An optimized molecular potential for carbon dioxide

Stability mechanism of crystalline CO2 and Xe

We explore the phase behaviors of simple molecular crystals in order to investigate the molecular basis of the stability mechanism relative to their liquid counterparts. The free energies of the face centered cubic crystals of Xe and CO2 are calculated as a collection of oscillators, and those of the liquids are from an equation of state via molecular dynamics simulations. The vibrational free energy in the solid is separated into the harmonic and anharmonic terms. The harmonic free energies decrease harshly with the expansion of the volume manifested as the large positive Grüneisen parameters, but the anharmonic free energies are positive and increase with volume, both of which originate from the deviation of the potential surface from the parabolic curve. The anharmonic free energies, though less significant in magnitude and destabilize the solids thermodynamically, serve to enhance their mechanical stability. The solid-liquid phase boundaries cannot be settled correctly without the exquisite balance between the two opposing contributions. A sharp contrast regarding the solid free energy is found in low-pressure ice, where the harmonic free energy does not decrease monotonically with volume and its anharmonic free energy is negative.

Thermophysical Properties and Phase Behavior of CO2 with Impurities: Insight from Molecular Simulations

Experimentally determining thermophysical properties for various compositions commonly found in CO2 transportation systems is extremely challenging. To overcome this challenge, we performed Monte Carlo (MC) and Molecular Dynamics (MD) simulations of CO2 rich mixtures to compute thermophysical properties such as densities, thermal expansion coefficients, isothermal compressibilities, heat capacities, Joule-Thomson coefficients, speed of sound, and viscosities at temperatures of (235-313) K and pressures of (20-200) bar. We computed thermophysical properties of pure CO2 and CO2 rich mixtures with N2, Ar, H2, and CH4 as impurities of (1-10) mol % and showed good agreement with available Equations of State (EoS). We showed that impurities decrease the values of thermal expansion coefficients, isothermal compressibilities, heat capacities, and Joule-Thomson coefficients in the gas phase, while these values increase in the liquid and supercritical phases. In contrast, impurities increase the value of speed of sound in the gas phase and decrease it in the liquid and supercritical phases. We present an extensive data set of thermophysical properties for CO2 rich mixtures with various impurities, which will help to design the safe and efficient operation of CO2 transportation systems.

CO2 ultrathin film growth on a monolayer of CO2 adsorbed on the NaCl(100) surface: sticking coefficient and IR-optical signatures in the ν3 region

CO2 ultrathin molecular films were grown onto a preadsorbed monolayer NaCl(100)/p(2 × 1)-CO2 at 40 K. Polarization infrared spectroscopy (PIRS) reveals that so-prepared films have better quality than directly grown films. A sticking probability of 0.74 ± 0.1 was deduced from the integrated IR absorption. The presence of the monolayer doublet in the film spectra suggests a Stranski-Krastanov film growth with locally varying film thicknesses on the surface. In the region of the ν3(12C16O2) band, fine structure was observed between the well-known transverse-optical (TO) and longitudinal optical (LO) bands. Two independent computational models were applied to analyze the nature of the observed fine structure. Both pair potential calculations in combination with a vibrational exciton model as well as plane-wave density functional theory (DFT) in combination with phonon calculations of IR intensities at the Γ-point reveal that a weak mode visible in s-polarization and p-polarization originates from a vibrational film excitation located near the substrate interface. A series of p-polarized weak bands appearing and partly disappearing upon film-growth is assigned to film stacks of unique local thickness.

Molecular Simulation Studies on the Vapor-Liquid Equilibrium of CO2 + 3,3,3-Trifluoropropene (R1243zf) Binary Mixtures

As fourth-generation refrigerants with great development prospects, hydrofluoroolefins (HFOs) can be mixed with other refrigerants, such as carbon dioxide (CO2), to form refrigerant mixtures with low global warming potential (GWP) and zero ozone depleting potential (ODP) while retaining the advantages of each component. Refrigerants can work together to achieve complementary benefits. Combinations of CO2 and HFOs can strengthen the thermodynamic properties of CO2 while inhibiting the flammability of HFOs. At present, relatively few studies have been conducted on the CO2 + 3,3,3-trifluoropropene (R1243zf) mixture. Besides experimental approaches, molecular simulation has grown in importance as a way to determine thermodynamic and transport properties in recent years. In this study, the Gibbs Ensemble Monte Carlo (GEMC) method was used to simulate the vapor-liquid equilibrium (VLE) properties of the CO2 + R1243zf binary mixture in the temperature range of 273.15 to 313.15 K, in which the three-site rigid TraPPE force field and a fully flexible transferable all-atom force field were selected to describe CO2 and R1243zf, respectively. By comparing the GEMC simulation results with the experimental data, it was found that the average deviation of pressure is 2.33%, the average deviation of liquid phase molar fraction Δx is 0.0099, and the mean deviation of gaseous phase molar fraction Δy is 0.0204. The simulation results accord well with the experimental data and the fitting data of the correlation model in the literature, indicating that the molecular models used for CO2 and R1243zf can reliably predict the VLE properties. Finally, the critical parameters of the mixture at a temperature of 313.15 K were predicted, and the radial distribution functions (RDFs) of pure R1243zf and the mixture at 273.15 K and 3.5 MPa were calculated and analyzed by molecular dynamics (MD) simulation.

Feasible Molecular Dynamics Simulation and Consistency of Critical Carbon Dioxide Thermophysical Properties

A method of approaching goal values is proposed for the simulation parameter determination. An economic-scale particle system is built for critical carbon dioxide (CO2) system simulations. The approaching method is applied to a critical CO2 system by trial simulation. With the simulation converging to experimental values, the obtained parameter settings help simulation to make practical sense. The measured values of thermal conductivity and viscosity are taken as approaching goal values. A molecular dynamics simulation of these two thermophysical properties is performed. The parameter settings are obtained after approaching the simulation for thermal conductivity and viscosity. As a verification, diffusion of critical CO2 is simulated by the obtained settings. Parameters obtained from the approaching method give rise to diffusion simulation results with experimental agreement. The achieved parameters can directly be used to simulate thermophysical properties of the critical CO2 system, providing reference settings for the simulation of the critical CO2 system. The superiority of critical CO2 in thermal and physical aspects is theoretically confirmed.

On the phase behaviors of CH4-CO2 binary clathrate hydrates: Two-phase and three-phase coexistences

We develop a statistical mechanical theory on clathrate hydrates in order to explore the phase behaviors of clathrate hydrates containing two kinds of guest species and apply it to CH4-CO2 binary hydrates. The two boundaries separating water and hydrate and hydrate and guest fluid mixtures are estimated, which are extended to the lower temperature and the higher pressure region far distant from the three-phase coexisting conditions. The chemical potentials of individual guest components can be calculated from free energies of cage occupations, which are available from intermolecular interactions between host water and guest molecules. This allows us to derive all thermodynamic properties pertinent to the phase behaviors in the whole space of thermodynamic variables of temperature, pressure, and guest compositions. It is found that the phase boundaries of CH4-CO2 binary hydrates with water and with fluid mixtures locate between simple CH4 and CO2 hydrates, but the composition ratios of CH4 guests in hydrates are disproportional to those in fluid mixtures. Such differences arise from the affinities of each guest species to the large and small cages of CS-I hydrates and significantly affect occupation of each cage type, which results in a deviation of the guest composition in hydrates from that in fluid on the two-phase equilibrium conditions. The present method provides a basis for the evaluation of the efficiency of the guest CH4 replacement to CO2 at the thermodynamic limit.

Molecular Mechanism of Conformational Crossover of Mefenamic Acid Molecules in scCO2

In this work, we studied conformational equilibria of molecules of mefenamic acid in its diluted solution in scCO₂ under isochoric heating conditions in the temperature range of 140-210 °C along the isochore corresponding to the scCO₂ density of 1.1 of its critical value. This phase diagram range totally covers the region of conformational transitions of molecules of mefenamic acid in its saturated solution in scCO₂. We found that in the considered phase diagram region, the equilibrium of two conformers is realized in this solution. In the temperature range of 140-180 °C, conformer I related to the first, most stable polymorph of mefenamic acid prevails. In the temperature range of 200-210 °C, conformer II, which is related to the second metastable polymorph becomes dominant. Based on the results of quantum chemical calculations and experimental IR spectroscopy data on the mefenamic acid conformer populations, we classified this temperature-induced conformational crossover as an entropy-driven phenomenon.

Densities, Viscosities, Thermal Expansivities, and Isothermal Compressibilities of Carbonated Hydroalcoholic Solutions for Applications in Sparkling Beverages

Densities, viscosities, isothermal compressibilities, and thermal expansivities of carbonated hydroalcoholic solutions relevant for sparkling beverages are evaluated by molecular dynamics simulations as a function of temperature and alcoholic degree. They are compared with available experimental data, among which new measurements of densities and viscosities are performed in that respect. The OPC water model seems to yield the most accurate results, and the choice of CO₂ model has little influence on the results. Theoretical densities obtained with the OPC model typically deviate by ∼2 kg m^-3 from experimental data. At low alcoholic degrees (<9% EtOH vol), experimental viscosities lie in between theoretical values derived from the Stokes-Einstein formula and the calculation of transverse current autocorrelation functions, but at higher alcoholic degrees (≥9% EtOH vol), the Stokes-Einstein relation leads to viscosities in quantitative agreement with experiments. Isothermal compressibilities estimated with a fluctuation formula roughly extend from 0.40 to 0.49 GPa^-1 in close agreement with the experimental range of values. However, thermal expansivities are found to significantly overestimate experimental data, a behavior that is partly attributed to the low temperature of maximum density of the OPC model. Despite this discrepancy, our molecular model seems to be suitable for describing several transport and thermodynamic properties of carbonated hydroalcoholic solutions. It could therefore serve as a starting point to build more realistic models for carbonated beverages, from fizzy drinks to sparkling wines.

Molecular Dynamics and Nuclear Magnetic Resonance Studies of Supercritical CO2 Sorption in Poly(Methyl Methacrylate)

The study of supercritical carbon dioxide sorption processes is an important and urgent task in the field of "green" chemistry and for the selection of conditions for new polymer material formation. However, at the moment, the research of these processes is very limited, and it is necessary to select the methodology for each polymer material separately. In this paper, the principal possibility to study the powder sorption processes using ¹³C nuclear magnetic resonance spectroscopy, relaxation-relaxation correlation spectroscopy and molecular dynamic modeling methods will be demonstrated based on the example of polymethylmethacrylate and supercritical carbon dioxide. It was found that in the first nanoseconds and seconds during the sorption process, most of the carbon dioxide, about 75%, is sorbed into polymethylmethacrylate, while on the clock scale the remaining 25% is sorbed. The methodology presented in this paper makes it possible to select optimal conditions for technological processes associated with the production of new polymer materials based on supercritical fluids.

Connecting Entropy Scaling and Density Scaling

It is shown that the residual entropy (entropy minus that of the ideal gas at the same temperature and density) is mostly synonymous with the independent variable of density scaling, identifying a direct link between these two approaches. The residual entropy and the effective hardness of interaction (itself a derivative at constant residual entropy) are studied for the Lennard-Jones monomer and dimer as well as a range of rigid molecular models for carbon dioxide. It is observed that the density scaling exponent appears to be related to the two-body interactions in the dilute-gas limit.

Molecular Characterization of Membrane Gas Separation under Very High Temperatures and Pressure: Single- and Mixed-Gas CO2/CH4 and CO2/N2 Permselectivities in Hybrid Networks

This work illustrates the potential of using atomistic molecular dynamics (MD) and grand-canonical Monte Carlo (GCMC) simulations prior to experiments in order to pre-screen candidate membrane structures for gas separation, under harsh conditions of temperature and pressure. It compares at 300 °C and 400 °C the CO₂/CH₄ and CO₂/N₂ sieving properties of a series of hybrid networks based on inorganic silsesquioxanes hyper-cross-linked with small organic PMDA or 6FDA imides. The inorganic precursors are the octa(aminopropyl)silsesquioxane (POSS), which degrades above 300 °C, and the octa(aminophenyl)silsesquioxane (OAPS), which has three possible meta, para or ortho isomers and is expected to resist well above 400 °C. As such, the polyPOSS-imide networks were tested at 300 °C only, while the polyOAPS-imide networks were tested at both 300 °C and 400 °C. The feed gas pressure was set to 60 bar in all the simulations. The morphologies and densities of the pure model networks at 300 °C and 400 °C are strongly dependent on their precursors, with the amount of significant free volume ranging from ~2% to ~20%. Since measurements at high temperatures and pressures are difficult to carry out in a laboratory, six isomer-specific polyOAPS-imides and two polyPOSS-imides were simulated in order to assess their N₂, CH₄ and CO₂ permselectivities under such harsh conditions. The models were first analyzed under single-gas conditions, but to be closer to the real processes, the networks that maintained CO₂/CH₄ and CO₂/N₂ ideal permselectivities above 2 were also tested with binary-gas 90%/10% CH₄/CO₂ and N₂/CO₂ feeds. At very high temperatures, the single-gas solubility coefficients vary in the same order as their critical temperatures, but the differences between the penetrants are attenuated and the plasticizing effect of CO₂ is strongly reduced. The single-gas diffusion coefficients correlate well with the amount of available free volume in the matrices. Some OAPS-based networks exhibit a nanoporous behavior, while the others are less permeable and show higher ideal permselectivities. Four of the networks were further tested under mixed-gas conditions. The solubility coefficient improved for CO₂, while the diffusion selectivity remained similar for the CO₂/CH₄ pair and disappeared for the CO₂/N₂ pair. The real separation factor is, thus, mostly governed by the solubility. Two polyOAPS-imide networks, i.e., the polyorthoOAPS-PMDA and the polymetaOAPS-6FDA, seem to be able to maintain their CO₂/CH₄ and CO₂/N₂ sieving abilities above 2 at 400 °C. These are outstanding performances for polymer-based membranes, and consequently, it is important to be able to produce isomer-specific polyOAPS-imides for use as gas separation membranes under harsh conditions.

Simulation of the carbon dioxide hydrate-water interfacial energy

HYPOTHESIS

Carbon dioxide hydrates are ice-like nonstoichiometric inclusion solid compounds with importance to global climate change, and gas transportation and storage. The thermodynamic and kinetic mechanisms that control carbon dioxide nucleation critically depend on hydrate-water interfacial free energy. Only two independent indirect experiments are available in the literature. Interfacial energies show large uncertainties due to the conditions at which experiments are performed. Under these circumstances, we hypothesize that accurate molecular models for water and carbon dioxide combined with computer simulation tools can offer an alternative but complementary way to estimate interfacial energies at coexistence conditions from a molecular perspective.

CALCULATIONS

We have evaluated the interfacial free energy of carbon dioxide hydrates at coexistence conditions (three-phase equilibrium or dissociation line) implementing advanced computational methodologies, including the novel Mold Integration methodology. Our calculations are based on the definition of the interfacial free energy, standard statistical thermodynamic techniques, and the use of the most reliable and used molecular models for water (TIP4P/Ice) and carbon dioxide (TraPPE) available in the literature.

FINDINGS

We find that simulations provide an interfacial energy value, at coexistence conditions, consistent with the experiments from its thermodynamic definition. Our calculations are reliable since are based on the use of two molecular models that accurately predict: (1) The ice-water interfacial free energy; and (2) the dissociation line of carbon dioxide hydrates. Computer simulation predictions provide alternative but reliable estimates of the carbon dioxide interfacial energy. Our pioneering work demonstrates that is possible to predict interfacial energies of hydrates from a truly computational molecular perspective and opens a new door to the determination of free energies of hydrates.

The Impact of Foaming Effect on the Physical and Mechanical Properties of Foam Glasses with Molecular-Level Insights

Foaming effect strongly impacts the physical and mechanical properties of foam glass materials, but an understanding of its mechanism especially at the molecular level is still limited. In this study, the foaming effects of dextrin, a mixture of dextrin and carbon, and different carbon allotropes are investigated with respect to surface morphology as well as physical and mechanical properties, in which 1 wt.% carbon black is identified as an optimal choice for a well-balanced material property. More importantly, the different foaming effects are elucidated by all-atomistic molecular dynamics simulations with molecular-level insights into the structure–property relationships. The results show that smaller pores and more uniform pore structure benefit the mechanical properties of the foam glass samples. The foam glass samples show excellent chemical and thermal stability with 1 wt.% carbon as the foaming agent. Furthermore, the foaming effects of CaSO₄ and Na₂HPO₄ are investigated, which both create more uniform pore structures. This work may inspire more systematic approaches to control foaming effect for customized engineering needs by establishing molecular-level structure–property–process relationships, thereby, leading to efficient production of foam glass materials with desired foaming effects.

Molecular insight into carbon dioxide hydrate formation from saline solution

Carbon dioxide hydrate has been intensively investigated in recent years because of its potential use as gas and heat storage materials. To understand the hydrate formation mechanisms, the crystallization of CO₂ hydrate from NaCl solutions was simulated at a molecular level. The influence of temperature, pressure, salt concentration and CO₂ concentration on CO₂ hydrate formation was evaluated. Results showed that the amount of the newly formed hydrate cages pressure went through a fast linear growth period followed by a relatively stable period. Pressure had little effect on CO₂ hydrate formation and temperature had a significant influence. The linear growth rate was greatly reduced as the temperature dropped from 255 to 235 K. The salt ion pairs could inhibit CO₂ hydrate formation, suggesting that we should choose the lower salinity areas if we want to storage CO₂ as gas hydrates in the seabed sediments. The observations in this study can provide theoretical support for the micro mechanism of hydrate formation, and provide a theoretical reference for the technology of hydrate based CO₂ storage.

In the process of the carbon dioxide hydrate formation in NaCl solution, it could form 5¹², 5¹²6² and 5¹²6³ cages, and the 5¹²6² cage and 5¹² cage number ratio was slightly above 3 : 1.

Toward In Silico Prediction of CO2 Diffusion in Champagne Wines

Carbon dioxide diffusion is the main physical process behind the formation and growth of bubbles in sparkling wines, especially champagne wines. By approximating brut-labeled champagnes as carbonated hydroalcoholic solutions, molecular dynamics (MD) simulations are carried out with six rigid water models and three CO₂ models to evaluate CO₂ diffusion coefficients. MD simulations are little sensitive to the CO₂ model but proper water modeling is essential to reproduce experimental measurements. A satisfactory agreement with nuclear magnetic resonance (NMR) data is only reached at all temperatures for simulations based on the OPC and TIP4P/2005 water models; the similar efficiency of these two models is attributed to their common properties such as low mixture enthalpy, same number of hydrogen bonds, alike water tetrahedrality, and multipole values. Correcting CO₂ diffusion coefficients to take into account their system-size dependence does not significantly alter the quality of the results. Estimates of viscosities deduced from the Stokes-Einstein formula are found in excellent agreement with viscometry on brut-labeled champagnes, while theoretical densities tend to underestimate experimental values. OPC and TIP4P/2005 water models appear to be choice water models to investigate CO₂ solvation and transport properties in carbonated hydroalcoholic mixtures and should be the best candidates for any MD simulations concerning wines, spirits, or multicomponent mixtures with alike chemical composition.

Unveiling Carbon Dioxide and Ethanol Diffusion in Carbonated Water-Ethanol Mixtures by Molecular Dynamics Simulations

The diffusion of carbon dioxide (CO2) and ethanol (EtOH) is a fundamental transport process behind the formation and growth of CO2 bubbles in sparkling beverages and the release of organoleptic compounds at the liquid free surface. In the present study, CO2 and EtOH diffusion coefficients are computed from molecular dynamics (MD) simulations and compared with experimental values derived from the Stokes-Einstein (SE) relation on the basis of viscometry experiments and hydrodynamic radii deduced from former nuclear magnetic resonance (NMR) measurements. These diffusion coefficients steadily increase with temperature and decrease as the concentration of ethanol rises. The agreement between theory and experiment is suitable for CO2. Theoretical EtOH diffusion coefficients tend to overestimate slightly experimental values, although the agreement can be improved by changing the hydrodynamic radius used to evaluate experimental diffusion coefficients. This apparent disagreement should not rely on limitations of the MD simulations nor on the approximations made to evaluate theoretical diffusion coefficients. Improvement of the molecular models, as well as additional NMR measurements on sparkling beverages at several temperatures and ethanol concentrations, would help solve this issue.

Closed-circulating CO2 sequestration process evaluation utilizing wastes in steelmaking plant

The wastes network system exploration in metallurgical process imposes of great significance for advancing green circular economy in steel plant. This paper originally proposes a closed-circulating CO₂ sequestering process for wastes appreciation and harmless disposal, and the effect of two circulation strategy, i.e. Slag circulation strategy and cold-rolling waste water(CRW) circulation strategy, on the CO₂ uptake efficiency, carbonation degree and desalination rate were systemically discussed. Then, their kinetics are analyzed by model and molecular simulation in detail, respectively. In addition, the energy consumption and the cost are simulated for comprehensively evaluating its superiority. The experimental and molecular simulation results all show that the peak values for both strategies could be achieved when circulation times is in the range of three to five. CRW circulation strategy has a better CO₂ uptake efficiency than slag circulation strategy, the CO₂ uptake efficiency is about 487kgCO₂/tslag and corresponding desalination rate is 48.9%, when CRW is circulated for five times at 60 °C and 20 L/g for 90 min. Adopting CRW circulation strategy, the CO₂ sequestration efficiency is averagely doubled comparing to previous results. 129%-183% energy consumption and 35.6% cost would be reduced, which represents that the proposed routine is economical to step forward to industrial application.

Impregnation of Poly(methyl methacrylate) with Carbamazepine in Supercritical Carbon Dioxide: Molecular Dynamics Simulation

Fully atomistic molecular dynamics simulations are employed to study impregnation of the poly(methyl methacrylate) (PMMA) matrix with carbamazepine (CBZ) in supercritical carbon dioxide. The simulation box consists of 108 macromolecules of the polymer sample with the polymerization degree of 100, 57 molecules of CBZ, and 242,522 CO₂ molecules. The simulation is performed at 333 K and 20 MPa. It is found that by the end of the simulation, the CBZ uptake reaches 1.09 wt % and 50 molecules are sorbed by PMMA. The main type of interaction between PMMA and CBZ is hydrogen bonding between the carbonyl oxygen of PMMA and the hydrogen atoms of the CBZ NH₂-group. At the polymer surface, CBZ exists not only in the molecular form, as inside the polymer and in the bulk solution, but also in the form of dimers and trimers. The energy of formation of the hydrogen-bonded complexes is estimated within ab initio calculations.

Giant Effect of Negative Compressibility in a Water-Porous Metal-CO2 System for Sensing Applications

When compressed, the size of ordinary materials reduces. The opposite effect, when a material or system increases (decreases) its volume upon compression (decompression), is called Negative Compressibility (NC). NC is extremely rare, while being attractive for a wide range of applications. Here we demonstrate, by both experiments and MD simulations, a pronounced effect of volumetric NC in a system consisting of water, porous metal and CO₂. This effect is achieved due to a new extrusion-adsorption cycle of water from-into a porous metal driven by a wetting-nonwetting transition due to the increase-decrease of CO₂ pressure. The heterogeneous nature of such a system leads to unprecedented NC of up to ∼ 90% in a narrow pressure range, meaning that almost a double volume increase (decrease) upon compression (decompression) is achieved. As long as the wetting-nonwetting transition is achieved, the proposed approach is not limited to water and a specific porous metal. An example of the application of this phenomenon is miniature sensors, particularly for threshold CO₂ pressure detection.

A potential for molecular simulation of compounds with linear moieties

The harmonic angle bending potential is used in many force fields for (bio)molecular simulation. The force associated with this potential is discontinuous at angles close to 180°, which can lead to numeric instabilities. Angle bending of linear groups, such as alkynes or nitriles, or linear molecules, such as carbon dioxide, can be treated by a simple harmonic potential if we describe the fluctuations as a deviation from a reference position of the central atom, the position of which is determined by the flanking atoms. The force constant for the linear angle potential can be derived analytically from the corresponding force constant in the traditional potential. The new potential is tested on the properties of alkynes, nitriles, and carbon dioxide. We find that the angles of the linear groups remain about 2° closer to 180° using the new potential. The bond and angle force constants for carbon dioxide were tuned to reproduce the experimentally determined frequencies. An interesting finding was that simulations of liquid carbon dioxide under pressure with the new flexible model were stable only when explicitly modeling the long-range Lennard-Jones (LJ) interactions due to the very long-range nature of the LJ interactions (>1.7 nm). In the other tested liquids, we find that a Lennard-Jones cutoff of 1.1 nm yields similar results as the particle mesh Ewald algorithm for LJ interactions. Algorithmic factors influencing the stability of liquid simulations are discussed as well. Finally, we demonstrate that the linear angle potential can be used in free energy perturbation calculations.

Thermodynamics of mixing methanol with supercritical CO2 as seen from computer simulations and thermodynamic integration

The changes in extensive thermodynamic quantities, such as volume, energy, Helmholtz free energy and entropy, occurring upon mixing liquid methanol with supercritical CO₂, are calculated using Monte Carlo simulations and thermodynamic integration for all eight combinations of four methanol and two CO₂ potential models in the entire composition range at 313 K. The obtained results are also compared with experimental data whenever possible. The transition of the system from liquid to a supercritical state is found to occur at this temperature around a CO₂ mole fraction value of 0.95 with all model combinations considered. This liquid to supercritical transition is always accompanied by positive Helmholtz free energy of mixing values and, consequently, by the non-miscibility of the two components. Furthermore, both this non-miscibility around the liquid to supercritical transition and also the miscibility of the two components below this transition, in the liquid regime, are found to be primarily of the energetic rather than entropic origin; the entropy of mixing turns out to be very close to zero, and around the liquid to supercritical transition even its qualitative behaviour is strongly model dependent. Finally, it is found that the methanol expansion coefficient is not sensitive to the details of the potential models, and it is always in excellent agreement with the experimental data. On the other hand, both the volume and the energy of mixing depend strongly on the molar volume of neat CO₂ in the model being used, and in this respect the TraPPE model of CO₂ [J. J. Potoff and J. I. Siepmann, AIChE J., 2001, 47, 1676] performs considerably better than that of Zhang and Duan [Z. Zhang and Z. Duan, J. Chem. Phys., 2005, 122, 214507].

Experimental and modeling evidence for structural crossover in supercritical CO_{2}

The physics of supercritical states is understood to a much lesser degree compared to subcritical liquids. Carbon dioxide, in particular, has been intensely studied, yet little is known about the supercritical part of its phase diagram. Here, we combine neutron scattering experiments and molecular dynamics simulations and demonstrate the structural crossover at the Frenkel line. The crossover is seen at pressures as high as 14 times the critical pressure and is evidenced by changes of the main features of the structure factor and pair distribution functions.

Conformational equilibria of pharmaceuticals in supercritical CO2, IR spectroscopy and quantum chemical calculations

In this work we demonstrate a self-consistent effective technique of analyzing the conformational equilibria of active pharmaceutical ingredient (API) molecules dissolved in supercritical carbon dioxide in a wide range of thermodynamic parameters of state. This approach can be useful for pharmaceutics when the crystalline forms of pharmaceuticals with a high purity degree and desirable polymorphism are produced using CO₂-based supercritical fluids technologies. Within this approach we use a combination of quantum chemical calculations and in situ IR spectroscopy. Quantum chemical calculations allow us to perform the initial conformational search and to determine the energy characteristics of the most stable conformers of API and the energy barriers of transitions between them. IR spectroscopy gives the information on the equilibrium of the most stable conformers of pharmaceuticals dissolved in scCO₂ in the thermodynamic parameter range of interest. Finally we validate our approach by applying it to the study of carbamazepine dissolved in scCO₂ being in permanent contact with an excess of crystalline carbamazepine as an example. The conformational search for carbamazepine molecules in scCO₂ was also performed using molecular dynamics simulation for comparison with the results obtained by the technique presented in this paper.

Computer simulation study of fluorocarbon phosphate surfactant based aqueous reverse micelle in supercritical CO2: roles of surfactant functional groups, ionic strength, and phase changes in CO2

Structural and dynamic properties of an aqueous micelle organized from fluorocarbon phosphate surfactant molecules in supercritical carbon dioxide (CO₂) are investigated via molecular dynamics computer simulations. The roles of the functional groups and ionic strength of the surfactants on the formation of reverse micelles in supercritical CO₂, and related water dynamics characterized as translational and reorientational dynamics, are systematically demonstrated by employing three different phosphate-based surfactants paired with sodium cations. The strong electrostatic interactions between the phosphate head groups and sodium cations result in formation of an aqueous core inside the surfactant aggregates, where water molecules are bonded together with loss of the tetrahedral hydrogen bonded network found in bulk water. It is found that all the three surfactants with CO₂-philic fluorocarbon double tails build up well-stabilized reverse micelles in supercritical CO₂, avoiding direct contacts between CO₂ and water molecules. Despite this, the surfactant with a carboxylic ester linkage between the phosphate head and fluorocarbon tail group tends to coordinate water molecules toward sustaining the inter-water hydrogen bonds, indicating better efficiency at covering the aqueous core with hydrophobic groups compared to one without a carboxylic ester group. As for water molecules confined in the reverse micelle, their translational and reorientational motions, and fluctuating dynamics of the inter-water hydrogen bonds, significantly slow down compared to bulk water at ambient temperature. The water dynamics become more restricted with an increase in ionic strength of the anionic surfactant; this is attributed to divalent surfactant heads and sodium cations being more tightly bound together with bonding to water compared to monovalent ones. Lastly, the structural and dynamic changes of the reverse micelle caused by a phase change in CO₂ are monitored with gradually decreasing temperature and pressure from the supercritical to gaseous state for CO₂. The average reverse micelle structure equilibrated in supercritical CO₂ is found to remain stable over a time period of 0.2 ms through a depressurization process to gaseous CO₂. We note that the diverse pathways of surfactant self-aggregation in gaseous CO₂ could be controlled by the preceding solvation procedure in the supercritical regime which governs the final aggregated structures in gaseous CO₂.

Molecular Dynamics Study of the Swelling of Poly(methyl methacrylate) in Supercritical Carbon Dioxide

The swelling of a poly (methyl methacrylate) in supercritical carbon dioxide was studied by means of full atomistic classical molecular dynamics simulation. In order to characterize the polymer swelling, we calculated various properties related to the density, structure, and dynamics of polymer chains as a function of the simulation time, temperature, and pressure. In addition, we compared the properties of the macromolecular chains in supercritical CO₂ with the properties of the corresponding bulk system at the same temperature and atmospheric pressure. It was shown that diffusion of CO₂ molecules into the polymer led to a significant increase in the chain mobility and distances between them. Analysis of diffusion coefficients of CO₂ molecules inside and outside the poly(methyl methacrylate) sample has shown that carbon dioxide actively interacts with the functional groups of poly (methyl methacrylate). Joint analysis of the radial distribution functions obtained from classical molecular dynamics and of the averaging interatomic distances from Car-Parrinello molecular dynamics allows us to make a conclusion about the possibility of formation of weak hydrogen bonds between the carbon dioxide oxygen atom and the hydrogen atoms of the polymer methyl groups.

Bulk viscosity of CO2 from Rayleigh-Brillouin light scattering spectroscopy at 532 nm

Rayleigh-Brillouin scattering spectra of CO₂ were measured at pressures ranging from 0.5 to 4 bars and temperatures from 257 to 355 K using green laser light (wavelength 532 nm, scattering angle of 55.7°). These spectra were compared to two line shape models, which take the bulk viscosity as a parameter. One model applies to the kinetic regime, i.e., low pressures, while the second model uses the continuum, hydrodynamic approach and takes the rotational relaxation time as a parameter, which translates into the bulk viscosity. We do not find a significant dependence of the bulk viscosity with pressure or temperature. At pressures where both models apply, we find a consistent value of the ratio of bulk viscosity over shear viscosity η_b/η_s = 0.41 ± 0.10. This value is four orders of magnitude smaller than the common value that is based on the damping of ultrasound and signifies that in light scattering only relaxation of rotational modes matters, while vibrational modes remain "frozen."

Stabilization of 2D graphene, functionalized graphene, and Ti2CO2 (MXene) in super-critical CO2: a molecular dynamics study

The stabilization of nanoparticles is a main concern to produce an efficient nanofluid.

A Systematic Protocol for Benchmarking Guest-Host Interactions by First-Principles Computations: Capturing CO2 in Clathrate Hydrates

Clathrate hydrates of CO₂ have been proposed as potential molecular materials in tackling important environmental problems related to greenhouse gases capture and storage. Despite the increasing interest in such hydrates and their technological applications, a molecular-level understanding of their formation and properties is still far from complete. Modeling interactions is a challenging and computationally demanding task, essential to reliably determine molecular properties. First-principles calculations for the CO₂ guest in all sI, sII, and sH clathrate cages were performed, and the nature of the guest-host interactions, dominated by both hydrogen-bond and van der Waals forces, was systematically investigated. Different families of density functionals, as well as pairwise CO₂ @H₂ O model potentials versus wavefunction-based quantum approaches were studied for CO₂ clathrate-like systems. Benchmark energies for new distance-dependent datasets, consisting of potential energy curves sampling representative configurations of the systems at the repulsive, near-equilibrium, and asymptotic/long-range regions of the full-dimensional surface, were generated, and a general protocol was proposed to assess the accuracy of such conventional and modern approaches at minimum and non-minimum orientations. Our results show that dispersion interactions are important in the guest-host stabilization energies of such clathrate cages, and the encapsulation of the CO₂ into guest-free clathrate cages is always energetically favorable. In addition, the orientation of CO₂ inside each cage was explored, and the ability of current promising approaches to accurately describe non-covalent CO₂ @H₂ O guest-host interactions in sI, sII, and sH clathrates was discussed, providing information for their applicability to future multiscale computer simulations.