Diffusion mechanism of dissolved gases in transformer mineral oil under furfural effect.

Chitosan nanocomposite drug delivery systems designed for the ifosfamide anticancer drug using molecular dynamics simulations

Molecular simulation of CO2 capturing by dual functionalized phosphonium-based amino acid ionic liquids

We investigate the adsorption and diffusion behaviors of CO2, CH4, and N2 in interfacial systems composed of a polymer of intrinsic microporosity (PIM-1) and amorphous silica using grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. We build model systems of mixed matrix membranes (MMMs) with PIM-1 chains sandwiched between silica surfaces. Gas adsorption analysis using GCMC simulations shows that gas molecules are preferentially adsorbed in microcavities distributed near silica surfaces, resulting in an increase in the solubility coefficients of CO2, CH4, and N2 compared to bulk PIM-1. In contrast, diffusion coefficients obtained from MD simulations and then calibrated using the dual-mode sorption model show different tendencies depending on gas species: CO2 diffusivity decreases in MMMs compared to PIM-1, whereas CH4 and N2 diffusivities increase. These differences are attributed to competing effects of silica surfaces: the emergence of larger pores as a result of chain packing disruption, which enhances gas diffusion, and a quadrupole-dipole interaction between gas molecules and silica surface hydroxyl groups, which retards gas diffusion. The former has a greater impact on CH4 and N2 diffusivities, whereas the latter has a greater impact on CO2 diffusivity due to the strong quadrupole-dipole interaction between CO2 and surface hydroxyls. These findings add to our understanding of gas adsorption and diffusion behaviors in the vicinity of PIM-1/silica interfaces, which are unobtainable in experimental studies.

Molecular dynamics (MD) simulation was employed to investigate diffusion behavior of small penetrants in rubbery-polymer-based hybrid membranes, using pervaporative removal of benzene from its dilute solution by poly(dimethylsiloxane) (PDMS) membranes filled with calix[4]arene (CA) as the model system. In our previous experimental investigation, the normalized permeation rate of benzene (NPRb) and separation factor (benzene/water) through PDMS−CA hybrid membranes did not follow the usual monotonous or single peak/valley change, but accompanied minimum and maximum values instead. In the present study, nonbonding interaction energy between PDMS and CA, mean-square displacement (MSD), free volume characteristics, and diffusion coefficients of benzene and water in pure PDMS and hybrid membranes were analyzed by molecular dynamics simulation. The simulation results revealed that MSD and fractional free volume (FFV) values were closely dependent on interaction energy. Diffusion coefficients of benzene and water at “infinite dilution” and saturated condition displayed the same changing tendency, although the values at saturated condition were a bit larger. Moreover, it was observed that diffusion coefficients were not only related to FFV but also affected by the interaction between CA and the penetrants. Overall, the MD results agreed well with the experimental results.

Quantum and dynamic investigations of Complex iron- alkaloid-extract Cytisine derivatives of Retama monosperma (L.) Boiss. Seeds as eco-friendly inhibitors for Mild steel corrosion in 1M HCl

Understanding the underlying processes associated with the viscoelasticity performance of ethylene-propylene-diene monomer (EPDM) during its service life is essential for assessing and predicting its waterproofing performance in underground infrastructure. The viscoelasticity of the polymer is closely related to its free volume, and both of these properties depend on multiple factors, such as temperature, stress magnitude, and strain level. To explore the fundamental viscoelastic behavior of EPDM using free volume as a proxy for viscoelasticity, this article investigates the influence of temperature, stress magnitude, and strain level, as well as their combined effect, on the free volume through molecular dynamics (MD) simulations. An EPDM cross-linked molecular model was built and verified by comparing the simulation values of glass transition temperature, mechanical properties, and gas diffusivity with the experimental results reported in the literature. Then, the dependence of EPDM’s fractional free volume on temperature, strain, and their combined effect was investigated via MD simulations, on the basis of which the applicability of various superposition principles was also evaluated.

In this research, molecular dynamics (MD) simulations were used to study the transport properties of small gas molecules in the butadiene-styrene copolymer (SBR). The condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS) force field was applied. The diffusion coefficients were obtained from MD (NVT ensemble) and the relationship between gas permeability; the chemical structure and free volume of butadiene-styrene copolymer were investigated. The results indicated that the diffusion coefficient of oxygen declined with increasing styrene content. The fraction of free volume (FFV) in butadiene-styrene copolymer was calculated. It was concluded that diffusion coefficient increased as the FFV increases, which is in accordance with the analysis of the small molecular hop through the free volume in polymer matrix. Subsequently, the glass transition temperatures of these copolymers were calculated by MD. The result showed that the glass transition temperature increased with increasing styrene content in polymer.

Dissolved Gas Analysis (DGA) of insulating oil is widelyused fordiagnosing transformer incipient faults. Moisture is a major contaminantand degradation byproduct of transformer insulating oil. In this paper,molecular dynamics simulation was used to study the influence of moistureon the diffusion movement of dissolved gases in the insulating oil.Cycloalkanes (C20H42), alkanes (C20H38), and aromatic hydrocarbons (C20H26) are selected as the basic structural units in the constructed transformeroil simulation system. 0%, 1%, 3%, and 5% moisture molecules are addedto insulating oil, respectively, and the insulating oil generatesseven kinds of gas molecules through cracking. With an anhydrous modelused as a benchmark, we researched the diffusion trajectory, the diffusioncoefficient (D), free volume (VF), and the moisture–gas interaction energy of eachgas molecule as a function of moisture content. Through this study,we found that the increase of moisture content enlarges the VF value of dissolved gas in insulating oil,which makes the gas in oil easier to diffuse. Besides, the moisturecan also alter the mean square displacement (MSD) of dissolved gases;the mutual energy of molecules is mainly affected by the electrostaticinteraction energy. This study can contribute to a better understandingof the influence of different moisture contents on the diffusion movementof dissolved gas in transformer oil from the micro level.

ABSTRACTAccurate diffusion coefficient data of additives in a polymer are of paramount importance for estimating the migration of the additives over time. This paper shows how this diffusion coefficient can be estimated for three plastic additives [2-(2ʹ-hydroxy-5ʹ-methylphenyl) (UV-P), 2,6-di-tert-butyl-4-methylphenol (BHT) and di-(2-ethylhexyl) phthalate (DEHP)] in polyethylene terephthalate (PET) using the molecular dynamics (MD) simulation method. MD simulations were performed at temperatures of 293–433 K. The diffusion coefficient was calculated through the Einstein relationship connecting the data of mean-square displacement at different times. Comparison of the diffusion coefficients simulated by the MD simulation technique, predicted by the Piringer model and experiments, showed that, except for a few samples, the MD-simulated values were in agreement with the experimental values within one order of magnitude. Furthermore, the diffusion process for additives is discussed in detail, and four factors – the interaction energy between additive molecules and PET, fractional free volume, molecular shape and size, and self-diffusion of the polymer – are proposed to illustrate the microscopic diffusion mechanism. The movement trajectories of additives in PET cell models suggested that the additive molecules oscillate slowly rather than hopping for a long time. Occasionally, when a sufficiently large hole was created adjacently, the molecule could undergo spatial motion by jumping into the free-volume hole and consequently start a continuous oscillation and hop. The results indicate that MD simulation is a useful approach for predicting the microstructure and diffusion coefficient of plastic additives, and help to estimate the migration level of additives from PET packaging.

Free volume and alcohol transport properties of PDMS membranes: Insights of nano-structure and interfacial affinity from molecular modeling

Natural ester, as a new environmentally green insulating oil, has been widely used in transformer. In an oil-immersed transformer, the normal aging, thermal failure, and discharge failure could easily lead to the decomposition of the oil-paper insulation system and produce different kinds of gases. Studying gas dissolution in natural ester and mineral oil could provide assistance in applying criteria to make a diagnosis of different kinds of faults in the transformer. In this paper, the molecular dynamics method was used to investigate the diffusion behavior of seven fault characteristic gases (including H2, CO, CH4, C2H2, CO2, C2H4, C2H6) in natural ester and mineral oil. The simulation parameters of free volume, interaction energy, mean square displacement, and diffusion coefficient were compared between the natural ester and mineral oil. Meanwhile, the influence of temperature on the diffusion of gas molecules in two kinds of oils was also analyzed. Results showed that the free volume, the interaction energy, and the relative molecular mass of gas molecules were the factors influenced by the diffusion of gas molecules in natural ester and mineral oil. The order of the diffusion coefficients of gas molecules in natural ester was as follows: H2 > CH4 > CO > C2H2 > C2H4 > CO2 > C2H6 and that in mineral oil was as follows: H2 > CH4 > CO> C2H2 > C2 H4 > C2H6 > CO2. By comparing the diffusion behavior of gas molecules in natural ester and mineral oil, it was found that the smaller free volume and higher interaction energy of gas molecules in natural ester were the major reasons for the gas molecules to be more difficult to diffuse in natural ester. The rising temperature could enhance the free volume and reduce the interaction energy between gas molecules and oil. The diffusion coefficient of gas molecules increased exponentially with the follow of temperature. However, the temperature didn’t affect the ordering of diffusion coefficient, free volume, and interaction energy of gas molecules in natural ester and mineral oil.

The diffusion behaviour of hydrogen, carbon monoxide, carbon dioxide, methane, acetylene, ethylene and ethane in oil and paper medium was examined using molecular dynamics to reveal the diffusion mechanism of gas molecules in transformer oil–paper insulation system at the microscopic level. These compounds are commonly used in the dissolved gas analysis of power transformers and produced during the ageing process of oil–paper composite insulating material. Two groups of models were constructed using molecular dynamics simulation software to simulate the diffusion behaviour of the aforementioned seven types of small gas molecules in oil and paper. The diffusion coefficients, displacement features, free volume characteristics and interaction energies of the gas molecules were investigated. In particular, the diffusion micro-mechanism of the gas molecules was observed. The differences in diffusion features among the gas molecules were discussed, and the factors influencing the diffusion of the gas molecules were compared. Simulation results indicate that the diffusion coefficients of gas molecules in cellulose is an order of magnitude lower than that in oil, and the diffusion coefficients of these gas molecules in the two types of insulation media have different orders. Free volume of gas molecules is the main factor that influences the diffusion behaviour in oil, whereas intermolecular interaction is the main influencing factor of diffusion behaviour in cellulose.

Molecular dynamics (MD) simulation was used to study the structural and diffusive properties of zeolitic imidazolate framework-8 (ZIF-8)/polydimethylsiloxane (PDMS), a novel alcohol-permselective mixed matrix membrane (MMM). Simulation models of one pure PDMS membrane and three ZIF-8/PDMS MMMs with increasing loadings were successfully constructed. Non-bond energy turned out to be a strong attractive interaction between the PDMS matrix and ZIF-8 cells. The morphology and mobility of PDMS chains were characterized by mean square displacement (MSD). The fraction of free volume (FFV) of the pure membrane and MMMs was calculated and showed declining trends with increasing ZIF-8 loadings. The diffusion coefficients of n-butanol and water molecules were calculated by the Einstein relation. first increased then decreased, while decreased with the increasing loadings. The mechanism of selective diffusion behaviour was investigated and it was found that the inner channels of ZIF-8 provided selective pathways for n-butanol. Diffusion coefficients were correlated with FFV and the results showed that the logarithm of demonstrated a good linear relation with the inverse FFV and was in agreement with the free volume theory, while showed a significant deviation in the case of MMM-1 due to the selective diffusion channels provided by ZIF-8.

Viscosity and viscosity index are the crucial properties of lubricant base stocks. Molecular dynamics simulation and quantum calculation were used to simulate the five isomers of C26H54 to study the intrinsic relationship between viscosity, viscosity index, and the molecular structure of isoalkanes. The results showed that the intermolecular interaction energy and the volume of rigid-like groups were the intrinsic factors that affected the viscosity and which could describe the viscosity quantitatively. The molecule conformation was studied by calculating the rotational energy barrier of the dihedral angle in the isoalkane molecule, and combined with molecular dynamics, the effect of temperature on the molecular conformation at 313K and 373K was further investigated. The α, β, and γ carbon atoms adjacent to the tertiary carbon in the isoalkane molecule were difficult to rotate due to steric hindrance and could be regarded as rigid-like groups at 313K. The tertiary carbon and the three adjacent carbon atoms formed a regular tetrahedral rigid-like group at 373K. The changes in the intermolecular interaction energy and the volume of the rigid-like group with temperatures could better describe the viscosity index and reveal the fundamental reasons that affect the viscosity and the viscosity index. The molecular-level understanding of the relationship between the molecular structure and properties of isoalkanes provided theoretical support and scientific guidance for designing isoalkane molecules with specific properties. Molecular dynamics simulation and quantum calculation were performed using Material Studio 8.0 software. The Amorphous Cell module was used to create an amorphous cell. The Foricite module was used for molecular dynamics simulation; the forcefield was assigned as COMPASS II. Nose-Hoover thermostat and Berendsen barostat were applied to maintain the temperature and pressure, respectively. To describe the non-bond interactions, the Ewald method was applied to calculate the van der Waals and electrostatic interactions. The Conformers module was used to study the conformation and the Dmol3 module was used to calculate the conformational energy with fine quality; the functional of GGA-PW91 and the basis set of DNP were used to calculate the energy.

Effect of side branch on gas separation performance of triptycene based PIM membrane: A molecular simulation study