Zealand (four), and Vietnam (ten). Eligible patients were adults (aged ≥18 years) with a clinical diagnosis of acute stroke in the previous 2–15 days, brain imaging consistent with ischaemic or haemorrhagic stroke, and a persisting neurological deficit that produced a modified Rankin Scale (mRS) score of 1 or more. Patients were randomly assigned 1:1 via a web-based system using a minimisation algorithm to once daily, oral fluoxetine 20 mg capsules or matching placebo for 6 months. Patients, carers, investigators, and outcome assessors were masked to the treatment allocation. The primary outcome was functional status, measured by the mRS, at 6 months. The primary analysis was an ordinal logistic regression of the mRS at 6 months, adjusted for minimisation variables. Primary and safety analyses were done according to the patient's treatment allocation. The trial is registered with the Australian New Zealand Clinical Trials Registry, ACTRN12611000774921. Findings: Between Jan 11, 2013, and June 30, 2019, 1280 patients were recruited in Australia (n=532), New Zealand (n=42), and Vietnam (n=706), of whom 642 were randomly assigned to fluoxetine and 638 were randomly assigned to placebo. Mean duration of trial treatment was 167 days (SD 48·1). At 6 months, mRS data were available in 624 (97%) patients in the fluoxetine group and 632 (99%) in the placebo group. The distribution of mRS categories was similar in the fluoxetine and placebo groups (adjusted common odds ratio 0·94, 95% CI 0·76–1·15; p=0·53). Compared with patients in the placebo group, patients in the fluoxetine group had more falls (20 [3%] vs seven [1%]; p=0·018), bone fractures (19 [3%] vs six [1%]; p=0·014), and epileptic seizures (ten [2%] vs two [<1%]; p=0·038) at 6 months. Interpretation: Oral fluoxetine 20 mg daily for 6 months after acute stroke did not improve functional outcome and increased the risk of falls, bone fractures, and epileptic seizures. These results do not support the use of fluoxetine to improve functional outcome after stroke. Funding: National Health and Medical Research Council of Australia