Decrease In Diffusion Coefficient Research Articles

Nanorod Diffusion near the Solid-Liquid Interface with Varied Wall Nonuniformity.

The complex diffusion behaviors of rod-shaped nanoparticles near the solid-liquid interface are closely related to various biological processes and technological applications. Despite recent advancements in understanding the diffusion dynamics of nanoparticles near some specific solid-liquid interfaces, systematical studies to tune the interfacial interaction or fabricating nonuniform wall to see their effects on the nanorod (NR) diffusion are still lacking. This work utilized molecular dynamics simulations to investigate the rotational and translational diffusion dynamics of a single NR near the solid-liquid interface. We constructed a patterned wall featuring adjustable nonuniformity, which was accomplished by modifying the interaction between NR and the wall, noting that the resulting nonuniformity limits both the translational and rotational diffusion of NR, evident from decreases in diffusion coefficients and exponents. By trajectory analysis, we categorized the diffusion modes of NRs near the patterned wall with varied nonuniformities into three types: Fickian diffusion, desorption-mediated flight, and in-plane diffusion. Furthermore, energy analysis based on the adsorption-desorption mechanism has demonstrated that the three diffusion states are driven by interactions between the NR and the wall, which are primarily influenced by rotational diffusion. These results could significantly deepen the understanding of anisotropic nanoparticle interfacial diffusion and would provide new insights into the transport mechanisms of nanoparticles within confined environments.

Cerebral Small Vessel Disease in Children and Adolescents with Type 1 Diabetes Presenting with Diabetic Ketoacidosis during COVID-19 Era

Abstract Aim Cerebrovascular insult (CVI) is a known and significant morbidity of diabetic ketoacidosis (DKA); yet, its prevalence is underestimated. Untreated cerebral hypoperfusion in DKA may lead to cerebral injury, arterial ischemic stroke, cerebral venous thrombosis, and hemorrhagic stroke. This study assesses the cerebral perfusion status among children and adolescents with DKA during and after the resolution of DKA. Methodology Forty children and adolescents with type 1 diabetes mellitus (T1DM) presenting with DKA were assessed for severity of DKA, diabetes-duration, insulin therapy, Rappaport coma/ Near coma scale, Rancho los amigos levels of cognitive functioning scale, glycated-hemoglobin (HbA1c), D-dimer, INR and brain magnetic resonance imaging (MRI) during DKA and repeated neuro cognitive scoring, laboratory and MRI after resolution of the DKA. Results The mean age of the children and adolescents with T1DM presenting with DKA was 11.40 ± 2.82 years; their median diabetes-duration was 3 year; their mean HbA1c was 13.54 ± 2%. There was significant increase in platelet (P = 0.005), D-dimer (P < 0.001), Creatinine (P < 0.001) and Pulse wave velocity (P = 0.004) during attack of DKA while there was significance decrease in Apparent diffusion coefficient (P < 0.001). There was 30% of children and adolescents with T1DM had ischemic MRI changes during DKA. In children and adolescent with ischemic changes there was significant increase in severity of DKA (P = 0.003), HbA1c (P = 0.016), INR (P = 0.002), D-dimer (P < 0.001), Creatinine (P = 0.003). Conclusion Significant cerebral hypoperfusion occur during the DKA which is correlated with the DKA severity and D-dimer levels.

Receptor binding and tortuosity explain morphogen local-to-global diffusion coefficient transition

Morphogens are intercellular signaling molecules providing spatial information to cells in developing tissues to coordinate cell fate decisions. The spatial information is encoded within long-ranged concentration gradients of the morphogen. Direct measurement of morphogen dynamics in a range of systems suggests that local and global diffusion coefficients can differ by orders of magnitude. Further, local diffusivity can be large, which would potentially abolish any concentration gradient rapidly. Such observations have led to alternative transport models being proposed, including transcytosis and cytonemes. Here, we show that accounting for tissue architecture combined with receptor binding is sufficient to hinder the diffusive dynamics of morphogens, leading to an order of magnitude decrease in the effective diffusion coefficient from local to global scales. In particular, we built a realistic in silico architecture of the extracellular spaces of the zebrafish brain using light and electron microscopy data. Simulations on realistic architectures demonstrate that tortuosity and receptor binding within these spaces are sufficient to reproduce experimentally measured morphogen dynamics. Importantly, this work demonstrates that hindered diffusion is a viable mechanism for gradient formation, without requiring additional regulatory control.

Effect of purity on solidification structure and micro-segregation in a nickel-based single crystal superalloy

The purity of nickel-based single crystal alloys plays an important role in the final service performance of turbine blades as it impacts the solidification structure, elemental segregation, and inclusions of castings. The highest purity of superalloys studied before was of altogether around 10 ppm of contents of N and O produced by the triple purification process (VIM+ESR+VAR). However, the impact mechanism still remains controversial, and meanwhile whether a lower content of N and O still keeps the impacts on castings is not yet clear due to the lack of raw materials. A superhigh purity of second generation of nickel-based single crystal superalloy DD98M with the sum of N and O contents as low as 4 ppm (hitherto the highest purity of the superalloy as published), home-produced by electron beam smelting (EBS), was put on focus in this work. The effect of three levels of purity (N-doped, commercial, and EBS, corresponding to 21, 10, and 4 ppm for the sum of N and O contents, respectively) on the microstructure evolution (including primary dendrite arm spacing (PDAS), eutectic, micropores, and γ′ phase) and the solute segregation behavior at dendritic scale and nano scale (γ/γ′ interfaces) were systematically studied and the influencing mechanisms were elucidated. The results show that the PDAS decreased from 406 μm to 322 μm with the reduction of O and N from 21 ppm to 4 ppm, which was irrelevant to precipitation of the nitride or oxide inclusions. Instead, it was due to the decrease of diffusion coefficient D or the increase of equilibrium coefficient k, or the decrease of the critical nucleation supercooling and the incubation time of dendrites. The micro-segregation, the number and volume fraction of eutectic, and the number and voltage percentage of shrinkage pores and gas pores were all greatly decreased with the increase of purity. These changes were closely related to the reduction of PDAS and the reduction of fluidity of the remained liquid in the interdendrites during solidification. The morphology and the size of the γ′ phase were independent from the purity, while the segregation of alloying elements at the γ/γ′ interface changed remarkably with the variation of the purity.

Adsorption of Diffusing Tracers, Apparent Tortuosity, and Application to Mesoporous Silica.

The apparent tortuosity due to adsorption of diffusing tracers in a porous material is determined by a scaling approach and is used to analyze recent data on LiCl and alkane diffusion in mesoporous silica. The slope of the adsorption isotherm at small loadings is written as β = qA/qG, where qA is the adsorption-desorption ratio and qG = ϵ/(as) - 1 is a geometrical factor depending on the range a of the tracer-wall interaction, the porosity ϵ, and the specific surface area s. The adsorption leads to a decrease of effective diffusion coefficient, which is quantified by multiplying the geometrical tortuosity factor τgeom by an apparent tortuosity factor τapp. In wide pores or when the adsorption barrier is high, τapp = β + 1, as obtained in previous works, but in narrow pores there is an additional contribution from frequent adsorption-desorption transitions. These results are obtained in media with parallel pores of constant cross sections, where the ratio between the effective pore width ϵ/s and the actual width is ≈0.25. Applications to mesoporous silica samples are justified by the small deviations from this ideal ratio. In the analysis of alkane self-diffusion data, the fractions of adsorbed molecules predicted in a recent theoretical work are used to estimate τgeom of the silica samples, which is ≫1 only in the sample with the narrowest pores (nominal 3 nm). The application of the model to Li+ ion diffusion leads to similar values of τgeom and to a difference of energy barriers of desorption and adsorption for those ions of ∼0.06 eV. Comparatively, alkane self-diffusion provides the correct order of magnitude of τgeom, with adsorption playing a less important role, whereas adsorption effects on Li+ diffusion are much more important.

Hydrogen isotope permeation and retention behavior in the RAFM steel manufactured by laser powder bed fusion

Laser powder bed fusion (LPBF) has emerged as a promising additive manufacturing technique for producing complex components across various industries. This method is particularly instrumental in fabricating reduced activation ferritic/martensitic (RAFM) steel, a candidate structural material for the tritium breeding blanket. Hydrogen transport behaviors in the conventional RAFM steel have been largely investigated; however, they are not well understood in the LPBF-RAFM steel. In this work, we studied the hydrogen isotope permeation and retention behavior in the RAFM steel fabricated by LPBF, utilizing the deuterium gas driven permeation and thermal desorption spectroscopy experiments. The results show that the hydrogen isotope permeability of the LPBF-RAFM steel is close to the conventionally fabricated RAFM steel, but the diffusion coefficient is greatly lower and the retention is markedly higher in the LPBF-RAFM steel compared to the conventional RAFM steel. The large number density of microvoids and TiC precipitates in the LPBF-RAFM steel could be the predominant factors that result in the decrease in hydrogen isotope diffusion coefficient and the increase in the total amount of deuterium retention.

Analysis of Pressure Characteristics of Ultra-High Specific Energy Lithium Metal Battery for Flying Electric Vehicles

The lithium metal battery is likely to become the main power source for the future development of flying electric vehicles for its ultra-high theoretical specific capacity. In an attempt to study macroscopic battery performance and microscopic lithium deposition under different pressure conditions, we first conduct a pressure cycling test proving that amplifying the initial preload can delay the battery failure stage, and the scanning electron microscope (SEM) shows that the pressure is effective in improving the electrode’s surface structure. Secondly, we analyze how differing pressure conditions affect the topography of lithium deposits by coupling the nonlinear phase-field model with the force model. The results show that the gradual increase in the external pressure is accompanied by a drop in the length of the dendrite and the migration curvature in the diaphragm, and the deposition morphology is gradually geared towards smooth and thick development, which can significantly reduce the specific surface area of lithium dendrite. However, as cyclic charging and discharging continue, the decrease in the electrolyte diffusion coefficient results in higher internal stress inside the battery, and thus the external pressure must be increased so as to achieve marked inhibitory effects on the growth of the lithium dendrite.

Nucleation and growth mechanism of Ni/SiC composite coatings electrodeposited with micro- and nano-SiC particles

To study the nucleation and growth mechanism of Ni/SiC composite coatings, electrodeposition was conducted in the Ni bath containing SiC particles with different sizes. Compared to the Ni coating electrodeposited from the bare Ni bath, the Ni/SiC composite coatings exhibited similar grain size but rougher surface morphology due to the incorporation of SiC particles. By means of chronoamperometry (CA) and linear sweep voltammetry (LSV) techniques, it was found that the introduction of SiC particles did not change the nucleation mode of Ni: the Scharifker-Hills three-dimensional instantaneous nucleation mode. However, the nucleation sites on the cathode were reduced due to the inert surface and low conductivity of SiC particles. At a step potential of −0.94 V, Ni presented the maximum nucleation density (N0) of 3.09 × 107 cm−2, which was reduced by one order of magnitude for the Ni/SiC. Furthermore, SiC particles in the bath would extend the diffusion path of Ni2+ ions, resulting in the decrease in diffusion coefficient from 1.04 × 10−7 cm2 s−1 (Ni) to 9.91 × 10−9 cm2 s−1 (Ni/50 nm SiC), 1.83 × 10−8 cm2 s−1 (Ni/500 nm SiC) and 2.29 × 10−8 cm2 s−1 (Ni/5 μm SiC), suggesting the “Blocking effect”. COMSOL simulation further confirmed the “Blocking effect”. The “Blocking effect” would lead to excessive interface gaps around 5 μm SiC particles due to the uneven electrodeposition of Ni. To address the issue, we proposed that adding non-ionic surfactants into the bath to lower the electrochemical reaction rate of Ni2+ ions and accordingly gained composite coatings with fine continuity.

Effect of modified graphene nanosheets on the properties of 222Rn and gamma ray multifunctional shielding polymer composite materials

AbstractDevelopment of shielding materials for 222Rn and γ radiation has crucial implications for ensuring the safety of individuals. This paper reports the synthesis of modified graphene nanosheets (MGNPs) via the reaction of KH560 with graphene nanosheets (GNPs), while melt blending and hot‐press molding technique were used to fabricate a multifunctional polymer composite shielding material, MGNP/WB/PMMA (polymethyl methacrylate). Successful synthesis of MGNP was confirmed by Fourier transform infrared spectroscopy, and scanning electron microscope (SEM) analysis was utilized to assess the distribution of functional fillers within the cross‐section of the polymer composite. Additionally, thermogravimetric analysis (TGA) demonstrated that MGNP and WB particles enhance the thermal stability of the polymer composite materials. Compared to pure PMMA, 98.7% decrease in the radon diffusion coefficient was observed for MGNP1.5 wt%/PMMA composite material. In addition to enhance the radon‐blocking characteristics of the polymer composites, inclusion of WB particles also boosts their shielding capacity against gamma radiation. The 222Rn diffusion coefficient of MGNP1.5 wt%/WB25wt%/PMMA polymer composite material decreased by 99.6% and at energies of 60 KeV, 80 KeV, 122 KeV, 365 KeV, the mass attenuation coefficient (MAC) for the composite material increased by 0.79, 1.97, 0.57, and 0.05 cm2/g, respectively, compared to pure PMMA.Highlights The PMMA composite materials doped with graphene nanosheets demonstrate outstanding resistance to 222Rn. Graphene nanosheets modified by KH560 exhibit favorable dispersion within PMMA. WB can further enhance the radon resistance of MGNP/WB/PMMA composite materials. The polymer composites exhibit excellent 222Rn and gamma ray shielding properties.

High-resolution Diffusion-weighted Imaging to Detect Changes in Tumor Size and ADC, and Predict Adverse Biopsy Histology during Prostate Cancer Active Surveillance.

Majority of men with low-risk prostate cancer can be managed with active surveillance (AS). This study evaluates a high-resolution diffusion-weighted imaging (HR-DWI) technique to predict adverse biopsy histology (AH), defined as Gleason score ≥7 on any biopsy or ≥3 increase in number of positive biopsy cores on systematic biopsies. We test the hypothesis that high-grade disease and progressing disease undergo subtle changes during even short intervals that can be detected by HR-DWI. In a prospective clinical trial, serial multiparametric MRIs, incorporating HR-DWI and standard DWI (S-DWI) were performed approximately 12 months apart prior to prostate biopsy (n = 59). HR-DWI, which uses reduced field-of-view and motion compensation techniques, was compared with S-DWI. HR-DWI had a 3-fold improvement in spacial resolution compared with S-DWI as confirmed using imaging phantoms. For detecting AH, multiparametric MRI using HR-DWI had a sensitivity of 75% and specificity of 83.9%, and MRI using S-DWI had a sensitivity of 71.4% and specificity of 54.8%. The AUC for HR-DWI was significantly higher (0.794 vs. 0.631, P = 0.014). Secondary analyses of univariable predictors of AH showed tumor size increase [OR 16.8; 95% confidence interval (CI): 4.06-69.48; P < 0.001] and apparent diffusion coefficient (ADC) decrease (OR 5.06; 95% CI: 1.39-18.38; P = 0.014) on HR-DWI were significant predictors of AH. HR-DWI outperforms S-DWI in predicting AH. Patient with AH have tumors that change in size and ADC that could be detected using HR-DWI. Future studies with longer follow-up should assess HR-DWI for predicting disease progression during AS. We report on a prospective clinical trial using a MRI that has three times the resolution of standard MRI. During AS for prostate cancer, two high-resolution MRIs performed approximately a year apart can detect tumor changes that predict the presence of aggressive cancers that should be considered for curative therapy such as prostatectomy or radiation.

Effect of guar gum on dynamic wetting the fat coal surface with octyl phenol polyoxyethylene ether solution

To study the effect of guar gum (GG) on the surface of octyl phenol polyoxyethylene ether dynamic wetting fat coal, the effect of GG addition concentration on physical parameters, sedimentation rate, contact angle, maximum dimensionless wetting length (Ldw(max)) and maximum dimensionless wetting area (Adw(max)) in mixed dust suppression solutions was studied by experimental and molecular dynamic simulation methods. The results show that adding GG does not significantly alter the mixed dust suppression solution surface tension, but it increases the solution viscosity. When the GG concentration is greater than 0.1%, the viscosity of the mixed dust suppression solution increasing extent becomes larger. The impact wetting experiment and contact angle experiment show that when GG concentration is 0.01%, the mixed dust suppression solution contact angle is the smallest, the Ldw(max) and Adw(max) are the largest, and the solution wettability is the best. Compared with the Ldw(max), the Adw(max) has a smaller standard deviation and higher repeatability. Molecular dynamics simulation shows that after adding a trace amount of GG to the water/OP surfactant/fat coal system, the relative concentration coincidence area and interaction energy absolute value between coal and water in the system increases, the water molecules diffusion coefficient decreases, and the system wettability is further enhanced, which further illustrates the experimental results accuracy.

Low-Temperature Characterization of a Nonaqueous Liquid Electrolyte for Lithium Batteries

Rechargeable batteries exhibit poor performance at low temperatures due to sluggish ion transport through the electrolytic phase. Ion transport is governed by three transport parameters—conductivity, diffusion coefficient, and the cation transference number with respect to the solvent velocity—and the thermodynamic factor. Understanding how these parameters change with temperature is necessary for designing improved electrolytes. In this work, we combine electrochemical techniques with electrophoretic NMR to determine the temperature dependence of these parameters for a liquid electrolyte, LiTFSI salt dissolved in tetraglyme between −20 and 45 °C. At colder temperatures, all species in the electrolyte tend to move more slowly due to increasing viscosity, which translates to a monotonic decrease in conductivity and diffusion coefficient with decreasing temperature. Surprisingly, we find that the field-induced velocity of solvent molecules at a particular salt concentration is a nonmonotonic function of temperature. The cation transference number with respect to the solvent velocity thus exhibits a complex dependence on temperature and salt concentration. The measured thermodynamic and transport properties are used to predict concentration gradients that will form in a lithium-lithium symmetric cell under a constant applied potential as a function of temperature using concentrated solution theory. The calculated steady current at −20 °C is lower than that at 45 °C by roughly two orders of magnitude.

Enhancing the Proton Exchange Membrane in Tubular Air-Cathode Microbial Fuel Cells through a Hydrophobic Polymer Coating on a Hydrogel.

The usage time of air-cathode microbial fuel cells (MFCs) is significantly influenced by the moisture content within the proton exchange membrane (PEM). Therefore, enhancing the water retention capability of the PEM by applying a hydrophobic polymer coating to its surface has extended the PEM's usage time by three times and increased MFCs' operational duration by 66%. Moreover, the hydrophobic nature of the polymer coating reduces contamination on the PEM and prevents anode liquid from permeating into the air cathode. Towards the end of MFC operation, the internal resistance of the MFC is reduced by 45%. The polymer coating effectively maintained the oxygen reduction reaction activity in the cathode. The polymer coating's ability to restrict oxygen transmembrane diffusion is demonstrated by experimental data showing a significant decrease in oxygen diffusion coefficient due to its presence. The degradation efficiency of the chemical oxygen demand from 16% to 35% increased by a factor of one.

High-performance of Mg2+ additive for addressing NH3 escape in inorganic ammonia carbon capture

The urgency to reduce the energy consumption of organic amine carbon capture, while the inorganic ammonia carbon capture (IACC) and co-production of ammonium fertilizer eliminates the desorption energy consumption. Inhabiting NH3 escape at the end of IACC process requires a large amount of water consumption, and there is a lack of corresponding process parameters and technical routes. In this paper, Mg2+, one of major components in desulfurization wastewater, was used as an NH3 inhibitor. It’s found that the addition of MgCl2 can increase the total NH3 absorption rate by 13.02 % and reduce the total NH3 escape rate by 87.02 %. Furthermore, the effects of six process parameters in the forward and reverse NH3 mass transfer experiments could be distinctly quantified by the normalization method to determine the differentiation between priority and others. It’s concluded that NH3 can enter the first solvent shell of Mg2+ to replace some H2O based on the first-principles calculations, which is the main cause for the decrease of NH3 diffusion coefficient. Finally, simulation with Aspen Plus was applied to construct the complete process route, and the Arrhenius formula for the Mg(OH)2 nucleation was used to refine the reaction kinetic parameters of the simulation. The heat and water balance of IACC could achieve within this system, and the excellent NH3 inhibition effect of added Mg2+ was verified, which could effectively keep from the NH3 escape within 3 ppm.

Compressive strength and chloride permeability of cement-based materials with high-volume compound mineral admixtures

Ground granulated blast-furnace slag (GGBFS) is well known to be capable of improving the performance of cement-based materials, but few studies focus on the impact of its large dosage on concrete containing basalt powder. In this study, the compressive strength and chloride permeability of cement-based materials with compound mineral admixtures (CMAs) containing high-volume GGBFS, basalt powder and desulfurisation gypsum were investigated. The results showed that the mortar strengths at 3, 7 and 28 days decreased with increasing GGBFS content, but that at 56 and 84 days increased with the addition of 45 wt% GGBFS. This is because the activity of GGBFS at an early stage has not been stimulated and the pozzolanic effect is exerted at a later stage. Moreover, cement replacement with up to 55 wt% GGBFS caused a significant decrease in the chloride diffusion coefficient of the mortar and concrete. Furthermore, the incorporation of GGBFS led to a remarkable refinement in pore structure of the hardened paste due to pozzolanic and filler effects. Therefore, the partial replacement of cement with high CMA contents (≥70 wt%) in concrete is desirable for ocean projects requiring low chloride permeability, and significantly reduces carbon dioxide emissions.

Grand canonical Monte Carlo and molecular dynamics investigation of hydrogen solubility and diffusivity in nonmetallic polyvinyl chloride, polyethylene and polyvinylidene fluoride pipes materials

To choose appropriate nonmetallic pipe materials for reducing hydrogen permeation, the hydrogen permeability in polyvinyl chloride, polyethylene, and polyvinylidene fluoride are studied and compared by molecular dynamics simulations. The findings demonstrate the solubility coefficient of hydrogen in polyvinyl chloride rises as the temperature decreases. However, in polyethylene and polyvinylidene fluoride, the trend is reversed. With the decrease of temperature, the diffusion and permeability coefficients decrease. The diffusion of hydrogen molecules follows the jumping mechanism. Among the materials studied, polyvinylidene fluoride is the most suitable material for hydrogen pipes due to its superior resistance to hydrogen permeation.

Mitigating chloride attack in cementitious materials without compromising other properties via the use of viscosity modifying admixture

To mitigate the harmful chemicals ingression into cementitious materials, traditional methods often aim to densify the pore structure, posing challenges for concrete dead-load or crack control. This paper proposes a facile method to mitigate chloride attack via increasing the pore solution viscosity in cementitious material without changing the matrix density or compromising other properties. The results indicate that the addition of viscosity modifying admixtures (VMAs) to the mortar, as an equivalent mass replacement of mixing water, can mitigate chloride ingress. Although it reduces the early age hydration heat, the mechanical strength at full age (3 d–28 d) and the porosity at 28 d remain unchanged when using an effective VMA (PEG1000). In addition, the matrix resistance to leaching is improved as well. These results may be attributed to the enhanced pore solution viscosity and the coexistence of VMA and C–S–H with decreased interlayer distance and shortened mean chain length. After 28 d of exposure, the most effective experimental group (PEG1000) showed approximately a 28 % decrease in chloride diffusion coefficient compared to the control group. This paper may inspire new technology to mitigate ions diffusion and promote its practical application.

Manifestation of site energy landscapes for ion transport in borate glasses.

The potential energy landscape of lithium borate glass of composition Li3B7O12 has been investigated by the charge attachment induced transport (CAIT) technique. Here, native lithium ions have been replaced by foreign alkali ions, M+ = K+, Rb+, Cs+. All experiments exhibit a pronounced decrease of native ion diffusion coefficients over more than 4 orders of magnitude with decreasing local population of Li+. The energy landscape is modelled by a site energy distribution (SED) with a concentration dependent Fermi energy of the native Li+ ions. The width of the populated part of the SED is found to be 250 meV (FWHM). The conclusion is made possible by a combination of a macroscopic ion replacement experiment with a Nernst-Planck-Poisson modelling of concentration depth profiles measured by secondary ion mass spectrometry (SIMS). Possible generalizations of macroscopic transport theory to match an Onsager ansatz are discussed.

Pore‐Scale Modeling of Water and Ion Diffusion in Partially Saturated Clays

AbstractAn accurate mechanistic understanding of solute diffusion in partially saturated clays is critical for assessing the safety of deep geological repositories for radioactive waste. In this study, a pore‐scale numerical framework is developed to simulate water and ion diffusion in partially saturated clays. First, the two‐phase Shan‐Chen Lattice Boltzmann method is employed to establish the liquid‐gas distribution in a reconstructed three‐dimensional pore geometry of a clay. An equivalent solute method is also developed and validated to improve the numerical stability of the solution at the liquid/gas interface corresponding to steep variations of the concentration and diffusion coefficient of the water tracer. By using a mobility‐distance relationship from molecular simulations, Fick's law is numerically solved to simulate water diffusion in nanopores, while the coupled Poisson‐Boltzmann‐Nernst‐Planck equations are solved to simulate ion diffusion under the influence of the electrical double layer (EDL). Our model reveals that the decrease of relative effective diffusion coefficients during the desaturation is more pronounced for ions than for water, due to the additional transport pathway of water tracers in the gas phase. The obtained effective diffusion coefficients of tritiated water and ions agree well with reported data from compacted sedimentary rocks. By comparing the local electric potential and the distribution of ion concentrations in single pores, the simulation results suggest that the EDL in unsaturated clays has a more complex influence on ion distribution than under fully water‐saturated conditions. This study provides critical insights into the coupled transport processes of solutes in partially saturated clays.

Molecular Dynamics Simulation Study of the Diffusion of Carbon Dioxide in Heptane: Application in Natural Gas Cleaning

Using solvents to remove carbon dioxide is an effective way to purify natural gas. Diffusion of solute in solvent is key to natural gas purification. In this study, molecular dynamics (MD) simulations of the diffusion of carbon dioxide in heptane at different temperatures and pressures were performed to simulate the conditions of industrial purification process. The diffusion coefficients were measured in two different configurations; pure solvent and binary solute-solvent systems. The diffusion coefficients of the binary system were observed to be in the order of 10-9 m2/s. An increased temperature was observed to increase the diffusion coefficient of the carbon dioxide in heptane while an increase in pressure reduced the value of the diffusion coefficient. The diffusion coefficient was also observed to follow an Arrhenius-type relationship with respect to temperature. The activation energy of the system increased from 9.228 kJ/mol to 11.139 kJ/mol with pressure increase. A linear relationship was detected between the diffusion coefficient and the viscosity of the system and an increased viscosity of the system results in a decreased diffusion coefficient. The results of the research showed that carbon dioxide behavior in heptane offers the theoretical backing for the development of a new natural gas desulphurization solvent.