Self-diffusion Coefficient Of Water Research Articles

Synthesis and crosslinking of partially disulfonated poly(arylene ether sulfone) random copolymers as candidates for chlorine resistant reverse osmosis membranes

Biphenol-based, partially disulfonated poly(arylene ether sulfone)s synthesized by direct copolymerization show promise as potential reverse osmosis membranes. They have excellent chlorine resistance over a wide range of pHs and good anti-protein and anti-oily water fouling behavior. Crosslinking of these copolymers that have high degrees of disulfonation may improve salt rejection of the membranes for reverse osmosis performance. A series of controlled molecular weight, phenoxide-endcapped, 50% disulfonated poly(arylene ether sulfone)s were synthesized. The copolymers were reacted with a multifunctional epoxy resin and crosslinked thermally. The effects on network properties of various factors such as crosslinking time, copolymer molecular weight and epoxy concentration were investigated. The crosslinked membranes were characterized in terms of gel fraction, water uptake, swelling and self-diffusion coefficients of water. The salt rejection of the cured membranes was significantly higher than that for the uncrosslinked copolymer precursors.

A Diffusive Anomaly of Water in Aqueous Sodium Chloride Solutions at Low Temperatures

Molecular dynamics simulations are presented for the self-diffusion coefficient of water in aqueous sodium chloride solutions. At temperatures above the freezing point of pure water, the self-diffusion coefficient is a monotonically decreasing function of salt concentration. Below the freezing point of pure water, however, the self-diffusion coefficient is a non-monotonic function of salt concentration, showing a maximum at approximately one molal salt. This suggests that sodium chloride, which is considered a structure-making salt at room temperature, becomes a structure-breaking salt at low temperatures. A qualitative understanding of this effect can be obtained by considering the effect of ions on the residence time of water molecules near other water molecules. A consideration of the freezing point depression of aqueous sodium chloride solutions suggests that the self-diffusion coefficient of water in supercooled sodium chloride solutions is always higher than that in pure (supercooled) water at the same temperature.

NMR investigation of water and methanol mobility in nanocomposite fuel cell membranes

Water and methanol transport behavior, morphology, and solvent adsorption of filler-free Nafion membrane, Nafion–SiO2, Nafion–TiO2, and two Nafion–Zr(HPO4)2 composites were investigated using nuclear magnetic resonance methods, including spin-lattice relaxation and pulsed-field-gradient spin-echo diffusion conducted under both variable temperature and variable hydrostatic pressure conditions and scanning electron microscopy analysis. A comparison between water and methanol self-diffusion coefficients reveals that the water mobility is higher than the methanol mobility in all the membranes. Additionally, the inclusion of inorganic fillers improves both the solvent uptakes and the transport properties of the composite membranes relative to filler-free Nafion, with the exception of one of the Nafion–Zr(HPO4)2 composite. Nafion–Zr(HPO4)2 composites were prepared by two different procedures, in situ and ex situ. Although phosphorus-31 magic-angle spinning nuclear magnetic resonance spectra show the same structures of the particles in both kinds of membranes, the morphology, solvent absorption properties, and solvent mobilities are very different.

Fast thermal desorption spectroscopy study of H∕D isotopic exchange reaction in polycrystalline ice near its melting point

Using fast thermal desorption spectroscopy, a novel technique developed in our laboratory, we investigated the kinetics of HD isotopic exchange in 3 microm thick polycrystalline H2O ice films containing D2O layers at thicknesses ranging from 10 to 300 nm at a temperature of -2.0+/-1.5 degrees C. According to our results over the duration of a typical fast thermal desorption experiment (3-4 ms), the isotopic exchange is confined to a 50+/-10 nm wide reaction zone located at the boundary between polycrystalline H2O and D2O ice. Combining these data with a theoretical analysis of the diffusion in polycrystalline medium, we establish the range of possible values for water self-diffusion coefficients and the grain boundary widths characteristic of our ice samples. Our analysis shows that for the grain boundary width on the order of a few nanometers, the diffusivity of D2O along the grain boundaries must be at least two orders of magnitude lower than that in bulk water at the same temperature. Based on these results, we argue that, in the limit of low concentrations of impurities, polycrystalline ice does not undergo grain boundary premelting at temperatures up to -2 degrees C.

Water Diffusion in a Synthetic Hectorite Clay Studied by Quasi-elastic Neutron Scattering

We present a quasi-elastic neutron scattering study of water dynamics confined in a model clay system, a synthetic hectorite with Na+ compensating counterions. As shown by water adsorption gravimetry and neutron/X-ray diffraction, the clay system has, unlike its natural counterparts, very well-defined swelling characteristics, with a clear appearance of a monohydrated and a bihydrated state. This simplifies to a great extent neutron scattering analysis and interpretation. Initially, microscopic relaxation times as well as long-range self-diffusion coefficients for water in Na-hectorite at ambient temperature are determined using the time-of-flight (TOF) and neutron spin echo (NSE) neutron scattering techniques, applying a simple model of isotropic (three-dimensional) translational diffusion. Results from the two techniques are in excellent agreement, giving diffusion coefficient of approximately 1.5 × 10-10 m 2 s-1 and 4.5 × 10-10 m 2 s-1 for the monohydrated and bihydrated state, respectively. Concentrat...

Measurement of anisotropy of pore diffusion in protein crystals by PFG NMR and by CLSM

Solvent (water) and solute (fluorescein and rhodamine B) diffusion in crystalline lysozyme protein, which represent a mesoporous material with an anisotropic pore structure, was studied by pulsed field gradient nuclear magnetic resonance (PFG NMR) and confocal laser scanning microscopy (CLSM), respectively. Water and fluorescent dyes reveal anisotropic intracrystalline diffusion behaviour. The averaged intracrystalline self-diffusion coefficient of water was found to be about one order of magnitude smaller than the value for bulk liquid water at the same temperature. However, the intracrystalline self-diffusion coefficients of water are about two orders of magnitude faster than the corresponding values of the fluorescent dyes. For water and fluorescein in tetragonal lysozyme crystals, the anisotropy ratio, which is defined as the ratio of the largest component of the diffusion tensor to its averaged orthogonal value, was about 0.16, whereas for orthorhombic crystals the anisotropy ratios measured by CLSM for fluorescein (0.15 ± 0.03) and rhodamine B (0.25 ± 0.05) were in the same order of magnitude but somewhat larger compared to the value for water (0.11 ± 0.04) measured by PFG NMR.

Water state and diffusion through lipid bilayers: Effect of hydration degree

The dependences of adsorbed water state (obtained from the variations in 1H NMR spectra with the angle between the bilayer normal and magnetic field direction) and water diffusion along the bilayer normal (measured using pulsed field gradient 1H NMR) on hydration degree have been studied in macroscopically oriented bilayers of dioleoylphosphatidylcholine. The angle dependences of the shape of NMR spectrum are qualitatively different only for water concentrations higher and lower than that achieved by hydration from saturated vapors (χeq, about 23%). At concentrations lower than χeq, all water in the sample either makes the hydration shells of the lipid polar heads or is in fast exchange with the shell water, so the spin-echo signal from water is detected only within a narrow range of angles close to the magic angle, 54.7°. At concentration exceeding χeq, the spin-echo signal from water is retained at all orientations, suggesting that a portion of water between bilayers (quasi-free water) slowly exchanges with water bound to the polar heads. There is an inverse dependence of the coefficient of water self-diffusion through the bilayer system on the hydration degree, which is described in the Tanner model with account of water self-diffusion in the hydrophobic part of the bilayer. Bilayer permeability, distribution coefficient of molecules between aqueous and lipid phases, and water self-diffusion coefficient in the hydrophobic region of the bilayer are estimated.

Synthesis and characterization of partially fluorinated hydrophobic–hydrophilic multiblock copolymers containing sulfonate groups for proton exchange membrane

A new hydrophobic–hydrophilic multiblock copolymer has been successfully synthesized based on the careful coupling of a fluorine terminated poly(arylene ether ketone) (6FK) hydrophobic oligomer and a phenoxide terminated disulfonated poly(arylene ether sulfone) (BPSH) hydrophilic oligomer. 19F and 1H NMR spectra were used to characterize the oligomers’ molecular weights and multiblock copolymer's structure. The comparison of the multiblock copolymer 13C NMR spectrum with that of the random copolymer showed that the transetherification side reaction was minimized in this synthesis. The morphologies of membranes were investigated by tapping mode atomic force microscopy (AFM), which showed that the multiblock membrane acidified by the high temperature method has sharp phase separation. Membrane properties like protonic conductivity, water uptake, and self-diffusion coefficient of water as a function of temperature and relative humidity (RH) were characterized for the multiblock copolymer and compared with ketone type random copolymers at similar ion exchange capacity value and Nafion ® controls. The multiblock copolymers are promising candidates for proton exchange membranes especially for applications at high temperatures and low relative humidity.

The effects of thermal annealing on commercial Nafion ® membranes

The effect of thermal annealing as a pretreatment procedure for Nafion ® membranes was investigated. Commercial membranes with different thicknesses and ages (all melt extruded films) were considered. After a short heat treatment at 165 °C, the proton conductivity, water permeability, equilibrium water sorption, and self-diffusion coefficient of water and protons increased in all films, with the greatest improvements occurring in thin films (<50 μm). Wide-angle X-ray measurements showed that the annealing procedure decreases the crystallinity of thick films and increases the crystallinity of thin films, in general. Small-angle X-ray measurements showed that annealing may alter the distribution of water within the films. Thermal annealing appears to ‘normalize’ the transport properties of different thicknesses and batches of Nafion ®, reducing the differences in values of measured transport properties between thin and thick membranes.

Structure and mobility in water plasticized poly(ethylene oxide)

The change in structure and mobility of poly(ethylene oxide) (PEO) containing 2 wt% of fumed silica and the water self-diffusion coefficient in concentrated PEO–water systems have been investigated at room temperature in the water weight fraction, w w, range 0–0.50 w/w. Pulsed field gradient nuclear magnetic resonance was used to measure the self-diffusion coefficients. Structure and mobility properties of PEO were measured with differential scanning calorimetry as well as with positron annihilation lifetime spectroscopy. The largest reduction of the degree of crystallinity of PEO was observed when w w was increased from 0.13 w/w to 0.50 w/w. Moreover, water induced relaxation of the PEO segments in the amorphous phase, which seemed to have been strained by the crystals during compression molding. The water self-diffusion coefficient increased logarithmically with increased water content below water weight fractions in the amorphous phase of 0.30 w/w and the water molecules were obstructed by the crystalline phase.

Study on physical and electrostatic interactions of counterions in poly(perfluorosulfonic) acid matrix: Characterization of diffusion properties of membrane using radiotracers

The self-diffusion coefficients of water and ions were used to study the physical (tortuosity) and electrostatic interactions of counterions in poly(perfluorosulfonic) acid membrane (Nafion-117) matrix. The self-diffusion coefficients of water ( D H 2 O m ) were measured in the water swollen Nafion-117 membrane with Zn 2+, Ca 2+, Sr 2+, and Fe 2+ counterions by analyzing the experimental exchange rates between tritium tagged water (HTO) in membrane and equilibrating water. In order to study the effects of equilibrating solution, the HTO-desorption rate profiles between the membrane samples in H + or Cs + forms and equilibrating solution containing CsCl or HCl (0.25 mol/L) were measured. It was observed that the HTO-exchange rate profile was slower in case of membrane sample in Cs +-from equilibrated with salt/acid solution than that equilibrated with deionized water in same ionic form. However, HTO-exchange rate profile did not alter in case of H +-form of membrane on equilibration with salt or acid solution. The variation of ln D H 2 O m with polymer volume function V p/(1 − V p), where V p is polymer volume fraction, indicated that: (i) D H 2 O m in the membrane with multivalent counterions was lower than that reported for membrane with monovalent counterions at same V p, and (ii) the linear trends observed in variation of ln D H 2 O m with V p/(1 − V p) for multivalent and monovalent counterions were significantly different. The values of D H 2 O m in membrane normalized with D H 2 O m at V p = 0 were taken as an estimate of the tortuosity factor for self-diffusion of ions in the membrane matrix. The self-diffusion coefficients of ions reported in the literature along with tortuosity factor obtained from D H 2 O m in the corresponding ionic forms of the membrane were analyzed to obtain the charge ( Z i ) independent electrostatic interaction parameter g( φ) of monovalent and divalent ions in the membrane. This analysis indicated that g( φ) also vary exponentially as a function of V p/(1 − V p) irrespective of charge on counterions. In order to study the influence of V p on diffusional transport rates of Na + and Cs + ions in membrane, a permeation experiment was carried out using H +-form of membrane having high water volume fraction. The diffusional transport rates of Cs + and Na + in H +-form of membrane were found to be similar indicating that the water volume fraction in membrane has strong influence on the parameters that govern the diffusion across the Nafion-117 membrane.

NMR studies of pore formation and water diffusion in self-hardening cut-off wall materials

Cut-off walls for the containment of polluted sites are vertical in-ground barriers of low hydraulic conductivity. To construct these barriers, self-hardening watery suspensions of a special cement-based hydraulic binder and a cement-stable bentonite are used. The formation of the pore structure during hardening of suspensions with different solid contents and the water self-diffusion in the resulting cut-off wall materials were studied by non-destructive 1H NMR techniques. It was found that an increased amount of hydrating solids in the suspension leads to a decrease in NMR relaxation times and self-diffusion coefficients of the pore water, indicating a reduction of the pore sizes and an enhancement of the diffusion resistance. The self-diffusion coefficients of the water in the hardened cut-off wall materials were determined to be about four orders of magnitude smaller than in bulk liquid water and two orders of magnitude smaller than in pure bentonite–water suspensions confirming the excellent diffusive resistance of the cut-off wall materials.

Transport Properties of Proton Exchange Membranes

Disulfonated poly(arylene ether sulfone) random copolymers have shown promise as potential candidates for proton exchange membranes. The chemical structure of these copolymers has served as a basis for the molecular design of new random and multiblock copolymers. In random copolymers the sulfonic acid is distributed statistically, while in the multiblock copolymers the chemical structure is designed to have co continuous hydrophilic and hydrophobic domains. The effect of these variables on the transport properties of the membranes was investigated. For the random copolymers, high proton conductivity was observed at higher IEC along with high methanol permeability. But for the multiblock copolymers, it was possible to combine high proton conductivity with low methanol permeability. Improved proton conductivity was observed under partially hydrated conditions for samples with higher block lengths. Self diffusion coefficient of water was also measured by NMR which provided better understanding of the transport process.

Hydration of alkali ions from first principles molecular dynamics revisited

Structural and dynamical properties of the hydration of Li(+), Na(+), and K(+) in liquid water at ambient conditions were studied by first principles molecular dynamics. Our simulations successfully captured the different hydration behavior shown by the three alkali ions as observed in experiments. The present analyses of the dependence of the self-diffusion coefficient and rotational correlation time of water on the ion concentration suggest that Li(+) (K(+)) is certainly categorized as a structure maker (breaker), whereas Na(+) acts as a weak structure breaker. An analysis of the relevant electronic structures, based on maximally localized Wannier functions, revealed that the dipole moment of H(2)O molecules in the first solvation shell of Na(+) and K(+) decreases by about 0.1 D compared to that in the bulk, due to a contraction of the oxygen lone pair orbital pointing toward the metal ion.

Synthesis and characterization of sulfonated poly(arylene ether) polyimide multiblock copolymers for proton exchange membranes

Novel multiblock copolymers, based on segmented sulfonated hydrophilic-hydrophobic blocks, were synthesized and investigated for their application as proton exchange membranes. A series of segmented sulfonated poly(arylene ether sulfone)-b-polyimide multiblock copolymers, with various block lengths, were synthesized via the coupling reaction between the terminal amine moieties on the hydrophilic blocks and naphthalene anhydride functionalized hydrophobic blocks. Successful imidization reactions required a mixed solvent system, comprised of NMP and m-cresol, in the presence of catalysts. Proton conductivity measurements revealed that the proton conductivity improved with increasing hydrophilic and hydrophobic block lengths. The morphological structure of the multiblock copolymers was investigated using tapping mode atomic force microscopy (TM-AFM). The AFM images of the copolymers demonstrated well-defined nanophase separated morphologies, with the changes in the block length having a pronounced effect on the phase separated morphologies of the system. The self diffusion coefficient of water, as measured by1H NMR, provided a better understanding of the transport process. Thus, the block copolymers showed higher values than Nafion, andcomparable proton conductivities in liquid water, as well as under partially hydrated conditions at 80°C. The new materials are strong candidates for use in PEM systems.

Feasibility of noninvasive evaluation of biophysical properties of tissue-engineered cartilage by using quantitative MRI

The application of tissue-engineered cartilage in a clinical setting requires a noninvasive method to assess the biophysical and biochemical properties of the engineered cartilage. Since articular cartilage is composed of 70–80% water and has dense extracellular matrixes (ECM), it is considered that the condition of the water molecules in the tissue is correlated with its biomechanical property. Therefore, magnetic resonance imaging (MRI) represents a potential approach to assess the biophysical property of the engineered cartilage. In this study, we test the hypothesis that quantitative MRI can be used as a noninvasive assessment method to assess the biophysical property of the engineered cartilage. To reconstruct a model of cartilaginous tissue, chondrocytes harvested from the humeral head of calves were embedded in an agarose gel and cultured in vitro up to 4 weeks. Equilibrium Young's moduli were determined from the stress relaxation tests. After mechanical testing, MRI-derived parameters (longitudinal relaxation time T1, transverse relaxation time T2, and water self-diffusion coefficient D) were measured. The equilibrium Young's modulus of the engineered cartilage showed a tendency to increase with an increase in the culture time, whereas T1 and D decreased. Based on a regression analysis, T1 and D showed a strong correlation with the equilibrium Young's modulus. The results showed that T1 and D values derived from the MRI measurements could be used to noninvasively monitor the biophysical properties of the engineered cartilage.

Molecular understanding of dynamical properties of the vapor/ethanol-aqueous-solution interface

The variation of dynamical properties of the vapor/ethanol-aqueous-solution interface with excess adsorption of ethanol is investigated by a molecular dynamics (MD) simulation. Five independent NVT (T = 298.15 K) constant MD runs were performed, where a slab of ethanol-aqueous-solution with ethanol mole fraction of 0.0052, 0.012, 0.024, 0.057 and 0.12, respectively, reached an equilibrium state of adsorption. The positional dependence of molecular self-diffusivity was evaluated by profiles of the self-diffusion coefficient of ethanol and water along the axis normal to the interface. The profile of the ethanol self-diffusion coefficient showed local extrema at the interface, the degree of which is weakened with increase of . Lateral self-diffusivity of an adsorbed ethanol molecule was also evaluated by the two-dimensional (2D) self-diffusion coefficient at the interface, which is a monotonically decreasing function of ethanol excess adsorption. Moreover, the desorption probability of an adsorbed ethanol molecule was estimated, which monotonically increases with ethanol excess adsorption. The molecular origin of the variation of these dynamical properties was explained by correlating them with the profile of the free energy change.

A-B-A-B-A Block Amphiphiles. Balance between Hydrophilic and Hydrophobic Segmentation

Six penta-block amphiphiles of the general structure A-B-A-B-A or B-A-B-A-B (where A = a hydrophilic ether and B = a hydrophobic carbon chain) were synthesized and examined via water solubilities, surface activities, cloud points, and self-diffusion coefficients (the latter obtained by Fourier transform pulsed gradient spin−echo NMR, abbreviated PGSE-NMR). It was found that segmentation can have a dramatic effect upon solute properties, including solubility, propensity to self-assemble, aggregation number, and cooperativity. These data are relevant to biological systems where segmentation is a widespread phenomenon.

Molecular dynamics study of methane in water: diffusion and structure

We present molecular dynamics simulation results for the diffusion coefficients and structure of water–methane mixtures in constant NPT ensembles, at T = 270, 300 K and P = 8.104 × 107 Pa. The systems we have studied consist of one, four and eight CH4 molecules and varying H2O molecules per unit cell, which correspond to methane concentration of 0.081, 0.324 and 0.643 mol/l, respectively. The intermolecular potentials used in all the simulations were the four-site TIP4P model of water [1] and the fitted Lennard-Jones 12-6 potential for CH4–H2O [2]. Our results show that the methane concentration has little impact on the structure of water and the formation of hydrogen bonds (H-bonds) between water molecules. The H-bond numbers, H-bond length and the H-bond angle are independent of the methane concentration at the temperatures and densities examined in this study. We also find that the number of H-bonds and angles are sensitive to the temperature. The rise of temperature produces a decrease in the number and an increase in the angle of the H-bonds. Enhanced structuring of the hydration-shell water molecules is indicated by an increase of the first and second peak in the water oxygen–oxygen radial distribution function as temperature is decreased. The self-diffusion coefficient of water is sensitive to the methane concentration and temperature.

Swelling Kinetics of Poly(N-isopropylacrylamide) Minigels

We synthesize poly(N-isopropylacrylamide) (PNIPAM) gels with different sizes in the micrometer scale by a slight variation of a recent emulsion polymerization method (ref 1). The procedure is different than that typically used for obtaining macroscopic PNIPAM hydrogels. The resultant minigel suspension is polydisperse thus allowing the swelling kinetics for different gel sizes to be studied; we do so at temperatures below the volume-transition temperature by wetting with water previously dried particles. The resultant swelling is followed by optical video microscopy. We find that the characteristic swelling time scales with the inverse of the particle dimension squared, in agreement with theoretical predictions (ref 2). The proportionality constant is the network diffusion coefficient D, which for the minigels under consideration appears to be in between that of PNIPAM macrogels and the self-diffusion coefficient of water.