Decrease In Water Diffusion Research Articles

Barrier properties of polyamide 12/montmorillonite nanocomposites: Effect of clay structure and mixing conditions

Hybrid films, composed of a PA12 polyamide matrix filled with 5 wt% of clays were melt processed under different mixing conditions to classically obtain montmorillonite-filled microcomposites and organo-modified montmorillonite-filled nanocomposites. The change in mixing shear rates allowed to notably modulating the fragmentation level for montmorillonite and the exfoliation level for modified montmorillonite. Barrier properties to nitrogen and water were altered with a reduction in PA12 chain mobility by confinement effect and geometrical constraints. Tortuosity effects induced by fillers were also highlighted. The water permeability and diffusivity were significantly decreased with exfoliated clays, while an increase was measured with an increased water solubility in presence of clay aggregates, both counterbalanced tortuosity effects. This trend was amplified when using low shear rates. According to filler used and mixing conditions, water sorption measurement has successfully demonstrated an increase in water solubility, due to filler affinity, which was surprisingly inconsistent with the change of water permeability. The occurrence of water plasticization phenomenon was also pointed out through the non-constant diffusivity, reflecting an increased free volume with water activity. Again, a decrease in water diffusivity was unexpectedly obtained, explained by tortuosity effects and ascribed to local water retention at the vicinity of clays, which creates water aggregates in films. These findings were also confirmed by a mathematical modeling approach of sorption process, discussing about the sorbed water access.

Effects of Superabsorbent Polymers on the Hydraulic Parameters and Water Retention Properties of Soil

Superabsorbent polymers (SAPs) are widely applied in dryland agriculture. However, their functional property of repeated absorption and release of soil water exerts periodic effects on the hydraulic parameters and water-retention properties of soil, and as this property gradually diminishes with time, its effects tend to be unstable. During the 120-day continuous soil cultivation experiment described in this paper, horizontal soil column infiltration and high-speed centrifugation tests were conducted on SAP-treated soil to measure unsaturated diffusivityDand soil water characteristic curves. The experimental results suggest that the SAP increased the water retaining capacity of soil sections where the suction pressure was between 0 and 3,000 cm. The SAP significantly obstructed water diffusion in the soil in the early days of the experiment, but the effect gradually decreased in the later period. The average decrease in water diffusivity in the treatment groups fell from 76.6% at 0 days to 1.2% at 120 days. This research also provided parameters of time-varying functions that describe the unsaturated diffusivityDand unsaturated hydraulic conductivityKof soils under the effects of SAPs; in future research, these functions can be used to construct water movement models applicable to SAP-treated soil.

Computational Modelling of Cation Contamination in PEFCs

Polymer electrolyte membrane (PEM) is a key component of polymer electrolyte membrane fuel cell (PEFC). It is vulnerable to cationic impurities that are present in the air and fuel stream or that may have originated from the cell, stack or balance of plant components. Almost all foreign cations have higher affinity to sulfonic acid group than proton and the presence of foreign cation in the membrane or catalyst layers results in reduced ionic conductivity, increased electro-osmotic drag, and reduced water content (1).In this study, a steady, one dimensional cation (Na+) contamination model of membrane electrode assembly (MEA) of PEFC (shown in figure 1) is developed by solving the coupled transport phenomena related to the foreign cation, water, and oxygen. Concentrated solution theory is utilized to solve the cation transport equation based on Serincan et al.’s model (2) and dilute solution theory for water, hydrogen, and oxygen transport. A spherical agglomerate model is considered in the cathode catalyst layer (CCL) to describe effects of foreign cations in the catalyst layer. To separate the effects of cations on different transport phenomena, first no contamination case is solved and then contamination effects are added one by one to the cation transport, water transport, and oxygen transport and finally to the Nernst equation.Cation (Na+) contamination in the air stream found to cause several detrimental effects on the performance of PEFC. The main effects are: (a) cation replaces proton and occupies sulfonic acid sites in ionomer. It can occupy up to 96% sites in the cathode catalyst layer (CCL); (b) It causes serious water depletion in the ionomer due to decrease in water diffusivity; (c) It decreases the oxygen diffusivity causing a decrease in dissolved oxygen concentration in CCL; and (d) It decreases the open circuit voltage (OCV) in CCL due to reduction in proton concentration in the ionomer phase. These contamination effects on each parameter decrease the protonic current density and combined effects result in a reduction of current density from 0.72 A/cm2 to 0.25 A/cm2at a cell voltage of 0.7 V (shown in figure 2). Acknowledgements The authors are grateful for the financial support of the Department of Energy (DOE)-EERE, DE-EE00000467 (University of Hawaii, prime contractor, Jean St-Pierre, PI).

Water diffusivity in a mixed phase binary system: Effective water diffusion coefficients in tetradecanol–eicosanol mixtures

Fourier-transform infrared attenuated total reflection spectroscopy is used to measure the effective water diffusion coefficients in pure tetradecanol between 313K and 338K and in liquid and liquid–solid mixtures of tetradecanol (C14H29OH) and eicosanol (C20H41OH) at 313K. The temperature dependent data in tetradecanol yields an activation energy for diffusion higher than that measured in pure alkanes. This is consistent with the critical role hydrogen bonds play in diffusion. Differential scanning calorimetry and infrared spectroscopy are used to construct the solid–liquid phase diagram for the tetradecanol–eicosanol system. Diffusion measurements in these mixtures are linked to the experimental phase diagram. Water diffusivity in liquid mixtures at low eicosanol concentrations (80 to 100mol percent tetradecanol) suggests that the measured diffusion coefficient of water is independent of the mixture's bulk viscosity. We find a six-fold decrease in water diffusivity when the solid fraction of the equilibrium mixture increases from zero to thirty percent. The decrease in water diffusivity results from a longer diffusion path length caused by solid portions of the mixture acting as barriers to diffusion.

Water sorption behavior in polyimide thin films controlled by inorganic additives

We investigated the decreased water diffusivity depending on the content and size of carbon black (CB) added to polyimide (PI) in order to synthesize composite films (PI-CB). CBs with different diameters were used: N110 (CB1, 11–19 nm), N326 (CB2, 26–30 nm) and N660 (CB3, 49–60 nm). The water diffusion coefficient of PI-CB decreased with increasing CB content from 2.67·10−9 for pure PI films to 0.22·10−9 cm2/s for PI-CB films containing 40 wt% CB1. Moreover, the decrease in water diffusivity with increasing CB content exhibited a linear relationship. The water diffusivity of PI-CB increased with the increasing size of CB at the same CB content. The water uptake of PI-CB had a linear relationship depending on the CB content, similar to water diffusivity; the water uptake of composite films decreased by 73% (from 3.67% to 0.98%) for PI-CB1 (40 wt%) films as compared to PI films. In addition, the water uptake varied not only with the CB content, but also by the size of the CB surface area. Open image in new window

Computational Modeling of Foreign Cation Contamination in PEFCs

A steady-state, one dimensional computational model is developed to predict the effect of foreign cation contamination in polymer electrolyte fuel cells (PEFCs). The model solves the coupled transport phenomena of cations, oxygen, hydrogen, and water. Foreign cation (i.e. Na+) contamination in the air stream found to significantly decrease the performance of PEFC. The main effects are: (1) foreign cation replaces the proton and occupies the sulfonic acid sites in the ionomer. It can occupy up to 98% of the sites in the ionomer phase of the cathode catalyst layer (CCL). (2) It causes severe water depletion in the ionomer due to decrease in water diffusivity and equilibrium water uptake. (3) It decreases the oxygen diffusivity causing a decrease in dissolved oxygen concentration in CCL. (4) It also decreases the equilibrium potential due to reduction in proton concentration in the ionomer phase in cathode. These contamination effects on each parameter decrease protonic current density and combined effects result in a reduction of current density from 0.72 A/cm2 to 0.25 A/cm2 at a constant cell voltage of 0.7 V.

Contribution of perfusion in piffusion-weighted 1H-MRI of intrahepatic and subcutaneous hepatocellular carcinoma in rat

Hepatocellular carcinoma (HCC) and liver metastases are an increasing problem worldwide. Non-invasive methods for understanding of HCC growth mechanisms are highly desirable. A biexponential model for analysis of non-invasive diffusion-weighted 1 H magnetic resonance imaging (MRI) provides important information about neoplastic transformation in capillary liver tissue perfusion and water molecular diffusion. Fast and slow components of water apparent diffusion coefficient (ADC) were separated in the normal rat liver, intrahepatic, and subcutaneous HCCs. MRI was acquired with a Varian 9.4 T horizontal bore system. The fast component of ADC (ADC fast ), which contributes 38% to total signal in the intrahepatic HCC, was significantly lower compared to normal liver value, while the slow component of ADC did not differ in liver, intrahepatic, and subcutaneous HCCs. A decrease in ADC fast may be caused by restricted perfusion in abnormal tumor microvessels. Thus, a reported earlier decrease in ADC in HCC compared to normal liver was mostly due to a decreased in tumor perfusion rather than a decrease in water diffusion. Subcutaneous HCC showed a very limited vasculature development, which makes the tumor perfusion extremely poor and hypoxic. Simultaneous monitoring of water ADC changes in orthotopic and subcutaneous HCCs may be useful, but a possibility of location-based physiological and metabolic differences must be recognized. Keywords: hepatocellular carcinoma, 1 H-MRI, apparent diffusion coefficient, perfusion, tumor location.

Swell Drying: Coupling Instant Controlled Pressure Drop DIC to Standard Convection Drying Processes to Intensify Transfer Phenomena and Improve Quality—An Overview

Increasing globalization is driving agricultural production in developing countries toward better recovery levels and high additional value. A great opportunity is thus offered to emerging economies. By adopting appropriate and improved methods that allow better preservation, transport, and storage and that conform to the rigorous requirements and standards of organoleptic and nutritional quality as well as stringent hygienic criteria, emerging economies can derive significant benefits. Drying of biological materials usually results in shrinkage. The impact of shrinkage is often highly prejudicial for both the function and use of these materials. On the one hand, shrinkage results in a sharp decrease in water diffusivity through the porous structure (for dehydration and rehydration processes). On the other hand, the compact structure of dried biological material is usually unsuitable in terms of organoleptic quality and is inadequate for grinding. Thus, it is becoming very important to include retexturing and structure-expanding processes in standard drying processes. To this end, the instant controlled pressure drop (DIC) technology has been proposed and successfully tested as a texturing process for partially dried materials, which should normally intensify the whole operation. The quality of the final product as well as the performance of the drying process can also be dramatically improved. Finally, for a large category of dried fruits and vegetables, and even freeze-dried materials, microbiological contamination and the presence of insects and larvae are serious problems. DIC technology can be used as an ultra-high-temperature treatment for a controlled decontamination. Such dried, expanded, and completely decontaminated products can be used as highly nutritional snacks. They can easily be ground to produce expanded granulated powders with excellent sensory, nutritional, functional, and textural properties.

Water-Diffusion Slowdown in the Human Visual Cortex on Visual Stimulation Precedes Vascular Responses

We used magnetic resonance imaging (MRI) to investigate the temporal dynamics of changes in water diffusion and blood oxygenation level-dependent (BOLD) responses in the brain cortex of eight subjects undergoing visual stimulation, and compared them with changes of the vascular hemoglobin content (oxygenated, deoxygenated, and total hemoglobin) acquired simultaneously from intrinsic optical recordings (near infrared spectroscopy). The group average rise time for the diffusion MRI signal was statistically significantly shorter than those of the BOLD signal and total hemoglobin content optical signal, which is assumed to be the fastest observable vascular signal. In addition, the group average decay time for the diffusion MRI also was shortest. The overall time courses of the BOLD and optical signals were strongly correlated, but the covariance was weaker with the diffusion MRI response. These results suggest that the observed decrease in water diffusion reflects early events that precede the vascular responses, which could originate from changes in the extravascular tissue.

The effect of combretastatin A4 disodium phosphate and 5,6-dimethylxanthenone-4-acetic acid on water diffusion and blood perfusion in tumours

Purpose. To evaluate the effect of the vascular disrupting drugs combretastatin A-4 disodium phosphate (CA4DP) and 5,6-dimethylxanthenone-4-acetic acid (DMXAA) on the intra/extracellular volume fraction of water and blood perfusion in tumours using MRI. Methods and materials. Mice with C3H mammary carcinomas underwent repeated MRI T2-weighted imaging, water-diffusion and perfusion measurements before and up to 6-hours following CA4DP and DMXAA treatment. Results. CA4DP treatment caused an increase in water diffusion in those tumour areas that presented the lowest blood perfusion, however this appeared only after five hours. The blood perfusion in highly perfused tumour regions decreased immediately after administration of CA4DP. DMXAA treatment caused an early decrease in water diffusion in the low-perfused tumour segments and followed by a subsequent decrease in the remaining part of the tumour. The blood perfusion decreased early in all parts of the tumour. Conclusion. The effect of CA4DP and DMXAA on tumour blood flow was comparable. The reduction in regional blood flow caused by CA4DP in the whole tumour segment occurred early, however, changes in ADC after DMXAA appeared more extended and earlier than after CA4DP treatment, especially in tumour areas already suffering from a low blood perfusion.

NMR Relaxation and Water Self-Diffusion Studies in Whey Protein Solutions and Gels

The changes in water proton transverse relaxation behavior induced by aggregation of whey proteins are explained in terms of the simple molecular processes of diffusion and chemical exchange. The water self-diffusion coefficient was measured in whey protein solutions and gels by the pulsed field gradient NMR method. As expected, water self-diffusion was reduced with increased protein concentrations. Whatever the concentration, the water molecules were free to diffuse over distances varying from 15 to 47 mum. Water diffusion was constant over these distances, demonstrating that no restrictions were found to explain the water hindrance. The modification in protein structure by gelation induced a decrease in water diffusion. The effects of protein concentration on water diffusion are discussed and modeled. Two approaches were compared, the obstruction effect induced by a spherical particle and the cell model, which considered two water compartments with specific self-diffusion coefficients.

Water absorption in polyaminosiloxane-epoxy thermosetting polymers

Water absorption is a common problem in epoxy-based composites, especially in interfacial regions. In contrast to epoxy resins, silicones are hydrophobic materials. Modification of epoxy thermosets with silicones should improve physical properties because of decrease in water diffusion. Bisphenol-A diglycidyl ether (DGEBA) was cured with a synthesised aminopropyl perfunctionalised siloxane. Water absorption was analysed by different techniques, including near infrared (FTIR) water absorbance at 5200 cm −1 and gravimetry. Fick’s law diffusion coefficients were calculated. Swelling equilibrium of the resin was also analysed. Absorbed water catalyses the curing reaction strongly, allowing to reach high conversions.

Transient decrease in water diffusion observed in human occipital cortex during visual stimulation.

Using MRI, we report the observation of a transient decrease of the apparent diffusion coefficient (ADC) of water in the human brain visual cortex during activation by a black and white 8-Hz-flickering checkerboard. The ADC decrease was small (<1%), but significant and reproducible, and closely followed the time course of the activation paradigm. Based on the known sensitivity of diffusion MRI to cell size in tissues and on optical imaging studies that have revealed changes in the shape of neurons and glial cells during activation, the observed ADC findings have been tentatively ascribed to a transient swelling of cortical cells. These preliminary results suggest a new approach to produce images of brain activation with MRI from signals directly associated with neuronal activation, and not through changes in local blood flow.

Bubble growth in highly viscous silicate melts during continuous decompression from high pressure

High melt viscosity is thought to hinder bubble growth in water-bearing silicate melts, and viscosities above ∼10 9 Pa s may prevent growth and viscously quench a bubbly liquid. To investigate the influence of melt viscosity ( η) on magma degassing, rhyolitic melts were experimentally saturated with water at high pressures and then decompressed at a rate of either 0.125 or 0.25 MPa s −1; viscosity ( η = 2.5 × 10 6–6.3 × 10 8 Pa s) was varied between experiments by changing the initial hydration pressures and temperatures. Dissolved water contents and bubble sizes and porosities indicate that melts degassed in equilibrium when η = 2.5 × 10 6 Pa s, whereas when η > 10 8 Pa s, the melts did not degas at all, despite pressure drops up to 50 MPa. The transition between efficient and inefficient degassing occurred when η = ∼10 7–8 Pa s. In all experiments, bubbles expanded in size in response to pressure drops, but the extent of expansion and the size of bubbles that expanded both decreased as viscosity increased (e.g., 0–40 μm bubbles expanded when η = 1.6 × 10 8 Pa s; 0–20 μm bubbles expanded when η = 6.3 × 10 8 Pa s). The shift from efficient to inefficient degassing probably resulted from the decrease in water diffusivity ( D H 2 O ) as temperature decreased, whereas the decrease in degree of bubble expansion at higher viscosities resulted from increasing viscous resistance. Our results confirm model predictions that bubble expansion will be arrested when η ∼10 9 Pa s, at decompression rates of 0.125 and 0.25 MPa s −1. Such rates are expected only in explosive volcanic eruptions, however, and so higher viscosities will be needed for the melt to resist bubble growth in effusive eruptions.

Early detection of periventricular leukomalacia by diffusion-weighted magnetic resonance imaging techniques

Periventricular leukomalacia (PVL), the principal form of brain injury in the premature infant, is characterized by overt focal necrotic lesions in periventricular white matter and less prominent, more diffuse cerebral white matter injury. The early detection of the latter, diffuse component of PVL is not consistently possible with conventional brain imaging techniques. We demonstrate the early detection of the diffuse component of PVL by diffusion-weighted magnetic resonance imaging (DWI). In a premature infant with no definite cerebral abnormality detectable by cranial ultrasonography or conventional magnetic resonance imaging, DWI showed a striking bilateral decrease in water diffusion in cerebral white matter. The DWI abnormality (ie, decreased apparent diffusion coefficient) was similar to that observed with acute cerebral ischemic lesions in adults. At 10 weeks of age, conventional magnetic resonance imaging and ultrasonography showed striking changes consistent with PVL, including the presence of small cysts. The observations indicate the importance of DWI in the early identification of the diffuse component of PVL and also perhaps the role of ischemia in the pathogenesis of the lesion. (J Pediatr 1999;134:631-4)

Water vapor permeability and diffusivity through methylcellulose edible films

A model edible film made with methylcellulose was studied for its water vapor barrier properties. The steady-state water vapor transmission rate (WVTR) increased with both the water vapor pressure gradient and the initial water content before permeation. A decrease in water diffusion with increasing moisture content was due to a clustering phenomenon of water molecules within the film. Water concentration profiles within the film were estimated from the sorption isotherm, and differed from the theoretical linear profile based on Fick's first law. The WVTR and the diffusivity depended strongly on the water concentration because of interactions between water molecules and the polymer matrix. Thus water vapor permeability (WVP) calculations used for synthetic polymeric packaging and based on Fick's and Henry's laws do not apply for methylcellulose edible films.

Water binding and mobility in the phosphatidylcholine/cholesterol/water lamellar phase

Measurements of hydration and water self diffusion in lamellar phases of the ternary system: phosphatidylcholine/cholesterol/water have been made using pulse NMR relaxation methods. Systems containing phosphatidylcholine and cholesterol in a 1 : 1 mol ratio with varying water contents are studied at 20.5°C. The results indicate that 12 water molecules corresponds to complete hydration of the phosphatidylcholine/cholesterol unit, and in the region of this hydration a 4-fold decrease in water diffusion occurs. The nature of the bound water and its relationship to phase stability and overall water mobility in the system are discussed. It is concluded that at the stoichiometric composition the diffusion decreases due to the relative immobility of the bound water. The implications in terms of permeability regulation in the aqueous channels by water content and hydration are cited.