Moleculaire vrijheden in een thermodynamische gevangenis

Mechanical, Maritime and Materials Engineerin

Partial molar properties from single molecular dynamics simulations

In this manuscript, we show how to compute partial molar properties (e.g. partial molar volumes, energies, and enthalpies) of fluid mixtures from single Molecular Dynamics simulations in the microcanonical or canonical ensemble, using only relatively simple post-processing of trajectories. The method uses least squares linear regression of local fluctuations of particle numbers and energies in combination with the Small System Method, and is in principle valid for any number of components and for any type of intermolecular interactions. For multicomponent systems, only a single simulation is needed for a given composition of the mixture. Simulations of a binary WCA mixture are used to illustrate the method, and to investigate the effect of system size.Engineering Thermodynamic

Molecular simulation of vapor-liquid equilibria using the Wolf method for electrostatic interactions

The applicability of the Wolf method for calculating electrostatic interactions is verified for simulating vapor-liquid equilibria of hydrogen sulfide, methanol, and carbon dioxide. Densities, chemical potentials, and critical properties are obtained with Monte Carlo simulations using the Continuous Fractional Component version of the Gibbs Ensemble. Saturated vapor pressures are obtained from NPT simulations. Excellent agreement is found between simulation results and data from literature (simulations using the Ewald summation). It is also shown how to choose the optimal parameters for the Wolf method. Even though the Wolf method requires a large simulation box in the gas phase, due to the lack of screening of electrostatics, one can consider the Wolf method as a suitable alternative to the Ewald summation in VLE calculations.Engineering Thermodynamic

Understanding interactions between capped nanocrystals: Three-body and chain packing effects

Self-assembly of capped nanocrystals (NC) attracted a lot of attention over the past decade. Despite progress in manufacturing of NC superstructures, the current understanding of their mechanical and thermodynamic stability is still limited. For further applications, it is crucial to find the origin and the magnitude of the interactions that keep self-assembled NCs together, and it is desirable to find a way to rationally manipulate these interactions. We report on molecular simulations of interacting gold NCs protected by capping molecules. We computed the potential of mean force for pairs and triplets of NCs of different size (1.8–3.7 nm) with varying ligand length (ethanethiol-dodecanethiol) in vacuum. Pair interactions are strongly attractive due to attractive van der Waals interactions between ligand molecules. Three-body interaction results in an energy penalty when the capping layers overlap pairwise. This effect contributes up to 20% to the total energy for short ligands. For longer ligands, the three-body effects are so large that formation of NC chains becomes energetically more favorable than close packing of capped NCs at low concentrations, in line with experimental observations. To explain the equilibrium distance for two or more NCs, the overlap cone model is introduced. This model is based on relatively simple ligand packing arguments. In particular, it can correctly explain why the equilibrium distance for a pair of capped NCs is always ?1.25 times the core diameter independently on the ligand length, as found in our previous work [Schapotschnikow, R. Pool, and T. J. H. Vlugt, Nano Lett. 8, 2930 (2008)]. We make predictions for which ligands capped NCs self-assemble into highly stable three-dimensional structures, and for which they form high-quality monolayers.Process and EnergyMechanical, Maritime and Materials Engineerin

Study on hexane adsorption in zeolite ITQ-29 by molecular simulation

Adsorption isotherms and isosteric heat of adsorption of n-hexane in zeolite ITQ-29 were simulated using the Configurational Bias Monte Carlo (CBMC) technique in the grand-canonical (? VT) ensemble and compared with experimental results published by Gribov et al. and obtained by IR spectroscopy where the fractional loadings of n-hexane in ITQ-29 are presented in units from integral intensities of the absorption bands [u.a.]. In this work we present the simulation loadings of n-hexane in ITQ-29 converted to fractional coverages and compared to the experimental results. The simulations were performed using a united atom force field. In addition, we calculated equilibrium adsorption isotherms of ethane and propane in ITQ-29 in excellent agreement with published experiments. This force field successfully reproduces adsorption properties of linear alkanes in cation-free LTA zeolite and is suitable for fast and accurate adsorption data predictions.Process and EnergyMechanical, Maritime and Materials Engineerin

iRASPA: GPU-accelerated visualization software for materials scientists

A new macOS software package, iRASPA, for visualisation and editing of materials is presented. iRASPA is a document-based app that manages multiple documents with each document containing a unique set of data that is stored in a file located either in the application sandbox or in iCloud drive. The latter allows collaboration on a shared document (on High Sierra). A document contains a gallery of projects that show off the main features, a CloudKit-based access to the CoRE MOF database (approximately 8000 structures), and local projects of the user. Each project contains a scene of one or more structures that can initially be read from CIF, PDB or XYZ-files, or made from scratch. Main features of iRASPA are: structure creation and editing, pictures and movies, ambient occlusion and high-dynamic range rendering, collage of structures, (transparent) adsorption surfaces, cell replicas and supercells, symmetry operations like space group and primitive cell detection, screening of structures using user-defined predicates, and GPU-computation of helium void fraction and surface areas in a matter of seconds. Leveraging the latest graphics technologies like Metal, iRASPA can render hundreds of thousands of atoms (including ambient occlusion) with stunning performance. The software is freely available from the Mac App Store.Engineering Thermodynamic

Thermal conductivity of aqueous solutions of reline, ethaline, and glyceline deep eutectic solvents; a molecular dynamics simulation study

Accurate knowledge and control of thermal conductivities is central for the efficient design of heat storage and transfer devices working with deep eutectic solvents (DESs). The addition of water is a straightforward and cost-efficient way of tuning many properties of DESs. In this work, the thermal conductivities of aqueous solutions of reline, ethaline, and glyceline are reported for the first time. The non-equilibrium molecular dynamics Müller-Plathe (MP) method was used, along with the well-established GAFF and SPC/E force fields for DESs and water, respectively. We show that thermal conductivities of neat DESs are in excellent agreement with available experimental data. The addition of 25 wt% water results in nearly 2 times higher thermal conductivities in all DESs. A further increase in the fraction of water to 75 wt%, causes an increase in the thermal conductivities of DESs ca. 3 times. This behaviour is mainly due to the change in the microscopic structure of the DESs (i.e. hydrogen bonding) upon the addition of water. Our simulations reveal that thermal conductivities of aqueous DESs do not significantly depend on temperature. We also show that thermal conductivities strongly depend on system-size. System-sizes bigger larger than ca. 5 nm should be used.Engineering Thermodynamic

Mass Transport Limitations in Electrochemical Conversion of CO2 to Formic Acid at High Pressure

Mass transport of different species plays a crucial role in electrochemical conversion of CO2 due to the solubility limit of CO2 in aqueous electrolytes. In this study, we investigate the transport of CO2 and other ionic species through the electrolyte and the membrane, and its impact on the scale-up process of HCOO−/HCOOH formation. The mass transport of ions to the electrode and the membrane is modelled at constant current density. The mass transport limitations of CO2 on the formation of HCOO−/HCOOH is investigated at different pressures ranges from 5–40 bar. The maximum achievable partial current density of formate/formic acid is increased with increasing CO2 pressure. We use an ion exchange membrane model to understand the ion transport behaviour for both the monopolar and bipolar membranes. The cation exchange (CEM) and anion exchange membrane (AEM) model show that ion transport is limited by the electrolyte salt concentrations. For 0.1 M KHCO3, the AEM reaches the limiting current density more quickly than the CEM. For the BPM model, ion transport across the diffusion layer on either side of the BPM is also included to understand the concentration polarization across the BPM. The model revealed that the polarization losses across the bipolar membrane depend on the pH of the electrolyte used for the CO2 reduction reaction (CO2RR). The polarization loss on the anolyte side decreases with an increasing pH, while, on the cathode side, it increases with increasing catholyte pH. With this combined model for the electrode reactions and the membrane transport, we are able to account for the various factors influencing the polarization losses in the CO2 electrolyzer. To complete the analysis, we simulated the full cell polarization curve and fitted with the experimental dataEngineering Thermodynamic

Thermal conductivity of carbon dioxide from non-equilibrium molecular dynamics: A systematic study of several common force fields

We report a systematic investigation of the thermal conductivity of various three-site models of carbon dioxide (CO2) using nonequilibrium molecular dynamics in the temperature range 300–1000 K and for pressures up to 200 MPa. A direct comparison with experimental data is made. Three popular CO2 force fields (MSM, EPM2, and TraPPE) and two flexible models (based on EPM2) were investigated. All rigid force fields accurately predict the equation of state for carbon dioxide for the given range of variables. They can also reproduce the thermal conductivity of CO2 at room temperature and predict a decrease of the thermal conductivity with increasing temperature. At high temperatures, the rigid models underestimate the thermal conductivity.Process and EnergyMechanical, Maritime and Materials Engineerin

Solubilities of CO₂, CH₄, C₂H₆, CO, H₂, N₂, N₂O, and H₂S in commercial physical solvents from Monte Carlo simulations

The removal of acid gas impurities from synthesis gas or natural gas can be achieved using several physical solvents. Examples of solvents applied on a commercial scale include methanol (Rectisol), poly(ethylene glycol) dimethyl ethers (Selexol), n-methyl-2-pyrrolidone (Purisol), and propylene carbonate (Fluor solvent). Continuous Fractional Component Monte Carlo (CFCMC) simulations in the osmotic ensemble were used to compute the Henry coefficients of the pure gases CO (Formula presented.), CH (Formula presented.), C (Formula presented.) H (Formula presented.), CO, H (Formula presented.), N (Formula presented.), N (Formula presented.) O, and H (Formula presented.) S in the aforementioned solvents. The predicted Henry coefficients are in good agreement with the experimental results. The Monte Carlo method correctly predicts the gas solubility trend in these physical solvents, which obeys the following order: H (Formula presented.) S > CO (Formula presented.) > C (Formula presented.) H (Formula presented.) > CH (Formula presented.) > CO > N (Formula presented.) > H (Formula presented.). The gas separation selectivities for the precombustion process and the natural gas sweetening process are calculated from the pure gas Henry coefficients. The CO (Formula presented.) /N (Formula presented.) O analogy is verified for the solubility in these solvents.Engineering Thermodynamic