Self-diffusion Coefficient Of Water Research Articles

1H NMR study on microstructure of a novel acrylamide/methacrylic acid template copolymer in aqueous solution

A novel acrylamide/methacrylic acid template copolymer was prepared using polyallylammonium chloride (PAAC) as a template. This copolymer contains acrylamide (PAM), phenoxy acrylate (POA), and acylic acid (PAA) blocks. The investigation by high resolution nuclear magnetic resonance (H-1 NMR) shows that intramolecular hydrogen bonds between the PAM and PAA blocks lead to compact molecular arrangement at quite low pH values, and the motion of the phenoxy side chain of the POA blocks is somewhat restricted. With the increase in pH value of the solution, the carboxylic acid of the PAA block gradually dissociates, which weakens hydrogen bonds between the PAM and PAA blocks. The decrease in D (w), self-diffusion coefficient of water, indicates the growth in aggregate size of the template copolymer. The cross peaks between amide protons and backbone protons shown in 2D nuclear overhauser spectroscopy (NOESY) spectra imply the existence of the intermolecular hydrogen bonding interaction between PAM and PAA blocks. After the carboxylic acid of the PAA block is completely dissociated in alkaline solution, the electrostatic repulsion of the carboxylic ion makes the molecular chain of the copolymer exhibit more outstretched. Consequently, the phenoxy groups (the side chain of the POA block) have more space to move.

On the mutual diffusion properties of ethanol-water mixtures

The structural organization, the number of hydrogen bonds (H bond), and the self- and mutual diffusion coefficients of ethanol-water mixtures were studied by molecular dynamics simulation. It was found that both the numbers of H bonds per water and per ethanol decrease as the mole fraction of ethanol increases. The composition dependences and the relationships between the self- and the mutual diffusion coefficients were further discussed. The self-diffusion coefficient of water has a large drop as the concentration of ethanol increases from 0 to 0.3 and then it nearly keeps constant, while that of ethanol has a minimum around ethanol mole fraction of 0.5. The mutual diffusion coefficient could be divided into two parts, the kinematic factor and the thermodynamic factor. Both the kinematic and thermodynamic factors for ethanol-water mixtures were calculated. It was found that the change trend of mutual diffusion coefficients with the composition is mainly dependent on the thermodynamic factors.

Molecular Dynamics Simulation of Na-montmorillonite and Na/Mg-montmorillonite Hydrates

Molecular dynamics (MD) is used to investigate the structural and diffusive properties of interlayer species, namely water and counterion speciations, of Na-montmorillonite and Na/Mg-montmorillonite. It is found that Na/Mg-montmorillonite exhibits a higher swellability compared with that of Na-montmorillonite for given water content. Especially with 48–72 water molecules of interlayer, the expansion of layer spacings of Na/Mg-montmorillonite is greater than that of Na-montmorillonite. Interlayer water molecules have a stronger tendency to solvate Mg 2+ compared to that of Na +. Inner-sphere complexes and outer-sphere complexes of Mg 2+ with water are observed, whereas only planar inner-sphere complexes of Na + with water are found for two-layer Na/Mg-montmorillonite hydrates. The results also indicate the different diffusion modes of Na + and Mg 2+ for Na/Mg montmorillonite: Na + remains close to the surfaces of the sheets, whereas Mg 2+ is easily hydrated and is located in the middle of the interlayer. Comparatively, Na + has a wider diffusion range and a higher self-diffusion coefficient than Mg 2+. Moreover, self-diffusion coefficient of interlayer water is higher for Na-montmorillonite than that for Na/Mg-montmorillonite at the same water content. Most of the results obtained in this study can be understood in terms of the strong solvation of Mg 2+ with water molecules, and Mg 2+ and Na + have different influences on the structure of interlayer. It can therefore be assumed that the replacement of a relatively small amount of interlayer Na + cations by Mg 2+ in Na-montmorillonite during the initial stage can substantially change the swelling and diffusion properties of interlayer species.

Water Dynamics and Retrogradation of Ultrahigh Pressurized Wheat Starch

The water dynamics and retrogradation kinetics behavior of gelatinized wheat starch by either ultrahigh pressure (UHP) processing or heat are investigated. Wheat starch completely gelatinized in the condition of 90, 000 psi at 25 degrees C for 30 min (pressurized gel) or 100 degrees C for 30 min (heated gel). The physical properties of the wheat starches were characterized in terms of proton relaxation times (T2 times) measured using time-domain nuclear magnetic resonance spectroscopy and evaluated using commercially available continuous distribution modeling software. Different T2 distributions in both micro- and millisecond ranges between pressurized and heated wheat starch gels suggest distinctively different water dynamics between pressurized and heated wheat starch gels. Smaller water self-diffusion coefficients were observed for pressurized wheat starch gels and are indicative of more restricted translational proton mobility than is observed with heated wheat starch gels. The physical characteristics associated with changes taking place during retrogradation were evaluated using melting curves obtained with differential scanning calorimetry. Less retrogradation was observed in pressurized wheat starch, and it may be related to a smaller quantity of freezable water in pressurized wheat starch. Starches comprise a major constituent of many foods proposed for commercial potential using UHP, and the present results furnish insight into the effect of UHP on starch gelatinization and the mechanism of retrogradation during storage.

Self-diffusion coefficients of water in Nafion-117 membrane with multivalent counterions

The tritium tagged water (HTO) exchange rates between water swollen Nafion-117 membrane and equilibrating water were measured to obtain self-diffusion coefficients of water ( D H 2 O m ) in the membrane samples with several multivalent counterions, viz. Ba 2+, Fe 3+, Y 3+, La 3+, Eu 3+ and Th 4+. The values of D H 2 O m were obtained by analyzing the isotopic exchange rates profiles using an analytical solution of Fick's second law. The values of D H 2 O m in these ionic forms of Nafion-117 membrane were found to be in the range of 1.8 × 10 −6–2.2 × 10 −6 cm 2/s. The water uptake capacity in the membrane samples with multivalent counterions was also not found to vary significantly. Since diffusion of water occurs through water channels in the membrane, the narrow spread of D H 2 O m values in membrane samples with multivalent counterions was attributed to nearly constant polymer volume fraction ( V p = 0.75 ± 0.02) in the membrane. The comparison of D H 2 O m values obtained in the present work with that reported for Nafion-117 membrane having same V p (0.75) with Na + counterions (2.85 × 10 −6 cm 2/s) indicated that the self-diffusivity of water is higher in monovalent forms of membrane. D H 2 O m represents tortuosity factor in membrane and hence it is dependent on V p and microstructure of the membrane. Therefore, the slight difference in D H 2 O m values in monovalent and multivalent forms of membrane with same V p could be attributed to minor difference in transport channels of membrane in these forms of Nafion-117 membrane.

The fragile-to-strong dynamic crossover transition in confined water: nuclear magnetic resonance results

By means of a nuclear magnetic resonance experiment, we give evidence of the existence of a fragile-to-strong dynamic crossover transition (FST) in confined water at a temperature T(L)=223+/-2 K. We have studied the dynamics of water contained in 1D cylindrical nanoporous matrices (MCM-41-S) in the temperature range 190-280 K, where experiments on bulk water were so far hampered by crystallization. The FST is clearly inferred from the T dependence of the inverse of the self-diffusion coefficient of water (1D) as a crossover point from a non-Arrhenius to an Arrhenius behavior. The combination of the measured self-diffusion coefficient D and the average translational relaxation time tau(T), as measured by neutron scattering, shows the predicted breakdown of Stokes-Einstein relation in deeply supercooled water.

Degradation of Perfluorinated Ionomer Membranes for PEM Fuel Cells during Processing with H2O2

As a typical degradation of the proton exchange membranes (PEMs) for fuel cells, formation of hydrogen peroxide on the cathode surface has been presented to be a key issue which leads to the decomposition of PEM. Using perflurosulfonated ionomeric membranes with different equivalent weights (, 1000, and 1100) as test samples, degradation of PEM was investigated systematically in practical fuel cell usage conditions (e.g., during the progress of treatment. Membranes were characterized for proton conductivity by ac impedance technique, pulsed-field-gradient spin-echo NMR, Fourier transform infrared spectroscopy, thermogravimetric analysis (TGA), and extensile experimentation. Durability studies over a period of operation revealed evident membrane degradation ascribed to the decomposition of sulfonic acid groups in pendant side chains. The products of cross-linked S–O–S (condensation sulfonates) were strongly demonstrated by IR spectroscopy as a result of long treatment times, which suggests oxidation provoked by . Proton conductivity and the water self-diffusion coefficient decreased significantly due to the loss of water inside the membranes. TGA revealed further changes in the membrane morphology, where the onset and decomposition temperatures of the membranes changed upon exposure to . Membranes with high EW showed a faster decomposition rate than the other ones, whereas the mass loss step showed the reverse case. Although the membranes still retained their bulk physical properties in that they remained flexible and plastic, the tensile analysis showed decreased tensile strength and increased elongation-to-break accompanied by an increased Young’s modulus, which suggests a mechanically weaker membrane after exposure to .

The effect of polymers on the phase behavior of balanced microemulsions: diblock-copolymer and comb-polymers

The effect of some amphipilic diblock-copolymers and comb-polymers on a balanced Winsor III microemulsion system is investigated with the quaternary system n-octyl-β-d-glucoside/1-octanol/n-octane/D2O as basis system. The diblock-copolymers are polyethyleneoxide-co-polydodecenoxide (PEOxPEDODOy) and polyethyleneoxide-co-polybutyleneoxide (PEOxPEBUy), constituted of a straight chain hydrophilic part and a bulky hydrophobic part. Addition of the diblock-copolymer leads to an enhancement of the swelling of the middle phase by uptake of water and oil; a maximum boosting factor of 6 was obtained for PEO111PEDODO25. Nuclear magnetic resonance diffusometry yields the self-diffusion coefficients of all the components in the system. The diffusion experiments provide information on how the microstructure of the bicontinuous microemulsion changes upon addition of the polymers. The reduced self-diffusion coefficients of water and oil are sensitive to the type of polymer that is incorporated in the film. For the diblock-copolymers, as mainly used here, the reduced self-diffusion coefficient of oil and water will respond to how the polymer bends the film. When the film bends away from water, the reduced self-diffusion of the water will increase, whereas the oil diffusion will decrease due to the film acting as a barrier, hindering free diffusion. The self-diffusion coefficient of the polymer and surfactant are similar in magnitude and both decrease slightly with increasing polymer concentration.

An NMR spectroscopic study of water and methanol transport properties in DMFC composite membranes: Influence on the electrochemical behaviour

Water and methanol transport properties through bare recast Nafion and composite Nafion membranes containing ceramic fillers characterised by different acid–base behaviour were investigated by nuclear magnetic resonance (NMR) spectroscopy. The water self-diffusion coefficients were determined for the different membranes in the temperature range from 25 to 150 °C; the methanol self-diffusion coefficients were recorded for the most performing composite membrane (Nafion–SiO 2) and compared to the bare Nafion membrane. Water and methanol transport characteristics of these membranes were correlated to the electrochemical behaviour in direct methanol fuel cells. NMR and electrochemical polarization data indicate that both electrical properties and transport mechanism are influenced by the surface properties of the inorganic fillers; there is a clear evidence that acidic fillers such as silica promote the proton transport in the membrane with respect to basic alumina.

Anharmonic dynamics: melting and mass transfer in liquid

In view of the anharmonic model of the lattice vibrations, the origin and development of the chaotic motion in the condensed state are analyzed. The distribution function for the instantaneous local mode is obtained which is used then to calculate the unstable mode fraction and mean-square translational velocity. The transition to the liquid dynamics is discussed and the expression for the temperature of melting is derived. The analytical expressions for both self-diffusion coefficient and viscosity connecting these quantities with the unstable mode fraction and mean-square translational velocity (for the diffusion) are obtained. The temperature dependencies of both the self-diffusion and viscosity coefficients of water and corresponding isotope effects are described in excellent agreement with the experiment over the wide temperature interval including both the supercooled and overheated states. It is shown the isotope effects on both properties are defined by the mass ratio. The pressure dependencies of the viscosity calculated at various temperatures reproduce the actual dependencies rather well.

Molecular modeling study of sulfonated SIBS triblock copolymers

An important class of thermoplastic elastomers involves polystyrene and polyisobutylene blocks (SIBS). Sulfonated SIBS Triblock Copolymers (S-SIBS) are of particular interest because of potential applications for fuel cell and textile applications, where breathable, protective clothing is required. We have used multiscale modeling to gain an understanding of the static and dynamic properties of these polymer systems at detailed atomistic levels. Quantum chemistry tools were used to elucidate the bonding of water molecules and sulfonate groups. In addition, molecular dynamics was applied to calculate the polymer density at various levels of sulfonation. The structures of polymer with hydronium ions and also water were studied and the mechanism of water self-diffusion was proposed. It was found that with increase of water content the hydronium ions move further away from sulfonate groups. The self-diffusion coefficients of water were found to reproduce well experimental trends. Two different distributions of sulfonate groups were studied: one blocky and another perfectly dispersed. In the case of the blocky architecture, the water clusters are connected at a lower sulfonation level, leading to increased water diffusion coefficients as compared to the dispersed architecture.

Fluid Self-Diffusion in Scots Pine Sapwood Tracheid Cells

The self-diffusion coefficients of water and toluene in Scots pine sapwood was measured using low field pulsed field gradient nuclear magnetic resonance (PFG-NMR). Wood chips of 8 mm diameter were saturated with the respective liquids, and liquid self-diffusion was then traced in one dimension orthogonal to the tracheid cell walls in the wood's radial direction. The experimental echo attenuation curves were exponential, and characteristic self-diffusion coefficients were produced for diffusion times spanning from very short times to times on the order of magnitude of seconds. Observed self-diffusion coefficients were decaying asymptotically as a function of diffusion time, an effect which was ascribed to the cell walls' restriction on confined liquid diffusion. The observed self-diffusion behavior in Scots pine sapwood was compared to self-diffusion coefficients obtained from simulations of diffusion in a square. Principles of molecular displacements in confined geometries were used for elucidating the wood's cellular structure from the observed diffusion coefficients. The results were compared with a mathematical model for diffusion between parallel planes.

Spatial hydration structures and dynamics of phenol in sub- and supercritical water

The hydration structures and dynamics of phenol in aqueous solution at infinite dilution are investigated using molecular-dynamics simulation technique. The simulations are performed at several temperatures along the coexistence curve of water up to the critical point, and above the critical point with density fixed at 0.3 g/cm3. The hydration structures of phenol are characterized using the radial, cylindrical, and spatial distribution functions. In particular, full spatial maps of local atomic (solvent) density around a solute molecule are presented. It is demonstrated that in addition to normal H bonds with hydroxyl group of phenol, water forms pi-type complexes with the center of the benzene ring, in which H2O molecules act as H-bond donor. At ambient conditions phenol is solvated by 38 water molecules, which make up a large hydrophobic cavity, and forms on average 2.39 H bonds (1.55 of which are due to the hydroxyl group-water interactions and 0.84 are due to the pi complex) with its hydration shell. As temperature increases, the hydration structure of phenol undergoes significant changes. The disappearance of the pi-type H bonding is observed near the critical point. Self-diffusion coefficients of water and phenol are also calculated. Dramatic increase in the diffusivity of phenol in aqueous solution is observed near the critical point of simple point-charge-extended water and is related to the changes in water structure at these conditions.

Magnetic Resonance Imaging (MRI) of Water Diffusion in 2-Hydroxyethyl Methacrylate (HEMA) Gels

ABSTRACTA fundamental study focusing on correlating the local water self-diffusion coefficient to the local free water content in 2-hydroxyethyl methacrylate (HEMA) gels was conducted. HEMA gels were synthesized with different nominal water content (50% to 90%). MRI measurements of local diffusion coefficient distribution and local water content profiles are conducted on a 600 MHz scanner. The local water content is measured via two spin-echo images with sufficiently long repetition time (TR) to eliminate T1-weighting and two values for the echo time (TE) in order to account for T2-weighting. The local diffusion coefficient is determined using a standard pulsed-field gradient spin-echo sequence. The measured local water content and diffusion coefficient data are compared with several single-parameter diffusion models for interstitial diffusion in the hydrogel (Makie-Meares, Stokes-Einstein, Brownian motion around overlapping spheres).

Electrical conductivity and self diffusion-NMR studies of the system: Water/sucrose laurate/ethoxylated mono-di-glyceride/isopropylmyristate

We studied the electrical conductivity and the self diffusion behaviors of cosmetic and pharmaceutical grade mixed nonionic surfactant microemulsion systems in order to determine the microstructural association changes. The systems studied were: water (W)/sucrose laurate (L1695)/ethoxylated mono-di-glyceride (EMDG)/isopropylmyristate (IPM) for the self diffusion investigations. For the electrical conductivity determination the water was replaced by an aqueous solution of 0.05 M NaCl. The system studied by electrical conductivity has a mixed surfactant content γ equals 31 wt% and a weight ratio of EMDG in the surfactants mixture δ equals 50 wt%. The mass fraction of NaCl in the aqueous phase, ɛ, equals 0.29 wt%. Electrical conductivity was determined as function of temperature and the mass fraction of oil in the mixture of water and oil (i.e. α (wt%)). The electrical conductivity ( κ) of the system varies from 4.3 × 10 −3 at α = 10 wt% to 1.3 × 10 −4 at α = 90 wt% and varies linearly with temperature. The system: W/L1695/EMDG/IPM was investigated using pulsed gradient spin echo-nuclear magnetic resonance (PGSE-NMR) in the following two mixed surfactant contents and distributions: [A] γ = 31 wt%, δ = 50 wt% and [B] γ = 26 wt%, δ = 25 wt%. It was found that the relative self-diffusion coefficients ( D/ D 0) of water and oil vary as α varies. In the case where γ = 31 wt%, δ = 50 wt%, the relative diffusion coefficients ( D/ D 0) decrease from 0.83 to 0.11 in the case of water, and increase from 0.001 to 0.36 in the case of oil as α varies from 5 to 85 wt%. The hydrodynamic radius ( R H) and the area ( a) per polar head group were estimated as 7.3 nm and 75 Å 2 for α = 10 wt%, γ = 31 wt%, and δ = 50 wt%.

Self-Diffusion Coefficient of Water in Tofu Determined by Pulsed Field Gradient Nuclear Magnetic Resonance

The self-diffusion coefficient of water in soybean protein dispersion and tofu was measured by pulsed field gradient (PFG) NMR. A soy protein isolate (SPI) dispersion (6 and 12%, w/w) in water, calcium cross-linked precipitate, and tofu were used for comparison. The self-diffusion coefficient of water (D) in the SPI dispersion, 2.23 x 10(-9) m2/s, was estimated lower than that of free water, 2.6 x 10(-9) m2/s at 25 degrees C, and decreased as the SPI concentration increased. It further decreased by the addition of calcium chloride, reflecting the obstruction effect induced by the precipitates in addition to the hydration and hydrodynamic interaction in the protein dispersion. The two water regions in tofu were interpreted by the two-site Kärger model: D1 and D2 of soft tofu were 2.26 (+/-0.11) x 10(-9) and 6.84 (+/-0.34) x 10(-11) m2/s, respectively. The relative amount of proton (water) was p1 = 0.98 and p2 = 0.02 at 100 ms of diffusion time. The self-diffusion coefficients of water decreased in pressed tofu, and their relative amounts of water changed to p1 = 0.93 and p2 = 0.07. It was suggested that D1 corresponded to obstructed water in the network structure and D2 corresponded to hydrated water on the surface layer of pores formed in the protein network of tofu. The pore sizes estimated from the diffusion length of obstructed water were 21.3 microm in soft tofu and 20.8 microm in pressed tofu. The removal of fat from pressed tofu led to a decrease in D2 from 6.26 (+/-0.31) x 10(-11) to 3.53 (+/-0.18) x 10(-11) m2/s, and the relative amount of hydrated water increased from 0.07 to 0.14, which indicated hydrophobic hydration.

Ionic and molecular Self-Diffusion in Ion-Exchange materials for fuel energetics studied by pulsed field gradient NMR

Pulsed field gradient nuclear magnetic resonance technique was applied to investigate the self-diffusion mechanism of water, alcohol molecules and Li− counterions in sulfocation exchangers with different structures of the polymeric matrix. It was shown that in the homogeneous perfluorinated sulfocation exchange membranes the ionic and water translation motions are controled by the hydrogen bond network forming in ionogenic channels at the high water content. At the low solvent content, the self-diffusion coefficients of methanol and ethanol are higher than the water self-diffusion coefficients. The influence of non-ion-exchange sorbed electrolyte on Li+ self-diffusion coefficients was observed in the heterogeneous sulfocation exchanger KU-23.

On the problem of field-gradient NMR measurements of intracrystalline diffusion in small crystallites—Water in NaA zeolites as an example

Necessary conditions for measuring intracrystalline diffusion in small crystal size systems via field-gradient NMR are discussed. As an illustrative case self-diffusion coefficients of water adsorbed in NaA zeolites (average crystal diameter about 1 μm) have been measured by 1H-NMR stimulated echoes in static magnetic field gradients of up to 180 T/m in the temperature range of 254–344 K. Obtaining intracrystalline diffusion coefficients necessitates a sufficiently high spatial resolution only provided by such large field gradients.

An NMR and SAXS investigation of DMFC composite recast Nafion membranes containing ceramic fillers

Small angle X-ray scattering (SAXS) and nuclear magnetic resonance (NMR) investigations of recast composite and bare Nafion membranes have been carried out. The self-diffusion coefficients of water and methanol have been determined over a wide temperature range by PFGSE 1H NMR method. The transport mechanism appears to be influenced by surface properties of inorganic fillers. Acidic silica filler appears to promote proton transport in the membrane with respect to basic alumina. An interaction of the silica surface with methanol molecules is also envisaged from the analysis of proton self diffusion coefficients of methanol. The SAXS analysis revealed a modification of the polymer structure immersed in pure methanol or methanol solution with respect to water. A significant increase of the average ion clusters dimension is observed for the composite SiO 2 membrane.

Phase equilibria of the mixed didodecyldimethylammonium bromide–sodium taurodeoxycholate–water system with a large solution region

The phase behavior over the entire concentration range for the system didodecyldimethylammonium bromide (DDAB)–sodium taurodeoxycholate (STDC)–water, at 25 °C, has been investigated, with emphasis on the DDAB-rich part. Polarizing microscopy, SAXS, 2H NMR and 1H self-diffusion NMR have been used in combination as probing techniques for phase behavior and microstructure. The system forms four major phases, all deriving from the respective binary surfactant systems. The two lamellar phases originating from the binary DDAB–water axis (D I and D II, at 3–30 and 83–91 wt.% DDAB, respectively) are only able to incorporate small amounts of STDC. The D II phase solubilizes a comparatively higher amount of bile salt (up to ca. 6 wt.%), while the D I phase takes up less than 0.25 wt.%. From the STDC–water axis, a solution phase and a “hexagonal-like” liquid crystalline phase are derived, at 0–26 and 37–60 wt.% of STDC, respectively. Heterogeneous regions are also indicated on the basis of NMR and SAXS data. The most striking feature is the large extension of the isotropic solution phase, which originates from the water corner and curves toward the DDAB-rich side of the phase diagram. Even though at the upper limit of the solution phase the amount of water is reduced to 10 wt.%, the measured water and DDAB self-diffusion coefficients exclude the possibility of reverse-type structures.