Heat pumps, which recycle waste heat, are a promising technology for reducing CO₂ emissions. Efficiently using low-grade waste heat remains challenging due to the limitations of standard heat exchangers and the need for more effective working fluids. This work introduces a multi-scale methodology that combines force field-based Monte Carlo simulations, quantum mechanics, and equations of state to explore the potential of formic acid as a new reactive fluid in thermodynamic cycles. Formic acid exhibits dimerization behavior, forming cyclic dimers in the gas phase, which can enhance the thermodynamic efficiency of heat recovery systems. The dimerization reaction of formic acid is crucial because it integrates chemical energy into thermodynamic processes, potentially improving the performance of heat pumps and other energy systems. The study implements umbrella sampling in Monte Carlo simulations to compute the thermodynamic properties of HCOOH dimerization, including equilibrium constants, enthalpy, and entropy. Results from two different methods to study dimer formation, namely the dimer counter method and the potential of mean force method, show strong agreement with the enthalpy of dimerization of −60.46 kJ mol⁻¹ and −62.91 kJ mol⁻¹, and entropy of −137.36 J mol⁻¹K⁻¹ and −146.98 J mol⁻¹K⁻¹, respectively. A very good agreement of the Monte Carlo results with Quantum Mechanics and experimental data validates the accuracy of the simulations. For phase equilibrium properties, the Peng–Robinson equation of state, coupled with advanced mixing rules, was applied and compared to Monte Carlo simulations in the Gibbs ensemble. This approach enabled the determination of the Global Phase Equilibrium of the system, vaporization enthalpy, phase composition, vapor and liquid densities of the coexisting phases, and entropy as a function of temperature. The agreement between the thermodynamic model and Monte Carlo simulations confirms the reliability of the methodology in capturing the phase behavior of the system. The findings demonstrate a promising approach for discovering and characterizing new reactive fluids, contributing to more efficient and sustainable energy technologies.

Cyclopentyl methyl ether (CPME) is a promising green solvent due to its eco-friendly properties; it is produced by adding methanol (MeOH) to cyclopentene. Separation of the resulting product mixture containing CPME and MeOH is critical, and it requires vapor-liquid equilibrium (VLE) data. In this work, isobaric VLE data were measured experimentally using an ebulliometer in a 60.0–101.3 kPa pressure range for a binary system of CPME + MeOH. VLE data were modeled using excess Gibbs (G (Formula presented.)) energy-based models such as Wilson, NRTL, and UNIQUAC. The formation of an azeotrope was analyzed. Flash point, surface tension, Gibbs adsorption, and thickness of surface layer were estimated using the Wilson model, which can help in determining molecule interaction and overall behavior of the system. Atmospheric and high-pressure distillation columns were designed using Aspen Plus to study the separation of CPME + MeOH, and an economic evaluation of the same was carried out.

Nitric oxide, NO, is a free radical that forms dimers, (NO)2, at its vapor–liquid coexisting temperatures. In this work, we developed an all-atom force field for NO and (NO)2. To assess the performance of this force field, we computed the vapor–liquid equilibrium (VLE) properties of the reactive NO–(NO)2 system, as well as those of pure NO and pure (NO)2, using Continuous Fractional Component Monte Carlo (CFCMC) simulations. We then compared the results with the available experimental data and predictions from two previously developed force fields. For the reactive NO–(NO)2 system, we performed CFCMC simulations in the reactive Gibbs ensemble in which the formation of NO dimers, 2NO ⇌ (NO)2, is considered. The predicted coexistence vapor–liquid densities, dimer mole fractions in the liquid phase, saturated vapor pressures, and heats of vaporization using our force field in the temperature range 120 K to 170 K are in excellent agreement with experimental values. In addition, we conducted a systematic parameter study to analyze the sensitivity of the new force field parameters and the isolated molecule partition functions of (NO)2 on the VLE properties of the reactive NO–(NO)2 system. The results indicate that the VLE properties of the reactive NO–(NO)2 system are affected by both the force field parameters of the involved species as well as the isolated molecule partition functions of (NO)2.

Adsorption simulations often assume a rigid framework, which can be exploited by replacing the expensive framework-adsorbate energy/force evaluation by interpolation of a precomputed energy grid. We present the implementation in RASPA3 of a triquintic interpolation algorithm by Boateng and Bradach and compare it to the tricubic algorithm of Lekien and Marsden. We extended the scheme to interpolation in fractional space to facilitate interpolation of non-rectangular frameworks and evaluated the accuracy. We find that the use of grids is advantageous for larger systems and/or large cutoffs, but generally the efficiency gains are modest (a factor of 2–5).

Computation of the excess entropy (Formula presented.) from the second-order density expansion of the entropy holds strictly for infinite systems in the limit of small densities. For the reliable and efficient computation of (Formula presented.) it is important to understand finite-size effects. Here, expressions to compute (Formula presented.) and Kirkwood–Buff (KB) integrals by integrating the Radial Distribution Function (RDF) in a finite volume are derived, from which (Formula presented.) and KB integrals in the thermodynamic limit are obtained. The scaling of these integrals with system size is studied. We show that the integrals of (Formula presented.) converge faster than KB integrals. We compute (Formula presented.) from Monte Carlo simulations using the Wang–Ramírez–Dobnikar–Frenkel pair interaction potential by thermodynamic integration and by integration of the RDF. We show that (Formula presented.) computed by integrating the RDF is identical to that of (Formula presented.) computed from thermodynamic integration at low densities, provided the RDF is extrapolated to the thermodynamic limit. At higher densities, differences up to (Formula presented.) are observed.

One of the most promising energy carriers for transport applications are hydrogen-based energy carriers. NaBH₄ is a hydrogen energy carrier and produces hydrogen bubbles when it is dissolved in water. The formation of hydrogen bubbles hinders experimental measurements of the thermodynamic and transport properties of aqueous NaBH₄ solutions at elevated temperatures. Accurate knowledge of these properties is essential for the NaBH₄ hydrolysis reactor modeling and design. Molecular dynamics (MD) simulations provide the option to study the thermodynamic and transport properties of NaBH₄ aqueous solutions without hindering hydrogen bubble formation. In this work, a new force field is developed for BH₄^-, namely, the Delft force field of BH₄^- (DFF/BH₄^-), which, combined with additional force fields, can accurately describe experimental densities and viscosities of 0 to 5 m (mol salt/kg water) NaBH₄, 0 to 3 m NaB(OH)₄, and 1 m NaOH aqueous solutions at 295 K within 1.8% and 10.8% maximum deviation, respectively. Empirical fitting correlations are created for densities, viscosities, and self-diffusivities obtained from the MD simulations of 0 to 5 m NaBH₄, 0 to 5 m NaB(OH)₄, and 0 to 1 m NaOH aqueous solutions at 295-363 K for NaBH₄ hydrolysis reactor modeling and design purposes.

Hydrate-based H2 storage is based on the mechanism of trapping H2 in water-based structures that are environmentally friendly and cost-efficient. Understanding the effects of common promoters on hydrate-based H2 storage at the molecular level is crucial for designing efficient storage systems, and for discovering novel promoters. Here, a series of μs-scale molecular dynamics simulations are performed to investigate the nucleation of binary H2 hydrates from gas-liquid two-phase solutions in the presence of various promoters, i.e., CH4, CO2, C2H6, C3H8, C5H10, and THF. The simulation results indicate that the H2 and promoter molecules first dissolve in water from the gas phase and then are absorbed on the cage-faces, promoting the nucleation and growth of binary H2 hydrates. THF is the most effective promoter for hydrate-based H2 storage, exhibiting high performance in converting H2 from the gas phase to hydrates. It is followed by CH4, C2H6, and CO2; C3H8 and C5H10 molecules are less effective H2 hydrate promoters. The presence of large promoter molecules enhances multi-occupied cage formation. The molecular insight into the nucleation of binary H2 hydrates with various promoters provided here not only contributes to a broader understanding of hydrate-based H2 storage but is expected to motivate further experimental and computational studies.

Accurate conductivity predictions of KOH(aq) are crucial for electrolysis applications. OH– is transferred in water by the Grotthuss transfer mechanism, thereby increasing its mobility compared to that of other ions. Classical and ab initio molecular dynamics struggle to capture this enhanced mobility due to limitations in computational costs or in capturing chemical reactions. Most studies to date have provided only qualitative descriptions of the structure during Grotthuss transfer, without quantitative results for the transfer rate and the resulting transport properties. Here, machine learning molecular dynamics is used to investigate 50,000 transfer events. Analysis confirmed earlier works that Grotthuss transfer requires a reduction in accepted and a slight increase in donated hydrogen bonds to the hydroxide, indicating that hydrogen-bond rearrangements are rate-limiting. The computed self-diffusion coefficients and electrical conductivities are consistent with experiments for a wide temperature range, outperforming classical interatomic force fields and earlier AIMD simulations.

Experimental screening of Metal Organic Frameworks (MOFs) for separation applications can be costly and time-consuming. Computational methods can provide many benefits in this process, as expensive compounds and a wide range of operating conditions can be tested while crucial mechanistic insights are gained. TAMOF-1, a recently developed MOF, stands out for its exceptional stability, robustness and cost-effective synthesis. Its good CO2 uptake capacity makes it a promising agent for flue gas separation applications. In this work, we combine experiments with simulations at the atomistic and numerical level to investigate the adsorption and separation of CO2 and N2. Using Monte Carlo simulations, we accurately reproduce experimental adsorption isotherms and elucidate the adsorption mechanisms. TAMOF-1 effectively separates CO2 from N2 because of preferential binding sites near Cu2+ atoms. To assess separation performance in equilibrium at different conditions along the entire isotherm pressure range, adsorbed mole fractions, selectivities, and the trade-off between selectivity and uptake (TSN) are calculated. The dynamic separation performance is assessed by breakthrough experiments and numerical simulations, demonstrating efficient dynamic separation of CO2 and N2, with CO2 being retained in the column.

Continuous Fractional Component Monte Carlo (CFCMC) and molecular dynamics (MD) simulations are performed to calculate the solubilities and self-diffusion coefficients of four light n-alkanes (methane, ethane, propane, and n-butane) in aqueous NaCl solutions as well as the thermodynamic properties of their corresponding hydrate crystals. Correction factors k_ij to the Lorentz-Berthelot combining rules for alkane groups (CH₃) and water are optimized (k_ij = 1.04) by fitting excess chemical potentials to experimental data at 1 bar and 298.15 K. Using these values of k_ij, we calculate the solubilities of the four alkanes in aqueous NaCl solutions with different molalities (0-6) mol/kg at different temperatures (278.15-308.15) K and pressures (1, 100, 200, 300) bar. The diffusion coefficients of the four alkanes in NaCl solutions (0-6) mol/kg are calculated at different temperatures (278.15-308.15) K and 1 bar and corrected for the finite-size effects. The lattice parameters of the corresponding hydrates with different guest molecules are computed using MD simulations at different temperatures (150-290) K and pressures (5-700) MPa. Isothermal compressibilities at 287.15 K and thermal expansion coefficients at 14.5 MPa for the corresponding hydrates are calculated. We present an extensive collection of thermodynamic data related to gas hydrates that contribute to a fundamental understanding of natural gas hydrate science.

We study important aspects of shape selectivity effects of zeolites for hydroisomerization of linear alkanes, which produces a myriad of isomers, particularly for long chain hydrocarbons. To investigate the conditions for achieving an optimal yield of branched hydrocarbons, it is important to understand the role of chemical equilibrium in these reversible reactions. We conduct an extensive analysis of shape selectivity effects of different zeolites for the hydroisomerization of C₇ and C₈ isomers at chemical reaction equilibrium conditions. The reaction ensemble Monte Carlo method, coupled with grand-canonical Monte Carlo simulations, is commonly used for computing reaction equilibrium of heterogeneous reactions. The computational demands become prohibitive for a large number of reactions. We used a faster alternative in which reaction equilibrium is obtained by imposing chemical equilibrium in the gas phase and phase equilibrium between the gas phase components and the adsorbed phase counterparts. This effectively mimics the chemical equilibrium distribution in the adsorbed phase. Using Henry’s law at infinite dilution and mixture adsorption isotherm models at elevated pressures, we calculate the adsorbed loadings in the zeolites. This study shows that zeolites with cage or channel-like structures exhibit significant differences in selectivity for alkane isomers. We also observe a minimal impact of pressure on the gas-phase equilibrium of these reactions at typical experimental reaction temperatures 400 − 700 K . This study marks initial strides in understanding the reaction product distribution for long-chain alkanes.

We explore the impact of force field parameters and reaction equilibrium on the scaling behavior towards the critical point in reactive binary systems, focusing on NO ₂/N ₂O ₄. This system can be considered as a special single-component system since NO ₂ and N ₂O ₄ are in chemical equilibrium via the chemical reaction 2NO ₂⇌N ₂O ₄. We simplify the system by representing both components as single LJ particles, achieving excellent agreement with densities computed using molecular simulations in which all-atom force fields were used. We investigate the effect of force field parameters (ɛ and σ) on phase behavior and show that the critical exponent β remains constant, which means that intermolecular interactions do not affect the scaling to the critical point when the chemical reaction takes place. We also investigate the sensitivity of the reaction equilibrium constant and show that even small changes in isolated molecule partition functions lead to large differences in chemical equilibria. We show that the critical exponent β is different for systems with different reaction equilibrium constants, so a careful parameterization of β is needed for an accurate computation of critical temperatures of reactive mixtures. We perform a screening of reactive binary mixtures for a wide range of ideal gas reaction equilibrium constants, revealing key insights into the thermodynamic behavior and critical properties. Thereby we facilitate the efficient screening of reactive binary mixtures for various applications. Our results emphasize the importance of accurately parameterizing β and provide valuable insights into the critical scaling behavior of complex reactive systems.

Hydrogen peroxide plays a key role in many environmental and industrial chemical processes. We performed classical Molecular Dynamics and Continuous Fractional Component Monte Carlo simulations to calculate thermodynamic properties of H₂O₂ in aqueous solutions. The quality of the available force fields for H₂O₂ developed by Orabi and English (2018) [67], and by Cordeiro (2014) [69] was systematically evaluated. To assess which water force field is suitable for predicting properties of H₂O₂ in aqueous solutions, four widely used water force fields were used, namely the TIP3P, TIP4P/2005, TIP5P-E, and a modified TIP3P force field. While the computed densities of pure H₂O₂ in the temperature range of 253 - 353 K using the force field by Orabi & English are in excellent agreement with experimental results, the densities using the force field by Cordeiro are underestimated by 3%. The TIP4P/2005 force field in combination with the H₂O₂ force field developed by Orabi & English can predict the densities of H₂O₂ aqueous solution for the whole range of H₂O₂ mole fractions in very good agreement with experimental results. The TIP4P/2005 force field in combination with either of the H₂O₂ force fields can predict the viscosities of H₂O₂ aqueous solutions for the whole range of H₂O₂ mole fractions in reasonably good agreement with experimental results. The computed diffusion coefficients for H₂O₂ and water molecules using the TIP4P/2005 force field with either of the H₂O₂ force fields are almost constant for the whole range of H₂O₂ mole fractions. Hydrogen bond analysis showed a steady increase in the number of hydrogen bonds with the solute concentrations in H₂O₂ aqueous solutions for all combinations except for the Cordeiro-TIP5P-E and Orabi-TIP5P-E systems, which showed a minimum at intermediate concentrations. The Cordeiro force field for H₂O₂ in combination with either of the water force fields can predict the Henry coefficients of H₂O₂ in water in better agreement with experimental values than the force field by Orabi & English.

The use of reactive working fluids in thermodynamic cycles is currently being considered as an alternative to inert working fluids, because of the preliminarily attested higher energy-efficiency potential. The current needs to simulate their use in thermodynamic cycles, which may operate in liquid, vapour or vapour-liquid state, are an accurate real-fluid equation of state and ideal gas thermochemical properties of each molecule constituting the mixture, to calculate the equilibrium constant. To this end, the appeal to a multi-scale theoretical methodology is paramount and its definition represents the objective of the present work. This methodology is applied and validated on the system N₂O₄ ⇌ 2NO₂. Firstly, the equations solved for simultaneous two-phase and reaction equilibrium are presented. Secondly, ideal gas thermochemical properties of N₂O₄ and NO₂ are computed at atomic scale by quantum mechanics simulations. Then, to apply the selected cubic equation of state, pure-component properties of the species forming the reactive mixture (critical point coordinates and acentric factor) are required as input. However, these properties are not measurable, since NO₂ and N₂O₄ do not exist in nature as pure components. To get around this difficulty, the methodology relies on molecular Monte Carlo simulations of the pure N₂O₄ and NO₂, as well as on the reactive N₂O₄ ⇌ 2NO₂, enabling the determination of those missing pure-component properties and thus the calculation, on a macroscopic scale, of the reactive mixture properties. Finally, the comparison of calculated mixture properties with available experimental data leads to validate the accuracy of the proposed methodology.

An in-depth review of the available experimental and molecular simulation studies of CO₂ diffusion in H₂O, which is a central property in important industrial and environmental processes, such as carbon capture and storage, enhanced oil recovery, and in the food industry is presented. The cases of both bulk and confined systems are covered. The experimental and molecular simulation data gathered are analyzed, and simple and computationally efficient correlations are devised. These correlations are applicable to conditions from 273 K and 0.1 MPa up to 473 K and 45 MPa. The available experimental data for diffusion coefficients of CO₂ in brines are also collected, and their dependency on temperature, pressure, and salinity is examined in detail. Other engineering models and correlations reported in literature are also presented. The review of the simulation studies focuses on the force field combinations, the data for diffusivities at low and high pressures, finite-size effects, and the correlations developed based on the Molecular Dynamics data. Regarding the confined systems, we review the main methods to measure and compute the diffusivity of confined CO₂ and discuss the main natural and artificial confining media (i.e., smectites, calcites, silica, MOFs, and carbon materials). Detailed discussion is provided regarding the driving force for diffusion of CO₂ and H₂O under confinement, and on the role of effects such as H₂O adsorption on hydrophilic confining media on the diffusivity of CO₂. Finally, an outlook of future research paths for advancing the field of CO₂ diffusivity in H₂O at the bulk phase and in confinement is laid out.

We present RASPA3, a molecular simulation code for computing adsorption and diffusion in nanoporous materials and thermodynamic and transport properties of fluids. It implements force field based classical Monte Carlo/molecular dynamics in various ensembles. In this article, we introduce the new additions and changes compared to RASPA2. RASPA3 is rewritten from the ground up in C++23 with speed and code readability in mind. Transition-matrix Monte Carlo is added to compute the density of states and free energies. The Monte Carlo code for rigid molecules is based on quaternions, and the atomic positions needed in the energy evaluation are recreated from the center of mass position and quaternion orientation. The expanded ensemble methodology for fractional molecules, with a scaling parameter λ between 0 and 1, now also keeps track of analytic expressions of dU/dλ, allowing independent verification of the chemical potential using thermodynamic integration. The source code is freely available under the MIT license on GitHub. Using this code, we compare four Monte Carlo (MC) insertion/deletion techniques: unbiased Metropolis MC, Configurational-Bias Monte Carlo (CBMC), Continuous Fractional Component MC (CFCMC), and CB/CFCMC. We compare particle distribution shapes, acceptance ratios, accuracy and speed of isotherm computation, enthalpies of adsorption, and chemical potentials, over a wide range of loadings and systems, for the grand canonical ensemble and for the Gibbs ensemble.

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.

Entropies for alkane isomers longer than C10 are computed using our recently developed linear regression model for thermochemical properties which is based on second-order group contributions. The computed entropies show excellent agreement with experimental data and data from Scott’s tables which are obtained from a statistical mechanics-based correlation. Entropy production and heat input are calculated for the hydroisomerization of C7 isomers in various zeolites (FAU-, ITQ-29-, BEA-, MEL-, MFI-, MTW-, and MRE-types) at 500 K at chemical equilibrium. Small variations in these properties are observed because of the differences in reaction equilibrium distributions for these zeolites. The effect of chain length on heat input and entropy production is also studied for the hydroisomerization of C7, C8, C10, and C14 isomers in MTW-type zeolite at 500 K. For longer chains, both heat input and entropy production increase. Enthalpies and absolute entropies of C7 hydroisomerization reaction products in MTW-type zeolite increase with higher temperatures. These findings highlight the accuracy of our linear regression model in computing entropies for alkanes and provide insight for designing and optimizing zeolite-catalyzed hydroisomerization processes.

A microscopic insight into hybrid CH₄ physisorption-hydrate formation in halloysite nanotubes (HNTs) is vital for understanding the solidification storage of natural gas in the HNTs and developing energy storage technology. Herein, large-scale microsecond classical molecular dynamics simulations are conducted to investigate CH₄ storage in the HNTs via the adsorption-hydration hybrid (AHH) method to reveal the effect of gas-water ratio. The simulation results indicate that the HNTs are excellent nanomaterials for CH₄ storage via the adsorption-hydration hybrid method. The CH₄ physisorption and hydrate formation inside and outside of the HNTs are profoundly influenced by the surface properties of the HNTs and the kinetic characteristics of CH₄/H₂O molecules. The outer surfaces of the HNTs exhibit relative hydrophobicity and adsorb CH₄ molecules to form nanobubbles. Moreover, CH₄ molecules adsorbed on the outer surface are tightly trapped between the hydrate solids and the outer surface, inhibiting their kinetic behavior and favoring CH₄ storage via physisorption. The inner surface of the HNTs exhibits extreme hydrophilicity and strongly adsorbs H₂O molecules; thus, CH₄ hydrate can form inside of the HNTs. It is more difficult for CH₄ and H₂O molecules inside of the HNTs to convert into hydrates than for those outside of the HNTs. A moderate gas-water ratio is advantageous for CH₄ physisorption and hydrate formation, whereas excessively high or low gas-water ratios are unfavorable for efficient CH₄ storage. These insights can help to develop an efficient CH₄ solidification storage technology.

Thermodynamic factors for diffusion connect the Fick and Maxwell-Stefan diffusion coefficients used to quantify mass transfer. Activity coefficient models or equations of state can be fitted to experimental or simulation data, from which thermodynamic factors can be obtained by differentiation. The accuracy of thermodynamic factors determined using indirect routes is dictated by the specific choice of an activity coefficient model or an equation of state. The Permuted Widom’s Test Particle Insertion (PWTPI) method developed by Balaji et al. enables direct determination of thermodynamic factors in binary and multicomponent systems. For highly dense systems, for example, typical liquids, it is well known that molecular test insertion methods fail. In this article, we use the Continuous Fractional Component Monte Carlo (CFCMC) method to directly calculate thermodynamic factors by adopting the PWTPI method. The CFCMC method uses fractional molecules whose interactions with their surrounding molecules are modulated by a coupling parameter. Even in highly dense systems, the CFCMC method efficiently handles molecule insertions and removals, overcoming the limitations of the PWTPI method. We show excellent agreement between the results of the PWTPI and CFCMC methods for the calculation of thermodynamic factors in binary systems of Lennard-Jones molecules and ternary systems of Weeks-Chandler-Andersen molecules. The CFCMC method applied to calculate the thermodynamic factors of realistic molecular systems consisting of binary mixtures of carbon dioxide and hydrogen agrees well with the NIST REFPROP database. Our study highlights the effectiveness of the CFCMC method in determining thermodynamic factors for diffusion, even in densely packed systems, using relatively small numbers of molecules.