Dynamic Behavior Of Water Research Articles

Low-frequency dynamics of water absorbed in Nafion membranes as a function of temperature

We performed a neutron scattering study to investigate the dynamical behaviour of water absorbed in Nafion at low hydration levels (λ = 6, λ = moles of water/moles of sulphonic acid sites) as a function of temperature in the range 200–300 K. To single out the signal of the confined water, the measurements were performed on samples hydrated with both H2O and D2O in the same temperature range. Due to the strong incoherent scattering cross section of hydrogen atoms with respect to deuterium, in the difference spectra the contribution from the Nafion membrane is subtracted out and most of the spectra originates from absorbed water. The estimated dynamical susceptibility exhibits features that resemble those of bulk water. In particular, the spectra display a bump at around 1 meV, possibly related to the α relaxation, the intensity of which is markedly affected by the temperature change. Two features due to the phonon-like collective hydrogen bond network dynamics are visible at approximately 7 meV and 25 meV.

Dynamics of water confined in fuel cell Nafion membranes containing zirconium phosphatenanofiller

A quasielastic neutron scattering investigation, to study the single particle dynamics of waterabsorbed in a Nafion/zirconium phosphate composite membrane hydrated at a saturationvalue, is herewith presented. The measurements were done on samples hydrated with bothH2O and D2O to properly select the spectral contribution of the confined water. Both the elasticincoherent structure factor (EISF) and the linewidth of the quasielastic component areevaluated as a function of the momentum transfer. Their trend suggests that themotion of the system hydrogen atoms can be schematized as a random jumpinginside a confining spherical region, which can be related to the boundaries of thecluster that water molecules form around the sulfonic and phosphate acid sites.The size of such a region, the characteristic time necessary to explore the regionand the number of mobile protons involved in this motion are similar to thoseestimated for water absorbed in a simple Nafion membrane at a saturation watercontent. Also the calculated jump diffusion coefficient resembles that of waterconfined in a simple Nafion membrane, and both are consistent with the value ofbulk water. The results indicate that the dynamical behaviour of water in Nafionmembranes is nearly unaffected by the presence of zirconium phosphate nanoparticles.

Temperature-Dependent Dynamics of Water Confined in Nafion Membranes

We performed a neutron scattering study to investigate the dynamical behavior of water absorbed in Nafion at low hydration level as a function of temperature in the range 200-300 K. To single out the spectral contribution of the confined water, the measurements were done on samples hydrated with both H(2)O and D(2)O. Due to the strong incoherent scattering cross section of hydrogen atoms with respect to deuterium, in the difference spectra, the contribution from the Nafion membrane is subtracted out and the signal originates essentially from protons in the liquid phase. The main quantities we extracted as a function of the momentum transfer are the elastic incoherent structure factor (EISF) and the line width of the quasielastic component. Their trend suggests that the motion of hydrogen atoms can be schematized as a random jumping inside a confining region, which can be related to the boundaries of the space where water molecules move in the cluster they form around the sulfonic acid site. Through the calculated EISF, we obtained information on the size of such a region, which increases up to 260 K and then attains a constant value. Above this temperature, the number of water protons that are dynamically activated in the accessible time window increases with a faster rate. The jump diffusion dynamics is characterized by a typical jumping time which is stable at 5.3 ps up to approximately 260 K and then gradually decreases. The ensemble of the findings indicates that, within the limits of the energy resolution of the present experiment, water absorbed in the Nafion membrane undergoes a dynamical transition at around 260 K. We discuss the possible relationship of this dynamical onset with the behavior of the electrical conductivity of the membrane as a function of the temperature.

Molecular dynamics studies of a model polymer–catalyst–carbon interface

Classical molecular dynamics simulations are performed to obtain insights into structural and dynamical behavior of water and O 2 transport through a model polymer membrane, and at the interface of such hydrated polymer with graphite-supported nanocatalyst platinum particles. Water clustering is observed near the membrane hydrophilic sites constituted by sulfonic groups, which due to their affinity with platinum, are located near the metallic surface. It is found that the diffusion of water through the model hydrated polymer membrane depends strongly on the level of membrane hydration due to contribution from various mechanisms whose relative weights change with the degree of hydration. Possible scenarios for O 2 diffusion are also analyzed.

Water Dynamics in Ionomer Membranes by Field-Cycling NMR Relaxometry

The dynamic behavior of water within two types of ionomer membranes, Nafion and sulfonated polyimide, has been investigated by field-cycling nuclear magnetic relaxation. This technique, applied to materials prepared at different hydration levels, allows the proton motion on a time scale of microseconds to be probed. The NMR longitudinal relaxation rate R(1) measured over three decades of Larmor angular frequencies omega is particularly sensitive to the host-water interactions and thus well-suited to study fluid dynamics in restricted geometries. In the polyimide membranes, we have observed a strong dispersion of R(1)(omega) following closely a 1/square root omega law in a low-frequency range (correlation times from 0.1 to 10 micros). This is indicative of a strong interaction of water with "interfacial" hydrophilic groups of the polymeric matrix (wetting situation). Variations of the relaxation rates with water uptake reveal a two-step hydration process: solvation and formation of disconnected aqueous clusters near polar groups, followed by the formation of a continuous hydrogen bond network. On the contrary, in the Nafion we observed weak variations of R(1)(omega) at low frequencies. This is typical of a nonwetting behavior. At early hydration stages, R(1)(omega) evolves logarithmically, suggesting a confined bidimensional diffusion of protons in the microsecond time range. Such an evolution is lost at higher swelling where a plateau related to three-dimensional diffusion is observed.

Supercooled Water in PVA Matrixes. II. A Molecular Dynamics Simulation Study and Comparison with QENS Results

Molecular dynamics (MD) simulations were carried out to elucidate the dynamic behavior of water confined in poly(vinyl alcohol), PVA, hydrogels. Model topology is supported by experimental network parameters, and simulation results are compared to an incoherent quasielastic neutron scattering (QENS) investigation carried out on PVA hydrogels. From the QENS dynamic scattering law (part I), a random jump model was adopted for the description of water diffusion to extract a microscopic diffusion coefficient and a residence time between two "jumps". In the present work, consistently with this framework, water diffusion parameters as diffusion coefficients and residence times have been evaluated using the mean square displacement of water in a time window of 10 ps and the time autocorrelation function of water hydrogen bonds. The calculated parameters are in good agreement with the experimental ones, giving confidence to this approach. Further developments are in progress to take into account a more realistic description of hydrogel structure in the molecular dynamics simulations.

Dynamics of Water Trapped between Hydrophobic Solutes

We describe the model dynamical behavior of the solvent between two nanoscopic hydrophobic solutes. The dynamics of the vicinal water in various sized traps is found to be significantly different from bulk behavior. We consider the dynamics at normal temperature and pressure at three intersolute distances corresponding to the three solvent separated minima in the free energy profile between the solutes with attractions. These three states correspond to one, two, and three intervening layers of water molecules. Results are obtained from a molecular dynamics simulation at constant temperature and pressure (NPT) ensemble. Translational diffusion of water molecules trapped between the two solutes has been analyzed from the velocity correlation function as well as from the mean square displacement of the water molecules. The rotational behavior has been analyzed through the reorientational dynamics of the dipole moment vector of the water molecule by calculating both first and second rank dipole-dipole correlation functions. Both the translational and reorientational mobilities of water are found to be much slower at the smaller separation and increases as the separation between solutes becomes larger. The occupation time distribution functions calculated from the trajectories also show that the relaxation is much slower for the smallest intersolute separation as compared to other wider separations. The sublinear trend in mean square displacement and the stretched exponential decay of the relaxation of dipolar correlation and occupation distribution function indicate that the dynamical behavior of water in the confined region between two large hydrophobic solutes departs from usual Brownian behavior. This behavior is reminiscent of the behavior of water in the vicinity of protein surface clefts or trapped between two domains of a protein.

Proton fluctuations and water diffusion in dextran chemical hydrogels studied by incoherent elastic and quasielastic neutron scattering

Proton fluctuations reporting local motions of the glycosidic linkages of chemically crosslinked dextran hydrogels with well defined pore-size distributions are studied by static and dynamic neutron-scattering approaches. The dependence of the dynamic behaviour of water on the pore sizes is also discussed.

Static and dynamic heterogeneities in water

The thermodynamic behaviour of water seems to be related to static heterogeneities. These static heterogeneities are related to the local structure of water molecules and, when properly characterized, may offer an economical explanation of thermodynamic data. 'What matters' most in determining some of the unusual properties of liquid water may be the fact that the local geometry of the liquid molecules is not spherical or oblong, but rather tetrahedral. In respect to static heterogeneities, this local geometry is critical. The dynamic behaviour of water seems to be related to dynamic heterogeneities, which seem to explain the dynamics of supercooled liquid water well.

Dynamic Behavior of Water within a Polymer Electrolyte Fuel Cell Membrane at Low Hydration Levels

Protonic conduction across the membrane of a polymer electrolyte fuel cell is intimately related to the dynamic behavior of water present within the membrane. To further the understanding of water dynamics in these materials, quasielastic neutron scattering (QENS) has been used to investigate the picosecond dynamic behavior of water within a perfluorosulfonated ionomer (PFSI) membrane under increasing hydration levels from dry to saturation. Evaluation of the elastic incoherent structure factor (EISF) reveals an increase in the characteristic length-scale of confinement as the number of water molecules in the membrane increases, tending to an asymptotic value at saturation. The fraction of elastic incoherent scattering observed at high Q over all hydration levels is well fit by a simple model that assumes a single, nondiffusing hydronium ion per membrane sulfonic acid site. The quasielastic component of the fitted data indicates confined dynamic behavior for scattering vectors less than 0.7 A(-1). As such, the dynamic behavior was interpreted using continuous diffusion confined within a sphere at Q < 0.7 A(-1) and random unconstrained jump diffusion at Q > 0.7 A(-1). As the number of water molecules in the membrane increases, the characteristic residence times obtained from both models is reduced. The increased dynamical frequency is further reflected in the diffusion coefficients predicted by both models. Between low hydration (2 H2O/SO3H) and saturation (16 H2O/SO3H), the continuous spherical diffusion coefficient changes from 0.46 +/- 0.12 to 1.04 +/- 0.12 (10(-5) cm2/s) and jump diffusion indicates an increase from 1.21 +/- 0.03 to 2.14 +/- 0.08 (10(-5) cm2/s). Overall, the dynamic behavior of water has been quantified over different length scale regimes, the results of which may be rationalized on the basis of the formation of water clusters in the hydrophilic domain that expand toward an asymptotic upper limit with increased hydration.

Molecular dynamics simulation of the coalescence of nanometer-sized water droplets in n-heptane.

Molecular dynamics simulations using a modified Drieding 2.21 force field were carried out to study the coalescence behavior of nanometer-sized water droplets in vacuum and in n-heptane. The coalescence mechanisms of the water droplets in the above-noted environments are fairly similar in a sense that the water droplets form a bridge linking the droplets before they merge. However, in the latter situation, due to the presence of n-heptane molecules in between the water droplets, the coalescence was observed to be slowed down considerably, especially in the first 10 ps of the process. However, once the bridge is formed, the water droplets, in both situations, spend about the same amount of time to form a single droplet. The maximum distance between the droplets above which coalescence does not occur was found to be 10 A. In terms of the dynamics, the diffusion coefficient of n-heptane in the emulsion system was very close to its value in the pure liquid form. This may be because n-heptane is the continuous phase. Nonetheless, the dynamic behavior of water in n-heptane is different from that of pure water during and after the coalescence. In particular, the self-diffusion coefficient of water molecules in n-heptane is about 20% higher than the experimental value of pure water. Due to the lack of strong attraction forces between water and n-heptane molecules, the n-heptane molecules were observed to orient themselves perpendicularly to the water/n-heptane interfaces so that the contacting area is minimized.

Dynamic and structural behavior of different rigid nonpolarizable models of water

The local structure and the dynamical behavior of water have been analyzed in two different regimes (the dense isochore of 0.995 g/cm2 and the supercritical isotherm of 673 K) through four rigid nonpolarizable models of water. An important change in the slope of temperature dependence of the self-diffusion coefficient at ρ=0.995 has been observed at T≈450 K, showing two main regions that are related to a change on the activation energy of the process (originated for a change of the structure of the first solvation shell from a tetrahedral to dodecahedral arrangement). The local orientational structure of water has been analyzed through the use of tetrahedral order parameters q. A direct relation between q and D has been observed for all models showing some kind of master curve up to 450 K at the 0.995 g/cm3 isochore. The structure of the system at short and large radial distances has been analyzed through a decomposition of the site–site radial distribution functions in terms of spherical harmonics, and a three-dimensional picture of the total pair distribution function has been reconstructed from this set of spherical harmonic projections.

MODELING OF WATER AND SEDIMENT DYNAMIC IN THE BASIN SCALE AND ITS APPLICATION TO THE ACTUAL BASIN

In this paper, an algorithm for long-term prediction of dynamic behavior of water and sediment in the basin scale using their physical process models was developed. In the algorithm, a basin is divided into the mountain watershed part and stream channel part and individual flow models are applied. Surface land slides and rainfall-runoff process from the mountain watershed are calculated by the kinematic wave model for subsurface-surface flow and slope stability analysis on a grid-based DEM (Digital Elevation Model). Sediment yield volume resulting from the surface land slides is calculated by slope stability analysis. Sedimentransport and bed variation in the stream channel can be predicted by debris flow, bed-load, suspended-load and wash-load formulae. The new method was verified by applying to Yahagi-dam basin, Japan.

Liquid-like dynamical behaviour of water in silica gel at 5 K

Vibrational spectra of silica gel with low concentrations of water, 3 and 10% were studied with inelastic neutron scattering (INS) at 5 K. The INS spectrum for the 3% water in silica gel shows dramatic changes compared to the spectra for other known ice phases in the energy transfer region of 2–150 meV. Its intermolecular librational band is shifted to lower energy by about 20 meV (or by a factor of 1.43) compared to ice-Ih, and has more in common with the librational band for water in the liquid state at 290 K. This indicates the softening of the corresponding transverse intermolecular force constants by a factor of 2 as compared to ice-Ih and could be understood by assuming that a low concentration, water in silica gel has a lot of weak and broken hydrogen bonds when compared to normal bulk ice.

Dynamic behaviour of water in hydrogel containing hydrophobic side chains as studied by pulse 1H NMR

1H NMR measurements on spin–lattice relaxation time ( T 1) and spin–spin relaxation time ( T 2) were carried out on water contained in water-swollen copolymer gels prepared from hydrophilic methacrylic acid and hydrophobic stearyl methacrylate (SMA) or lauryl methacrylate (LMA). The degree of swelling of the copolymer gels decreases drastically with increasing component of hydrophobic monomers. 1H T 1 and T 2 of water show linear relationship with the cube root of the degree of swelling. Motion of water is restrained with the decreasing network size and the amount of hydrophobic groups in the network. T 1 measurements with temperature change revealed that the hydrogen bondings in water are decreased by introducing hydrophobic groups into the hydro gel..

Glasslike dynamical behavior of the plastocyanin hydration water.

The dynamical behavior of water around plastocyanin has been investigated in a wide temperature range by molecular dynamics simulation. The mean square displacements of water oxygen atoms show, at long times, a t(alpha) trend for all temperatures. Below 150 K, alpha is constant and equal to 1; at higher temperatures it drops to a value significantly smaller than 1, and thereafter decreases with increasing temperature. The occurrence of such an anomalous diffusion matches the onset of the dynamical transition observed in the protein. The intermediate scattering function of water is characterized, at high temperature, by a stretched exponential decay evolving, at low temperature, toward a two step relaxation behavior, which becomes more evident on increasing the exchanged wave vector q. Both the mean square displacements and the intermediate scattering functions show, beyond the ballistic regime, a plateau, which progressively extends for longer times as long as the temperature is lowered, such behavior reflecting trapping of water molecules within a cage formed by the nearest neighbors. At low temperature, a low frequency broad inelastic peak is observed in the dynamical structure factor of hydration water; such an excess of vibrational modes being reminiscent of the boson peak, characteristic of disordered, amorphous systems. All these features, which are typical of complex systems, can be traced back to the glassy character of the hydration water and suggest a dynamical coupling occurring at the macromolecule-solvent interface.

Studies on dynamic behavior of water in crosslinked chitosan hydrogel

The dynamic behavior of water in crosslinked chitosan (cr-CS) was studied at the molecular level by proton nuclear magnetic resonance (1H-NMR) spectroscopy and positron annihilation lifetime spectroscopy (PALS). Proton line width was used as a direct parameter of local mobility during swelling. The dynamic behavior of water in hydrogel can be described by the two following processes: physical movement along free volume cavities in network and binding with hydrophilic groups of hydrogel. The formation of bound water and the effect of water on polymer network were correlated with the relaxation of hydrogel network. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:983–988, 1998

Studies on dynamic behavior of water in crosslinked chitosan hydrogel

Studies on dynamic behavior of water in crosslinked chitosan hydrogel

Keeping the shape but changing the charges: A simulation study of urea and its iso-steric analogs

As a first step in simulating solvent denaturation, we compare two possible potentials for urea; one based directly on a parameterization for proteins and another generated from ab initio, quantum calculations. Our results, which are derived from numerous, 1 ns simulations, indicate that both potentials reproduce essentially the same observed water structure (as evident in radial distribution functions). However, even though the quantum potential better approximates dimer energies, it is unable to simulate the dynamic behavior of water (as evident in measurements of diffusion) as well as the potential based on protein parameters. To understand its behavior in aqueous solution, we compare the urea simulations with those of solute molecules that possess the same planar, Y-shape as urea but are progressively more hydrophobic. We find that adding urea to a solution increases the number of hydrogen bonds, while adding any of the Y-shaped analogs decreases the number of hydrogen bonds. Moreover, in contrast to the Y-shaped analogs, which aggregate more as they become less polar, we find that urea mixes well in solution and has little tendency to aggregate. For our analysis of aggregation, we used a novel approach based on Voronoi polyhedra as well as the traditional method of radial distribution functions. In conclusion, we discuss how urea’s unique behavior in comparison to its Y-shaped analogs has clear implications for models of urea solvation and mechanisms of urea protein denaturation.

Nuclear magnetic relaxation as a selective probe of solute-solvent and solute-solute interactions in multi-component mixtures

Intermolecular nuclear magnetic relaxation may represent a powerful tool for the detection and study of weak, nonspecific molecular interactions in liquid solutions. Two intermolecular relaxation mechanisms, the electric quadrupole-field gradient interaction and the magnetic dipole-dipole relaxation are briefly described and recent applications are presented. The quadrupolar relaxation of 131Xe in water is shown, where the Xe atom can be regarded as an ideal hydrophobic model particle. Information about the dynamic behavior of water near the dissolved Xe is derived. 21Ne and 131Xe relaxation is used to probe electric field gradients acting in molecular liquids. By 81Br and 131Xe relaxation an attractive interaction between anions and Xe in aqueous solution is found. Finally, the application of intermolecular dipole-dipole interaction with respect to self-association of small hydrophobic particles in aqueous solutions is demonstrated. Salt effects on self-association are detected and their connection to the Hofmeister series is discussed.