Diffusion Coefficient Of Water Molecules Research Articles

Physicomechanical and antimicrobial characteristics of hydrogel based on poly(vinyl alcohol): Performance improvement via inclusion of chitosan‐modified nanoclay

ABSTRACTThe effect of a chitosan‐modified nanoclay (CMNC) on the physical, mechanical, and antimicrobial properties of poly(vinyl alcohol) (PVA) hydrogels prepared by the electron beam irradiation method is reported in comparison with pristine nanoclay (PNC). The X‐ray diffraction (XRD) results confirm that the chitosan modification process of nanoclay led to an enhancement in the clay gallery spacing. The inclusion of nanoclays in the PVA matrix decreased the gel content while it increased the swelling degree of the hydrogels. Both PNC and CMNC played a role, depending on their amounts, in swelling of the hydrogel. The swelling kinetic studies revealed a diffusion‐controlled swelling process. The diffusion coefficient of water molecules in hydrogels was decreased in the presence of PNC, while it increased with CMNC. Rheological investigations verified the influential role of nanoclays in decreasing the chemical crosslink density of the hydrogel. CMNC exhibited a higher reinforcing effect on hydrogel mechanical properties than PNC did, although the rheological analysis, in agreement with the XRD results, indicated a better dispersion of PNC in the PVA matrix. According to the antimicrobial tests, perfect inhibition of bacteria growth was obtained only for the hydrogels with CMNC. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47444.

Non-affine deformation of free volume during strain dependent diffusion in polymer thin films

We employ molecular dynamics simulations to understand the influence of non-affine deformation and recovery of free volume on the diffusion behavior of water molecules in polymers, as a function of tensile strain. This study is analogous to strain dependent diffusion of water in polymer thin films which undergo folding in response to water, which is not completely explored. Results reveal that diffusion coefficient of water molecules increases upto 20% strain followed by gradual reduction at higher strain. Non-affine deformation analysis indicates the coupled behavior of relaxation of polymer chains and recovery of free volume. Relaxation process at lower strain coalesces the free volume regions, enhancing the diffusion coefficient. Meanwhile, larger strain dissociates the highly deformed free volume regions. Interestingly, the dissociated regions undergo negligible changes in orientation and shape resulting in reduced diffusion coefficient. The above mechanisms coupled with hydrogen bonds are primarily responsible for shape change in polymer films.

NMR studies on the coupling of ion and water dynamics on various time and length scales in glass-forming LiCl aqueous solutions.

We combine 1H, 2H, and 7Li NMR methods to investigate the dynamics of water molecules and lithium ions in LiCl aqueous solutions over wide ranges of time and length scales down to their glass transitions. Structural relaxation times τ and self-diffusion coefficients D reveal that water and lithium dynamics are faster for lower salt content at ambient temperatures, while the differences vanish upon cooling when fractional freezing leads to similar salt concentrations in the remaining liquid phases. Relaxation times and diffusion coefficients of water molecules agree with those of lithium ions in the weakly supercooled regime, indicating that the dynamics are strongly coupled. Furthermore, non-Arrhenius temperature dependence is found and the Stokes-Einstein relation is obeyed in this temperature range. However, we observe various decoupling phenomena for the motion of the constituents and for dynamics on different length scales in the deeply supercooled regime. Most notably, the rotational motion of the water molecules does not follow the glassy slowdown of the studied salt solutions below ∼145 K, but it rather resembles that in nanoscopic confinement, molecular solutions, and high-density amorphous ice at low temperatures. This common low-temperature water dynamics is characterized by large-angle reorientation and Arrhenius temperature dependence.

Glioma Screening and Classification Based on Magnetic Resonance

In order to demonstrate the effect of perfusion magnetic resonance (MR) and diffusion tensor magnetic resonance imaging (MRI) in evaluating glioma, arterial spin labeling (ASL) imaging is used to calculate the hemodynamic parameter cerebral blood flow. Diffusion tensor imaging (DTI) is mainly used to analyze the diffusion coefficient of water molecules in the brain, obtaining anisotropic scores, average diffusion coefficients, diffusivity parameters, etc. The separability of these parameters is studied by analyzing the distribution of single parameters, single-model multi-parameters and multi-model multi-parameters. Parameters obtained in this paper are basically consistent with those reported in the previous literature, and the statistical analysis of each parameter is also basically the same as that of predecessors.

Water molecules migration at oil–paper interface under the coupling fields of electric and temperature: a molecular dynamics study

ABSTRACTMoisture is an important factor affecting the insulation properties of transformers. Due to the limitations of macroscopic experimental methods, the diffusion of water at oil–paper interface cannot be accurately measured. Therefore, molecular dynamics method was used in this work to establish oil–paper layer model of 105 atoms. Through jointly analysing the aggregation degree, diffusion coefficient, free volume as well as radial distribution function of water molecules, the diffusion mechanism of water molecules at oil–paper interface was studied. The results show that when the initial water content in paper was high, water molecules would accumulate at oil–paper interface to form the local high-water region during heating. The polarisation of the electric field strengthened the hydrogen bonding interaction between water molecules and increased the probability of occurrence of the high-water region. Meanwhile, electric field reduced the free volume and diffusion coefficient of water molecules and rendered its diffusion coefficient anisotropic. What’s more, when the electric field was combined with the temperature field, the electric field played a leading role in the diffusion of water molecules while the temperature field was less affected. Diffusion coefficients of water molecules at different temperatures from molecular dynamics simulations were well consistent with experimental results, which verified the rationality of the model.

Physical properties of tofu gel probed by water translational/rotational dynamics

The dynamic behavior of water molecules in tofu gels is observed using dielectric spectroscopy (DS) and pulsed field gradient nuclear magnetic resonance (PFG-NMR) methods. The breaking stress of tofu gels are dependent on the heating time, which affects the water structures characterized by the dielectric relaxation time, τ. On the other hand, the inhomogeneity of the gel structures decreases with increasing coagulant concentration was reflected as decreasing of the dielectric broadening parameter, β, that related for a fluctuation of the dynamic water structure. In addition, a decrease in the translational diffusion coefficient of water molecules is obtained with increasing coagulant concentration suggesting a reduction of the free space for the diffusional motion of water molecules. Negative correlations between the broadening parameter, β, and the inverse of diffusion coefficient, 1/D, are found in tofu gels and soymilk with distinct correlation coefficients of −0.95 and −0.87, respectively. These results suggest that water dynamics analysis and complementary analysis using DS and PFG-NMR measuring techniques can be an effective tool to evaluate food gel physical properties. Further these technique could also explain the molecular mechanisms of gel and liquid structures and properties via dynamic water structures.

Molecular dynamics simulation to assess the effect of temperature on diffusion coefficients of different ions and water molecules in C-S-H

Diffusion is a particle transportation process beginning from one point of a system to another through random molecular motion. This process depends on various parameters like temperature, concentration gradient, and particle size. The objective of this article is to assess the variation of diffusion coefficients of water molecules, chloride and sodium ions against different temperatures in calcium silicate hydrates (C-S-H) through molecular dynamics simulation. A uniform sodium chloride solution is modeled between cement hydrate layers with no concentration gradient. In such a solution, temperature could affect diffusion process in a significant manner. The two most important crystalline mineral analogues of C-S-H, tobermorite and jennite, are applied in this simulation. Diffusion coefficients of different ions and water molecules are found in different temperatures. It is revealed that diffusion coefficient is higher at high temperatures. Activation energies of chloride and sodium ions transport in cement hydrates are calculated through Arrhenius law. Output values of diffusion coefficients and activation energies are compared to previous experimental and simulation results in the related literature. A multi-scale analysis is run to estimate the penetration depth of $\mbox{Cl}^{-}$ ions in cement paste through Fick’s second law.

Morphofunctional state of the brain in chronic heart failure

The aim of the study was to examine the dependence of functional and morphological changes in the brain and factors of the severity of chronic heart failure. Materials and methods: 54 patients with chronic heart failure of ischemic genesis, I-IV functional class were cognitive tests, echocardiography, nuclear magnetic resonance imaging of the brain. Results. With an increase in the functional class of chronic heart failure, the worst results of Veksler-5 and 7 cognitive subtests are noted, a smaller thickness of gray matter of parietal lobes, lower diffusion coefficients of water molecules in the white matter of the parietal and occipital lobes, and a smaller thickness of the middle legs of the cerebellum. Comparative analysis of the central nervous system state indices in patients with chronic heart failure with different fraction of left ventricular ejection did not show significant differences. The conclusion. The established results confirm the significance of the functional class of chronic heart failure as a marker of cognitive dysfunction and pathological changes in both gray and white matter of the brain, whereas the size of the left ventricular ejection fraction is obviously less useful in this respect.

Connecting diffusion and entropy of bulk water at the single particle level

The relation between the dynamic (e.g., diffusion) and thermodynamic (e.g., entropy) properties of water and water-like liquids has been an active area of research for a long time. Although several studies have investigated the diffusivity and entropy for different systems, these studies have probed either the configurational entropy or the excess entropy of the overall system. In this study, we focus on the entropy of water at a single molecule level at different temperatures. We have used a method developed in our group to calculate the translational and rotational entropy of individual water molecules at various temperatures. We find that the single water translational and rotational entropy exhibit a transition at around 240 K. The translational entropy of individual water molecules shows a consistent variation with change in temperature whereas the variation in the case of rotational entropy is much smaller at different temperatures. We have also calculated diffusion coefficients of water molecules at these temperatures. We find that diffusion also shows the well-known fragile to strong crossover transition at around the same temperature where transition in entropy values has been seen. We have calculated both kinetic and thermodynamic fragilities and crossover points using diffusion and single water translational entropy values. Finally, we correlate the diffusion and translational entropy of individual water molecules using an analog of the Adam-Gibbs relation. SYNOPSIS Translational and rotational entropy of individual water molecules have been calculated at different temperatures. The thermodynamic fragility in terms of translational entropy has been measured. The thermodynamics have been connected with the dynamics by correlating the diffusion coefficient with translational entropy at different temperatures by using analog of Adam-Gibbs relation.

Molecular Dynamics Simulation of Water Transport Mechanisms through Nanoporous Boron Nitride and Graphene Multilayers.

In this study, molecular dynamics simulations are used to investigate water transport mechanisms through hourglass-shaped pore structure in nanoporous boron nitride (BN) and graphene multilayers. An increase in water flux is evidenced as the gap between the layers increases, reaching a maximum of 41 and 43 ns-1 at d = 6 Å in BN and graphene multilayers, respectively. Moreover, the BN multilayer exhibits less flux compared to graphene due to large friction force and energy barrier. In BN, the friction force dramatically increases when the layers are strongly stacked (d = 3.5 Å), whereas it would be independent of the layer separation when the layers are sufficiently spaced (d ≥ 5 Å). In contrast, it was shown that the friction force is independent of the layer spacing in graphene. On the other hand, water molecules across the BN exhibits larger energy barriers compared to graphene when the layers are highly spaced at d = 8 Å. Consistent with the result reported for the flux, the axial diffusion coefficient of water molecules in graphene increases with layer spacing, reaching a maximum of 6.8 × 10-5 cm2/s when the layers are spaced at d = 6 Å.

Molecular and condition parameters dependent diffusion coefficient of water in poly(vinyl alcohol): a molecular dynamics simulation study

Diffusion behavior of water molecules in poly(vinyl alcohol) (PVA) was investigated by molecular dynamics (MD) simulation, and the diffusion coefficient of water molecules in PVA was calculated based on the Einstein’s relation. The effects of polymerization degree, temperature, density, and pressure on the diffusion coefficient were conducted to gain insights into their diffusion mechanisms under different processing conditions. The results showed that the simulation data of diffusion coefficient was consistent with experimental values. With the increase of polymerization degree of PVA, the diffusion coefficient began with a sharp decrease and then became stable, while the diffusion coefficient increased with increasing temperature. It was also found that PVA matrix with a smaller cell density had a larger free volume, and then, the diffusion coefficient was larger. The H2O sorption isotherm could be described by the Langmuir sorption isotherm, and the diffusion coefficient decreased with the increase of pressure.

Molecular dynamics simulation of effect of glycerol monostearate on amorphous polyethylene in the presence of water.

Polyethylene (PE) is used widely as an electrical insulating material. However, the deterioration of its insulating ability is accelerated by exposure to a humid environment. To prevent the influence of water molecules, mixing 2,3-dihydroxypropyl octadecanoate-known as glycerol monostearate (GMS)-into PE has been proposed. However, the physical mechanism underlying the effect of GMS remains unclear. In the present study, the behavior of water molecules in amorphous PE with and without GMS molecule(s) was investigated in terms of diffusion and clustering using molecular dynamics (MD) simulations. Analyzing the mean square displacement (MSD), diffusion coefficient and radial distribution function (RDF) of the water molecules revealed that GMS contributed to suppressing the diffusion and dispersion of water molecules. Furthermore, it was demonstrated that, in the case of 4wt% GMS and less than 2wt% water, GMS contributes to reducing the diffusion coefficient of water molecules but does not change the glass transition temperature (T g) of the system drastically.

Морфофункциональное состояние головного мозга при хронической сердечной недостаточности

The aim of the study was to examine the dependence of functional and morphological changes in the brain and factors of the severity of chronic heart failure. Materials and methods: 54 patients with chronic heart failure of ischemic genesis, I-IV functional class were cognitive tests, echocardiography, nuclear magnetic resonance imaging of the brain. Results. With an increase in the functional class of chronic heart failure, the worst results of Veksler-5 and 7 cognitive subtests are noted, a smaller thickness of gray matter of parietal lobes, lower diffusion coefficients of water molecules in the white matter of the parietal and occipital lobes, and a smaller thickness of the middle legs of the cerebellum. Comparative analysis of the central nervous system state indices in patients with chronic heart failure with different fraction of left ventricular ejection did not show significant differences. The conclusion. The established results confirm the significance of the functional class of chronic heart failure as a marker of cognitive dysfunction and pathological changes in both gray and white matter of the brain, whereas the size of the left ventricular ejection fraction is obviously less useful in this respect.

Anisotropy in geometrically rough structure of ice prismatic plane interface during growth: Development of a modified six-site model of H2O and a molecular dynamics simulation.

This paper presents a modified version of the six-site model of H2O [H. Nada and J. P. J. M. van der Eerden, J. Chem. Phys. 118, 7401 (2003)]. Although the original six-site model was optimized by assuming the cut-off of the Coulomb interaction at an intermolecular distance of 10 Å, the modified model is optimized by using the Ewald method for estimating the Coulomb interaction. Molecular dynamics (MD) simulations of an ice-water interface suggest that the melting point of ice at 1 atm in the modified model is approximately 274.5 K, in good agreement with the real melting point of 273.15 K. MD simulations of bulk ice and water suggest that the modified model reproduces not only the structures and density curves of ice and water, but also the diffusion coefficient of water molecules in water near the melting point at 1 atm. Using the modified model, a large-scale MD simulation of the growth at an ice-water interface of the prismatic plane is performed to elucidate the anisotropy in the interface structure during growth. Simulation results indicate that the geometrical roughness of the ice growth front at the interface is greater in the c-axis direction than in the direction normal to the c-axis when it is analyzed along the axes parallel to the prismatic plane. In addition, during the growth at the interface, the transient appearance of specific crystallographic planes, such as a {202¯1} pyramidal plane, occurs preferentially at the ice growth front. The effect of different ensembles with different simulation systems on the anisotropy in the interface structure is also investigated.

Diffusion study for chloride ions and water molecules in C-S-H gel in nano-scale using molecular dynamics: Case study of tobermorite

Porous materials such as concrete could be subjected to aggressive ions transport. Durability of cement paste is extremely depended on water and ions penetration into its interior sections. These ions transport could lead different damages depending on reactivity of ions, their concentrations and diffusion coefficients. In this paper, chloride diffusion process in cement hydrates is simulated at atomistic scale using molecular dynamics. Most important phase of cement hydrates is calcium silicate hydrate (C-S-H). Tobermorite, one of the most famous crystal analogues of C-S-H, is used as substrate in the simulation model. To conduct simulation, a nanopore is considered in the middle of simulation cell to place water molecules and aggressive ions. Different chloride salts are considered in models to find out which one is better for calculation of the transport properties. Diffusion coefficients of water molecules and chloride ions are calculated and validated with existing analytical and experimental works. There are relatively good agreements among simulation outputs and experimental results.

Structural and water diffusion of poly(acryl amide)/poly(vinyl alcohol) blend films: Experiment and molecular dynamics simulations

To study the effects of composition ratios and temperature on the diffusion of water molecules in PVA/PAM blend films, five simulation models of PVA/PAM with ten water molecules at different composition ratios (4/0, 3/1, 2/2, 1/3, 0/4) were constructed and simulated by using a molecular dynamics (MD) simulation. The diffusion behavior of water molecules in blends were investigated from the aspects of the diffusion coefficient, free volume, pair correlation function (PCF) and trajectories of water molecules, respectively. And the hydrophilicity of blend composite was studied based on the contact angle and equilibrium water content (EWC) of the blend films. The simulation results show that the diffusion coefficient of water molecules and fractional free volume (FFV) of blend membranes increase with the addition of PAM, and a higher temperature can also improve the diffusion of water molecules. Additionally, the analysis of PCFs reveals the main reason why the diffusion coefficient of water in blend system increases with the addition of PAM. The measurement results of contact angle and EWC of blend films indicate that the hydrophilicity of blend films decreases with the addition of PAM, but the EWC of blends increases with the addition of PAM.

Mathematical Modeling of the Drying Process of Chrome Shavings

This paper described the mathematical modeling of chrome shavings drying by performing a microscopic balance for water content to describe the unidimensional transfer of water within the solid at a certain temperature. The deterministic model was solved using Laplace transformation and separation of variables methods in order to determine the average free moisture content at different intervals of time during the drying period. Agreement was observed between both, experimental and modeling data. The effective diffusion coefficient of water molecules during the drying of chrome shavings in a tunnel dryer was determined to be 5.8 x 10-6 m2/h.

G0500304 固体表面近傍におけるNaCl水溶液の拡散に関する分子動力学シミュレーション

Water scarcity is a serious problem around the world in the 21st century. Reverse osmosis (RO) which is a technology that uses a semipermeable membrane is used to remove ions from sea water. However, this method has a problem that separation performance falls gradually with the operating time due to fouling and concentration polarization at the membrane surface. In this study, molecular dynamics simulations were carried out to investigate the effects of interaction force between solid and liquid on the diffusion of water molecules near the solid surface. The results show that the strong interaction force decreases the diffusion coefficient of water molecules in the vicinity of the solid surface, and there is a strong correlation between the diffusion coefficient of water molecules and the contact angle of water on the solid surface.

Analysis of pH-dependent Structure and Mass Transfer Characteristics of Polydopamine Membranes by Molecular Dynamics Simulation

Detailed atomistic structures are constructed for polydopamine membranes containing different amounts of catechol and quinone groups to investigate the effect of pH value in the membrane casting solution on sorption and diffusion of small gas molecules (water and propylene) in the membranes. Interactions between dopamine oligomers are calculated, and it is found that the interactions decrease from −2356.52kJ·mol−1 in DOP-1 to −1586.69kJ·mol−1 in DOP-3 when all of the catechol groups are converted to quinone groups. The mobility of polymer segments and free volume properties of polydopamine membranes are analyzed. The sorption quantities of water and propylene in the membrane are calculated using Grand Canonical Monte Carlo method. The sorption results show that water adsorbed in DOP-1, DOP-2 and DOP-3 are 17.3, 18.6 and 20.0mg water per gram polymer, respectively, and no propylene molecule can be adsorbed. The diffusion behavior of water molecules in the membrane is investigated by molecular dynamics simulation. The diffusion coefficients of water molecules in DOP-1, DOP-2 and DOP-3 membranes are (1.80±0.52)×10−11, (3.40±0.64)×10−11 and (4.50±0.92)×10−11m2·s−1, respectively. The predicted sorption quantities and diffusion coefficients of water and propylene in the membrane present the same trends as those from experimental results.

INTERRELATION OF ATHEROSCLEROSIS RISK FACTORS AND CENTRAL NERVOUS SYSTEM CONDITION IN PATIENTS WITH CHRONIC HEART FAILURE OF ISCHEMIC ORIGIN

Aim. Chronic heart failure is one of the factors of the development and progression of cognitive disorders. To study the relationships between risk factors of atherosclerosis and condition of the central nervous system in patients with chronic heart failure of ischemic etiology. Material and methods. Cognitive tests, echocardiography, ultrasound investigation of carotid arteries, MRI of the brain and measurement of homocystein, CRP, lipid profile in venous blood were performed in 57 patients with I–IV functional class of chronic heart failure. Program STATISTICA7 (methods of incremental regression analysis) were used to assess the results. Results. Strong interactions between worsening of cognitive functions and increase of homocystein level, increase of thickness of IMC, decrease of low-density lipoprotein cholesterol were revealed. Decrease of thickness of the cerebral grey matter and increase of apparent diffusion coefficient of water molecules of the brain were associated with increased C-reactive protein levels, cholesterol of low density lipoproteins and high-density lipoproteins and with decrease of the thickness of carotid IMC. The decrease of thickness of middle legs of cerebellum was associated with reduced levels of low-density and high-density lipoprotein cholesterol. Conclusion. The relationship between risk factors of atherosclerosis, cognitive functions, and morphological changes of the brain was revealed.