Free Diffusion Coefficient Research Articles

Two-state free-volume model for anomalous dynamics in supercooled water macromolecules undergoing liquid–liquid phase transition

Modelling anomalous dynamics of complex liquid water is a huge challenge due to its various condensed molecular structures and the associated liquid–liquid phase transitions (LLPTs). In this study, we considered, for the first time, the influences of free volume and entropy on LLPTs for both low-density liquid (LDL) and high-density liquid (HDL) waters. Firstly, we proposed a two-state free-volume model and combined with the Adam-Gibbs models to investigate the dynamic equilibria with free diffusion coefficient, viscosity, glass transition temperature and the anomalous dynamics in the two-state water. Free-energy equation was then developed to explore the constitutive relationships of free volume, entropy and anomalously dynamic behaviors. Finally, analytical results of the proposed two-state free-volume model were compared with the experimental data of two-state water reported in literature, and good agreements between them were demonstrated. This study offers a new physical insight into free volume for the anomalous dynamics and LLPTs in the two-state water.

Analysis of service life and reliability of C50CASC structures in the splash zone of the South China Sea

This paper presents the results of a reliability analysis of the service life of hydraulic structures made of C50 coral aggregate seawater concrete (CASC) in the splash zone of the South China Sea. The analysis was conducted using the ChaDuraLife V1.0 software, which is based on reliability theory and modified chloride diffusion theory. Several parameters such as the surface free chloride concentration (Cs), chloride binding capacity (R), free chloride diffusion coefficient (Df), apparent chloride diffusion coefficient (Da), and time-dependent index (m) were considered in this analysis. The (1-m) boundary condition is adopted in this study. The effect of concrete cover thickness on the service life of CASC structures was discussed. The influence of rebar types and external protective measures on the service life was also investigated. Durability recommendations for C50CASC hydraulic structures in the splash zone were proposed. The minimum concrete cover thickness needed to achieve the service life of 30a to 100a was determined to provide theoretical support for the application of CASC in island and reef engineering. The results found that the best technical measures for extending the service life were the application of 2205 duplex stainless steel bars and silane protective coating on the surface of concrete structures. A concrete cover thickness of 6–7 cm could achieve the service life of 50a, while a thickness of 8.5–10 cm, could achieve the service life of 100a.

Gas transport model and numerical solution in roof rock based on the theory of free gas diffusion

Accurately identifying the source of coalbed methane is of great practical significance in preventing gas concentration overruns and improving extraction efficiency. At present, it is widely believed that underground gas mainly comes from coal desorption, and the mechanism of gas flow inside the coal seam is explored in depth, while the gas released inside the rock is often neglected, and there is insufficient understanding of its internal transportation law. In this study, the density gradient theory (FGDGD) in the coal matrix was used as a guide to characterize the adsorption and transport properties of gases within the rock. The anthracite and mudstone from the same roadway in Yuxi mine were selected for the experiments, and coal rock samples with particle sizes of 0.425 mm–0.25 mm were ground. The adsorption constant test and adsorption process experiments were carried out under the constant volume condition, and the transport behavior of gas within the rock was simulated and numerically solved, and the content and flow behavior of gas in the rock were discussed in detail. The results are as follows: (i) The simulation curves based on the density gradient model fit well with the experimental data for both coal and rock downholes, indicating that the gas transport law inside the rock downhole is also in accordance with the density gradient theory, which broadens the scope of application of the theory. (ii) The free gas diffusion coefficients Dks (rock) are smaller than Dks (coal) for the gases in the rock matrix and coal matrix, which may be due to the difference in the pore structure of the rock matrix, resulting in lower adsorption sites than those of the coal. (iii) Although the gas content inside the rock is relatively low compared with that of coal, it is significant for reducing the prediction error of gas influx in engineering practice. Therefore, it is necessary to consider the storage of gas in rocks and its migration behavior when studying the sources of gas emissions in engineering fields.

Compatibility and interaction between C6F12O[sbnd]N2 gas mixture and sealing rubber materials

The C6F12O gas mixture has good insulating properties and can replace the strong greenhouse gas sulfur hexafluoride (SF6) in power equipment. To ensure the safe operation of the equipment, the compatibility between the C6F12O/N2 gas mixture and commonly sealing rubber materials Ethylene-Propylene-Diene Monomer (EPDM), Nitrile Butadiene Rubber (NBR), and Fluororubber (FKM)) is an important factor. In this paper, the compatibility between the C6F12O/N2 gas mixture and rubber materials at different temperatures is studied. Moreover, the free volume fraction, diffusion coefficient, and radial distribution function are calculated. It is found that there exists a chemical reaction between the C6F12O/N2 gas mixture and rubbers, which leads to the appearance of folds or crystalline particles and the accumulation of a large number of F elements on the surface of NBR and EPDM. The simulation results show that the diffusion of the C6F12O/N2 gas mixture in the rubber is enhanced with the increase of temperature, and the diffusion coefficient of C6F12O in NBR is the largest, reaching 33.439 × 10−11m2/s. The experimental and theoretical results show that compatibility between the C6F12O/N2 gas mixture and FKM is better. FKM will have a better application prospect in C6F12O/N2-containing power equipment.

Molecular dynamics simulation and experimental verification of the effects of vinyl silicone oil viscosity on the mechanical properties of silicone rubber foam.

The molecular motion trajectories of silicone rubber foam (SRF) at various vinyl silicone oil viscosities were studied via molecular dynamics (MD) simulation from the perspective of all atomic molecules. The influence of different viscosities of vinyl silicone oil on interaction, compatibility, and aggregation degree of molecules was determined based on the mean square displacement, diffusion coefficient, binding energy, solubility parameter, radial distribution function, and radius of gyration. The mechanical properties of the SRF were also experimentally verified. Results revealed that as the viscosity of vinyl silicone oil increased, the mean square displacement, fractional free volume, diffusion coefficient, and solubility parameter of the system decreased, whereas its larger radius of gyration increased. Moreover, the radial distribution function showed a weaker relative interaction between molecular chains. The calculated binding energy demonstrated that the system had better compatibility at a viscosity of 0.45 Pa s. This study provided a deeper insight into the relation between the viscosity of vinyl silicone oil and mechanical properties of the SRF. As the viscosity of vinyl silicone oil increased, the changing trend in MD simulation results of elastic modulus, shear modulus, bulk modulus, and Poisson's ratio was consistent with the experimental results. The MD simulations can promote theoretical predictions and scientific basis for the design of the SRF with desired performances.

Mechanical properties and pore network connectivity of sodium montmorillonite as predicted by a coarse-grained molecular model

The study of ionic transport through hydrated sodium montmorillonite (Na-MMT)—the main swelling component of bentonite—is of significant interest to better understand its ability to contain contaminants. From a macroscopic viewpoint, porosity and tortuosity can be viewed as scaling factors connecting diffusion coefficients in the clay material to free diffusion coefficients of ions in water. In this work, the mechanical properties, porosity and tortuosity of Na-MMT were calculated using a bottom-up approach. First, the constructed coarse-grained mesoscopic models of Na-MMT were fitted to all-atom molecular dynamics simulations. Thirty different models—each containing one thousand 120 Å-wide hexagonal Na-MMT platelets—were generated. The dry densities considered herein range from 0.8 to 1.3 g/cm3. Second, calculated the models' elastic constants were in excellent agreement with experimental values reported in the literature. Third, each system's porosity, pore size distribution and pore network were analysed. Fourth, the pore network information was used to create a 3D image of hydrated Na-MMT, and random walk simulations were employed to evaluate its tortuosity. Finally, the porosity and tortuosity values estimated using the models were compared to macroscopic experimental values describing tritium and iodide diffusion in Na-MMT dominant bentonite. The values obtained using the models were fairly consistent with experimental values reported in the literature.

In Vivo Microstructure Imaging in Oropharyngeal Squamous Cell Carcinoma Using the Random Walk With Barriers Model.

Apparent diffusion coefficient is not specifically sensitive to tumor microstructure and therapy-induced cellular changes. To investigate time-dependent diffusion imaging with the short-time-limit random walk with barriers model (STL-RWBM) for quantifying microstructure parameters and early cancer cellular response to therapy. Prospective. Twenty-seven patients (median age of 58 years and 7.4% of females) with p16+/p16- oropharyngeal/oral cavity squamous cell carcinomas (OPSCC/OCSCC) underwent MRI scans before therapy, of which 16 patients had second scans at 2 weeks of the 7-weeks chemoradiation therapy (CRT). 3-T, diffusion sequence with oscillating gradient spine echo (OGSE) and pulse gradient spin echo (PGSE). Diffusion weighted images were acquired using OGSE and PGSE. Effective diffusion times were derived for the STL-RWBM to estimate free diffusion coefficient D0 , volume-to-surface area ratio of cellular membranes V/S, and cell membrane permeability κ. Mean values of these parameters were calculated in tumor volumes. Tumor microstructure parameters were compared with clinical stages of p16+ I-II OPSCC, p16+ III OPSCC, and p16- IV OCSCC by Spearman's rank correlation and with digital pathological analysis of a resected tissue sample. Tumor microstructure parameter responses during CRT in the 16 patients were assessed by paired t-tests. A P-value of <0.05 was considered statistically significant. The derived effective diffusion times affected estimated values of V/S and κ by 40%. The tumor V/S values were significantly correlated with clinical stages (r = 0.47) as an increase from low to high clinical stages. The in vivo estimated cell size agreed with one from pathological analysis of a tissue sample. Early tumor cellular responses showed a significant increase in D0 (14%, P = 0.03) and non-significant increases in κ (56%, P = 0.6) and V/S (10%, P = 0.1). Effective diffusion time estimation might impact microstructure parameter estimation. The tumor V/S was correlated with OPSCC/OCSCC clinical stages. 1 TECHNICAL EFFICACY STAGE: 1.

Identification of chloride diffusion coefficient in concrete using physics-informed neural networks

The corrosion of steel caused by chloride is one of the important causes of the deterioration of reinforced concrete structures. In order to describe the transport behavior of chlorides in concrete and to make quantitative durability designs based on it, it is necessary to determine the chloride diffusion coefficient correctly. However, the commonly used analytical method is not suitable for the free chloride diffusion coefficient considering the nonlinear chloride binding capacity of concrete. In this study, by comparing the results of solving forward problems involving chloride transport equations using finite element method (FEM) and solving corresponding inverse problems using physics-informed neural networks (PINNs), it is verified that PINNs can be applied to identify the chloride diffusion coefficient in non-steady-state immersion tests and accelerated chloride migration tests considering no chloride binding, Langmuir binding, or Freundlich binding. The robustness of PINNs is verified by training with 5% and 10% noise data as observed data. The results of interpolation and extrapolation using PINNs trained successfully show its excellent generalization performance.

A Molecular Dynamics Simulation Study on the Diffusion Coefficients of the •OH, •H, and •HO2 Free Radicals Related in the Hydrogen Production Process in Supercritical Water

Supercritical water (SCW) gasification is a clean and efficient utilization method of coal or biomass for hydrogen generation. However, the intrinsic mechanisms in this complex process are still unclear, in which radical diffusion studies are essential. For a severe SCW environment, relevant experiments are difficult. From this perspective, molecular dynamics (MD) simulations have become an efficient and convenient approach to investigate the process in SCW. Therefore, free radical diffusion in a SCW environment was studied by means of MD simulations in this study. And, the effects of temperature and pressure on the free radical diffusion coefficients were investigated in this work. The results illustrated that a higher temperature in a SCW environment can enhance the diffusion of free radicals in this study, especially for the •H radical, therefore facilitating and promoting reactions for hydrogen production. Lower pressure could promote the diffusion coefficients of free radicals. This work is expected to help understand the diffusion properties of free radicals in SCW better and may provide theoretical support for accomplishing efficient complete coal gasification for hydrogen production.

Thermodynamics and transport properties of valine and cysteine peptides in water

The study of thermodynamic and transport properties of amino acids gives access to knowledge about interactions between distinct molecules necessary to understand a variety of biological processes. In this work, we have performed molecular dynamics simulations of valine and cysteine peptides with lengths of (n) = 1, 2, 4, 8 & 16 monomers in an aqueous environment to estimate the solvation free energy and diffusion coefficients. We modeled our system using OPLS-AA parameter sets and TIP3P water at 310 K temperature. Solvation free energy of both the peptides have been reported using two thermodynamic integration (TI) based methods: TI and TI-CUBIC. The solvation free energy of both the peptides increases with the chain length. The estimated values using both methods are in good agreement with a statistical error of < 3%. The solvent accessible surface area (SASA) of solutes and hydrogen bonds between solute and solvent have also been presented to get more insights on solvation processes. The increase in solvation energy of both peptides with chain length is further corroborated by increase in the number of hydrogen bond (between peptide and water) and SASA with the chain length. In addition, the self diffusion coefficient of both solute and solvent along with their binary diffusion coefficient have been estimated for both the peptides. Furthermore, the effect of chain length on solvation free energy as well as diffusion coefficients have been studied.

Performance evaluation of waste cooking oil at different stages and rejuvenation effect of aged asphalt through molecular dynamics simulations and density functional theory calculations

• Representative models of waste cooking oil at different stages are determined. • When the triglyceride is converted to free acid, the positive electrostatic potential, and cohesive energy density increase. • After chemical treatment, waste cooking oil can improve rejuvenation effects on fraction free volume and diffusion characteristics of aged asphalt. Waste cooking oil (WCO) has received more and more attention as an asphalt rejuvenator. The properties of WCO itself will change with the use of time, affecting its rejuvenation effect on aged asphalt. This paper explored the property changes of WCO with the cooking process and the rejuvenation effect on aged asphalt through molecular dynamics (MD) simulations and density functional theory (DFT) calculations. First, the representative molecules of WCO at different stages were determined. The polarity, basic properties, and solubility parameters of WCO at different stages were discussed. Finally, the thermodynamics properties, binding energy, free volume, and diffusion characteristics of WCO rejuvenated asphalt at different stages were compared. With the cooking process, the triglyceride (TG) is converted into fatty acid (FA), positive maximum electrostatic potential and electrostatic interaction increase, FA has stronger adsorption capacity leads to the reduction of model energy. After chemical treatment, the dipole moment and cohesive energy density of treated fatty acid (TFA) decrease. WCO at different stages can restore asphalt properties, and the surface free energy of asphalt can be better restored by FA. WCO at different stages mainly affects non-bond energy. Compared with FA, the incorporation of TFA has a better rejuvenation effect on the free volume fraction and diffusion coefficient of aged asphalt. The diffusion coefficients in asphalt are respectively: TFA > TG > FA.

Free Energy Landscapes, Diffusion Coefficients, and Kinetic Rates from Transition Paths.

We address the problem of constructing accurate mathematical models of the dynamics of complex systems projected on a collective variable. To this aim we introduce a conceptually simple yet effective algorithm for estimating the parameters of Langevin and Fokker-Planck equations from a set of short, possibly out-of-equilibrium molecular dynamics trajectories, obtained for instance from transition path sampling or as relaxation from high free-energy configurations. The approach maximizes the model likelihood based on any explicit expression of the short-time propagator, hence it can be applied to different evolution equations. We demonstrate the numerical efficiency and robustness of the algorithm on model systems, and we apply it to reconstruct the projected dynamics of pairs of C60 and C240 fullerene molecules in explicit water. Our methodology allows reconstructing the accurate thermodynamics and kinetics of activated processes, namely free energy landscapes, diffusion coefficients, and kinetic rates. Compared to existing enhanced sampling methods, we directly exploit short unbiased trajectories at a competitive computational cost.

Waterborne polyurethane/silica nanocomposites based on electrostatic interaction: Interfacial interactions and properties

To study the effect of electrostatic interaction on the interface and mechanical properties of nano-composites, two kinds of silica nanoparticles with positive and negative charges were prepared and introduced into waterborne polyurethane (WPU) emulsion with negative charges to prepare composite emulsion and film. Molecular dynamics simulation and instrumental characterization were used to investigate the properties of nanoparticles and hybrid films. Molecular dynamics simulation results showed that WPU/SiO2(+) nanocomposite demonstrated higher interfacial free energy, lower free volume fraction and diffusion coefficient than those of WPU/SiO2(−) nanocomposite. The mechanical properties of the composite films indicated that there is no positive effect on the mechanical properties as the charges of SiO2 and WPU keep the same. Therefore, the mechanical properties of the composite films can be effectively enhanced by introducing positively charged SiO2, and the optimized introduction amount is 1 %. SEM images of the cross-section for the film show that SiO2(+) exhibits higher interfacial interaction strength with the WPU matrix than SiO2(−). Accordingly, molecular dynamics simulation and instrumental analysis afford similar results.

In preprints: morphogens in motion.

In preprints: morphogens in motion.

Construction of the small intestine on molecular dynamics simulation and preliminary exploration of drug intestinal absorption prediction

In this study, molecular dynamics simulation was applied to the construction of the small intestinal epithelial cell membrane and prediction of drug absorption. First, we constructed a system of a small intestinal epithelial cell membrane that was close to the real proportion and investigated the effects of temperature, water layer thickness, and ionic strength on membrane properties to optimize environmental parameters. Next, three drugs with different absorptivity, including Ephedrine (EPH), Quercetin (QUE), and Baicalin (BAI), were selected as model drugs to study the ability of drugs through the membrane by the free diffusion and umbrella sampling simulation, and the drug permeation ability was characterized by the free diffusion coefficient D and free energy barrier (△G) in the processes. The results showed that the free diffusion coefficient D and △G orders of the three drugs were consistent with the classical experimental absorption order, indicating that these two parameters could be used to jointly characterize the membrane permeability of the drugs.

Seawater resistance mechanism of reinforcing bar–concrete composite structures restored by repair material incorporating multiple admixtures in a simulated marine splash zone

To improve the durability of concrete structures in marine environment, a marine cementitious repair material (MCRM) incorporating silica fume (SF), water-reducing admixtures (WRA), ground granulated blast furnace slag (GBFS), and desulfurized gypsum (DG) was prepared and its properties were investigated, and the seawater resistance mechanism of reinforcing bar–concrete composite structures restored by the MCRM was revealed in the simulated marine splash zone. The results show that incorporation of the SF-WRA-GBFS-DG composite materials increased the compressive strength, decreased the free chloride diffusion coefficient, enhanced the bond strength of the mortars, and improved the seawater resistance of the restored composite structures. The increase in mechanical properties and the decrease in chloride permeabilities were attributed to the combined effect of the SF-WRA-GBFS-DG composite materials, which increased hydration products such as C–S–H gel and refined the pore structure of the samples. Further, it also reduced the drying shrinkage of the mortars, which was attributed to an increase in the ettringite content, and consumed calcium hydroxide to form additional C–S–H gel, thereby decreasing the quantity of crystals at the adhesive interface. These results have important engineering significance in the safety of deteriorated reinforcing bar–concrete composite structures used in a marine environment.

Diffusion of benzene and tetrachloroethylene through saturated cement paste

Diffusion of highly volatile organic compounds (VOCs) through buried concrete infrastructure components, such as pipes and culverts, can occur if these components come in contact with contaminated groundwater or soil. Among various VOCs, benzene and tetrachloroethylene (PCE), are the most common contaminants. This paper aims at measuring the effective diffusion coefficients of benzene and PCE in cement paste. The free diffusion coefficients of VOCs in simulated pore solution were also measured and used in empirical models.Results show that VOCs diffusion coefficients decreased with decreasing w/c because of reduced porosity and increased tortuosity of the pore network as well as increased ionic strength of the pore solution. The increased ionic strength of the pore solution reduced the solubility limit of VOCs in the pore solution and decreased the free diffusion coefficient of VOCs in the pore solution. Also, among all models, phenomenological model provided the most accurate estimates.

Assessment of the size selectivity of peritoneal permeability by the restriction coefficient to protein transport.

Transport of serum proteins from the circulation to peritoneal dialysate in peritoneal dialysis patients mainly focused on total protein. Individual proteins have hardly been studied. We determined serum and effluent concentrations of four individual proteins with a wide molecular weight range routinely in the standardised peritoneal permeability analysis performed yearly in all participating patients. These include β2-microglobulin, albumin, immunoglobulin G and α2-macroglobulin. The dependency of transport of these proteins on their molecular weight and diffusion coefficient led to the development of the peritoneal protein restriction coefficient (PPRC), which is the slope of the relation between the peritoneal clearances of these proteins and their free diffusion coefficients in water, when plotted on a double logarithmic scale. The higher the PPRC, the more size restriction to transport. In this review, we discuss the results obtained on the PPRC under various conditions, such as effects of various osmotic agents, vasoactive drugs, peritonitis and the hydrostatic pressure gradient. Long-term follow-up of patients shows an increase of the PPRC, the possible causes of which are discussed. Venous vasculopathy of the peritoneal microcirculation is the most likely explanation.

Iodide and chloride ions diffusivity, pore characterization and microstructures of concrete incorporating ground granulated blast furnace slag

Innovative approaches are under research to study the resistance of chloride ion penetration in concrete containing chloride ions, to minimize the impact of chloride ion penetration test errors in coastal reinforced concrete (RC), which is helpful to the design of coastal RC structures. In this study, the diffusion depth, free ion concentration and diffusion coefficient of chloride, iodide ions with different curing ages and GGBFS content were measured by the Rapid Chloride Migration Test (RCM) and Rapid Iodide Migration tests (RIM). The SEM-EDS and MIP were used to analyze the microstructures, pore size distribution and the hydrated products. The results show that the performance of GGBFS concrete against the diffusion of corrosive ions is affected by the curing age and the content of GGBFS. With the increase of GGBFS content, especially concrete with 60% GGBFS, the influence of chloride, iodide ion penetrating into concrete gradually becomes smaller. The long-age curing system is more conducive to the concrete resistance to the migration and diffusion of chloride, iodine ions. Compared with the ordinary concrete, the total porosity of concrete mixed with GGBFS is lower, the internal microstructures have fewer cracks and defects, the density is better, and the diffusion coefficient of chloride and iodide ions is also lower. In addition, using the concept of corrosive ion adjustment coefficient (conversion coefficient of diffusion between chloride ion and iodide ion) and applying the data regression analysis (DRA), it is found that there is a good quadratic parabolic function relationship between the GGBFS content and the ions adjustment coefficient.

Effective free water diffusion coefficient of cement paste internally cured with superabsorbent polymers

A hybrid model, comprised of analytical and phenomenological models, was developed to quantify the effective free water diffusion coefficient of cement hydration with superabsorbent polymers (SAP) as internal curing. NMR/MRI measurements were used to estimate the water types and distribution, and the cement degree of hydration. Employing reverse engineering, the estimated temporal free water distribution in cement hydration with SAP was reproduced using 1-D finite element transient flow model and a tortuosity model. The computed free water diffusion coefficient and effective diffusion length of cement paste with 0.2% and 0.3% SAP is 5.48 × 10−12 m2/s and 4.46 × 10−12 m2/s, and 0.25 mm and 0.19 mm, respectively. The computed efficacy of SAP dropped by 8% and 27%, for 0.2% and 0.3% SAP content respectively, when accounting for particles agglomeration.