Water Diffusion Mechanism Research Articles

Water diffusion in fully hydrated porcine stratum corneum

The microscopic mechanisms of water diffusion in fully hydrated porcine stratum corneum (SC) have been studied by a combination of incoherent quasielastic neutron scattering (QENS) and pulsed field gradient-nuclear magnetic resonance (PFG-NMR) for two sample orientations. The presence of three types of water in fully hydrated SC is inferred on the basis of water sorption isotherm data, i.e., (a) bound and (b) weakly bound hydration water forming layers between adjacent lipid bilayers of SC, as well as (c) bulk water probably located in the corneocytes and in intercellular regions. Water self-diffusion coefficients for motions parallel and perpendicular to the membrane plane of D∥=3.30×10−10 m2/s and D⊥=1.56×10−10 m2/s, respectively, were determined by PFG-NMR and assigned to the translational diffusion of weakly bound water. QENS measurements have been carried out using different samples hydrated with H2O and D2O, respectively, in order to separate the contribution of SC from that of the water. The QENS data for both sample orientations and two different energy resolutions can be fitted by a model which accounts for the microscopic dynamics of all three aforementioned types of water. This analysis establishes rotational diffusion coefficients for bound and weakly bound hydration water of 0.025 and 0.030 meV, respectively. Furthermore, the QENS data prove the presence of bulk water in fully hydrated SC samples.

The role of ions diffusion in the cathodic delamination rate of polyester coated phosphatized steel

A very important mechanism of organic coating deterioration is the cathodic delamination. The basic knowledge about the diffusion mechanism of ions, oxygen and water through the coatings and through the defects in the coatings is not yet sufficient to improve the performance of organic coatings by developing more resistant coatings, and more research is necessary. In this study a particular electrochemical cell has been developed in order to keep separated the solution in contact with an artificial defect (inner solution) and the solution in contact with the rest of the intact organic coating (outer solution). In this way it is possible to polarize the sample in the cathodic region and to control the environment (ions and their concentration, oxygen, water activity) in the defect and around the defect. With this cell samples of polyester coated phosphatised steel were studied. The cathodic delamination was measured, in different solutions, by measuring the distance of delamination from the artificial defect after different times of exposure to cathodic polarisation. The experimental results are discussed in terms of two parameters: the induction time (time necessary to notice the beginning of delamination) and the delamination rate (mm2 of delamination per hour). On comparing the experimental data it was possible to obtain new and interesting information on the delamination mechanism and to determine also the rate controlling step of the phenomenon for our materials, which is found to be the cation diffusion through both the coating and the defect. The proposed mechanisms were confirmed also by SEM observation and surface analysis.

Study of water movement in concrete

An investigation on the movement of water in 100% Portland cement concretes is presented. The water diffusion and sorptivity tests, together with the accelerated water permeability test, are performed to determine the transport parameters that characterise the mechanisms of water diffusion, sorptivity and permeability in concrete respectively. The effects of pore humidity, ambient temperature, environmental relative humidity, applied hydrostatic pressure gradient and water-cement ratio are also explored. The results obtained from the tests on water diffusion, sorptivity and permeability show good agreement with those reported in the literature. The study of water movement has practical implications on the prediction of transport and distribution of aggressive chemical agents in concrete, as well as on the development of rational and quantitative durability assessment for concrete structures exposed to different climatic and environmental conditions.

Study of water movement in concrete

An investigation on the movement of water in 100% Portland cement concretes is presented. The water diffusion and sorptivity tests, together with the accelerated water permeability test, are performed to determine the transport parameters that characterise the mechanisms of water diffusion, sorptivity and permeability in concrete respectively. The effects of pore humidity, ambient temperature, environmental relative humidity, applied hydrostatic pressure gradient and water-cement ratio are also explored. The results obtained from the tests on water diffusion, sorptivity and permeability show good agreement with those reported in the literature. The study of water movement has practical implications on the prediction of transport and distribution of aggressive chemical agents in concrete, as well as on the development of rational and quantitative durability assessment for concrete structures exposed to different climatic and environmental conditions.

Molecular dynamics simulation study of the mechanisms of water diffusion in a hydrated, amorphous polyamide

Atomistic, molecular dynamics simulations of water diffusion in a hydrated, amorphous polyamide have been carried out to study the effects of polymer dynamics, cross-linking, and hydrogen bonding interactions on the molecular mechanisms of diffusion. This polymer was selected as a model for the discriminating layer of FT-30 reverse osmosis membranes, used commercially for water desalination. Analysis of the configurations generated during the simulations shows that, at the relatively high water content studied, a continuous water phase is formed which permeates the polymer and consists of more than 90% of all waters of hydration. Water diffusion takes place by distinct “jump-like” movements between weakly localized sites in this continuous phase. The jump length is on average ∼3Å, independent of the system studied, and is most likely defined by the local, cooperative rearrangement of water molecules. However, the jump frequency, or equivalently, the rate of water diffusion varies from system to system, and depends on polymer dynamics and cross-linking density, and more significantly, on water–water hydrogen bonding interactions.

The relationship between water absorption characteristics and the mechanical strength of resin-modified glass-ionomer cements in long-term water storage.

The purpose of this study is to elucidate the water absorption characteristics of resin-modified glass-ionomer cements and to also investigate the relationship between the characteristics and mechanical strength after long-term water storage. The mechanism of water diffusion in these cements is also discussed. Water absorption was measured using a gravimetric analysis for 12 m, while the diffusion coefficient was calculated using Fick's law of diffusion. Water solubility was determined based on the weight of the residue in the immersed water. The compressive and diametral tensile strength were measured at 1, 2, 6, and 12 m. A correlation was observed between the diffusion coefficient and equilibrium water uptake, which thus suggests the water in the cements to diffuse through micro-voids in accordance with the 'Free volumetric theory'. A correlation was seen between the solubility and diffusion coefficient of the cements. The deterioration ratio, defined as the ratio of the strength at 12 m versus that at 1 m, was also calculated. Finally, a negative correlation was observed between the deterioration ratio of the compressive strength and the diffusion coefficients of the cements.

Stratum corneum thickness and apparent water diffusivity: facile and noninvasive quantitation in vivo.

Water diffusivity across human stratum corneum (SC) is independent of position in the membrane, i.e., water transport is homogeneous in this structurally heterogeneous membrane. In this study, repeated adhesive tape-stripping of the SC and concomitant measurements of transepidermal water loss (TEWL) allows facile determination of SC thickness and deduction of an apparent average water diffusion coefficient across the membrane. The results demonstrate that simple, noninvasive two-point measurements of TEWL, and cumulative SC weight removed by tape-stripping, can be used to provide accurate estimates of SC thickness and apparent water diffusivity across the membrane, in vivo in man. The results obtained are in good agreement with a more detailed analysis of the data and with previously published values using this and other, less convenient, techniques. While the approach does not permit the molecular mechanism of water diffusion across the SC to be deduced (and hence diffusivity values must be prefaced by the word "apparent"), it does lend itself to facile application at other body sites, and to diseased or (for example) penetration-enhancer perturbed skin.

Thermal and diffusion processes in laser-induced stress relaxation and reshaping of cartilage

The thermodynamic characteristics of the ‘bound-to-free’ phase transformation of water are studied by means of differential microcalorimetry and FTIR spectroscopy. This phase transition controls the stress relaxation and reshaping of cartilage which we have observed previously under moderate laser heating. It is shown that the FTIR spectrum of bound water in cartilage differs from that of free water in cartilage, and that both show differences to the FTIR spectrum of pure water. The proportion of bound water in cartilage is found to be of the order 4%. We have examined water liberation and absorption kinetics and found them to be controlled by diffusion through the tissue and also by the bound-to-free water transformation. The theoretical modelling and experimental data allowed calculation of diffusion coefficients and the activation energy for water transfer. The latter was found to be close to the heat of phase transformation of water. We have established that the drying and wetting processes in cartilage are reversible (fully or in a part, depending on the temperature of preliminary drying) and can be described by identical solutions of the diffusion problem, with coefficients of water diffusion being the same. The mechanism of water diffusion in cartilaginous tissue is also discussed.

The mechanisms of water diffusion in polymerized silicate melts

Diffusion of water was experimentally investigated for melts of albitic (Ab) and quartz-orthoclasic (Qz29Or71, in wt %) compositions with water contents in the range of 0 to 8.5 wt % at temperatures of 1100 to 1200 °C and at pressures of 1.0 and 5.0 kbar. Apparent chemical diffusion coefficients of water (D water) were determined from concentration-distance profiles measured by FTIR microspectroscopy. Under the same P-T condition and water content the diffusivity of water in albitic, quartz-orthoclasic and haplogranitic (Qz28Ab38 Or34, Nowak and Behrens, this issue) melts is identical within experimental error. Comparison to data published in literature indicates that anhydrous composition only has little influence on the mobility of water in polymerized melts but that the degree of polymerization has a large effect. For instance, Dwater is almost identical for haplogranitic and rhyolitic melts with 0.5–3.5 wt % water at 850 °C but it is two orders of magnitude higher in basaltic than in haplogranitic melts with 0.2–0.5 wt % water at 1300 °C. Based on the new water diffusivity data, recently published in situ near-infrared spectroscopic data (Nowak 1995; Nowak and Behrens 1995), and viscosity data (Schulze et al. 1996) for hydrous haplogranitic melts current models for water diffusion in silicate melts are critically reviewed. The NIR spectroscopy has indicated isolated OH groups, pairs of OH groups and H2O molecules as hydrous species in polymerized silicate melts. A significant contribution of isolated OH groups to the transport of water is excluded for water contents above 10 ppm by comparison of viscosity and water diffusion data and by inspection of concentration profiles from trace water diffusion. Spectroscopic measurements have indicated that the interconversion of H2O molecules and OH pairs is relatively fast in silicate glasses and melts even at low temperature and it is inferred that this reaction is an active step for migration of water. However, direct jumps of H2O molecules from one cavity within the silicate network to another one can not be excluded. Thus, we favour a model in which water migrates by the interconversion reaction and, possibly, small sequences of direct jumps of H2O molecules. In this model, immobilization of water results from dissociation of the OH pairs. Assuming that the frequency of the interconversion reaction is faster than that of diffusive jumps, OH pairs and water molecules can be treated as a single diffusing species having an effective diffusion coefficient The shape of curves of Dwater versus water content implies that increases with water content. The change from linear to exponential dependence of Dwater between 2 and 3 wt % water is attributed to the influence of the dissociation reaction at low water content and to the modification of the melt structure by incorporation of OH groups.

Swelling/deswelling of anionic copolymer gels

Studies of dynamic and equilibrium swelling of ionic gels are important in understanding the diffusion of physiologically important fluids in materials for site-specific controlled drug delivery applications. The dynamic and equilibrium swelling properties of dry glassy poly(2-hydroxyethyl methacrylate-comethacrylic acid) and poly(2-hydroxyethyl methacrylate-co-acrylic acid) polymeric networks were studied as a function of pH, ionic strength, nature of the counterion and buffer composition. The mechanism of water diffusion in these gels became more anomalous as the pH of the swelling medium increased and as the ionic strength decreased at a constant pH ≥ p K a,gel. The mechanism of water diffusion was Fickian in all unbuffered swelling media at pH 4.0, which is lower than the pK a,gel. The p K a,gel of these gels was between 5.5 and 6. At pH 4.0, the diffusion mechanism was independent of ionic strength. This swelling behaviour is explained in terms of the concept of ion osmotic swelling pressure and ion exchange kinetics.

Water diffusion and sorption in amorphous macromolecular systems and foods

Understanding the mechanisms of water sorption and diffusion in amorphous polymers is of particular importance in relation to our ability to interpret related transport phenomena in foods. Penetrant transport in rubbery or glassy polymers may be classified as Fickian, non-Fickian (anomalous), Case II or super Case II transport, depending on the thermodynamic activity and temperature of the system in consideration. Additional structural parameters influencing the transport mechanism include the molecular weight, degree of crosslinking and degree of branching of the polymer, and its thermal and solvent expansion coefficients. The mechanisms of water transport can be determined by a variety of experimental techniques, the simplest one of which is gravimetry. In gravimetric studies, the amount of water absorbed in the polymer is plotted versus time according to equation M t M ∞ = kt 1 . If the exponent n is 0·5 (for planar systems) the diffusion is Fickian. Non-Fickian behavior is observed for 0·5<n<1·0, with a limit of Case II transport for n = 1·0. Non-Fickian and Case II transport are indicative of coupling of diffusional and relaxational mechanisms. Relaxation is related to a transition from rubbery to glassy state. Major relaxational mechanisms are indicative of stresses formed in the polymer during swelling. These observations translate into important consequences for water transport in food products, as the mechanism of water transport may influence the performance of food products in processing and use.

Hydrothermal effects on the physico‐chemical properties of pure and glass fiber reinforced polyester and vinylester resins

AbstractThe effect of aging by immersion in water at 60°C on polyester and vinylester resins, unreinforced and reinforced with glass fibers, was investigated. Changes in weight, glass transition temperature, secondary relaxations, and storage modulus, as well as loss modulus were measured, and the environmental effects on these parameters were assessed. Moreover, the chemical structure changes of the materials during aging were studied by the use of IR spectra. The evolution of the physico‐chemical and the mechanical properties as a function of the aging time was established. The results indicate the importance of the matrix morphology, the structure regularity, the presence of the fibers on the water diffusion mechanisms, and the thermomechanical properties through gravimetric changes, DSC, DMA and IR experiments.

La diffusion reactive de l'eau dans le verre de silice les implications du modele classique

When gaseous H 2 18O reacts on a plane substratum of hydrated silica glass, the elementary composition of which is uniform and constant, 18O concentration profiles are not consistent with an error function complement. We recall the accepted model for the reactive diffusion mechanism of water and we demonstrate a relation allowing the calculation of the apparent self-diffusion coefficient of oxygen atoms from any experimental 18O concentration profile. This coefficient is related to that of hydrogen atoms for the same experiment. Concerning diffusion of gaseous H 2O into silica glass initially free of OH groups, the elementary composition varies at any position, but we can suppose that the hydration equilibrium of silica glass by molecularly dissolved water at this place is reached. Thus we obtain a relation between the apparent diffusion coefficient of hydrogen atoms in these conditions, and that corresponding to the same, but uniform and constant, elementary composition.

Studies of water permeability and proteins of erythrocyte membranes in patients with Duchenne muscular dystrophy.

The characteristics of water permeability of erythrocytes from 54 Duchenne muscular dystrophy (DMD) patients and age-matched controls have been determined by a pulse nuclear magnetic resonance (NMR) technique. A decreased permeability of erythrocyte membrane in DMD was definitely found at all temperatures between 15 and 42 degrees C, with normal values for the activation energy of water diffusion. No differences between DMD and control subjects in the pattern of erythrocyte membrane polypeptides separated by two-dimensional electrophoresis could be detected. The findings are discussed in relation to the molecular mechanisms of water diffusion across erythrocyte membrane and the problem of erythrocyte membrane abnormalities in DMD. A new interpretation of erythrocyte membrane alterations is proposed based on the recent findings regarding the molecular pathology of DMD.

Decreased water permeability of erythrocyte membranes in patients with duchenne muscular dystrophy

The water proton relaxation times of erythrocytes (T'2a) from 15 Duchenne muscular dystrophy (DMD) patients and age-matched controls have been determined by a pulse nuclear magnetic resonance technique. Statistically significant higher values of T'2a were noted for the DMD erythrocytes, both in the presence or absence of p-chloromercuribenzene sulfonate, a potent inhibitor of water diffusion through erythrocyte membrane. As the values of T'2a are inversely related to the water permeability, the results indicate a decreased permeability of erythrocyte membrane in DMD. The findings are discussed in relation to the molecular mechanisms of water diffusion across erythrocyte membrane and in regard to the general problem of membrane abnormalities in DMD.

Water in epoxy resins Part II. Diffusion mechanism

AbstractA model is proposed for the diffusion mechanism of water in glassy epoxy resins. The polymer network is assumed to consist of two regions in which water molecules possess different mobilities. By considering the distribution of water molecules among these regions it is possible to describe the concentration dependency of the diffusion coefficient in the sorption and resorption processes. The diffusion coefficient becomes constant when the sorption temperature is close to the effective glass transition temperature of the epoxy‐water binary mixture. An explanation of this effect is also provided by the model.

The physico-chemical mechanism of mediated transport I. Facilitated diffusion

On the basis of the currently accepted model for the cell membrane structure, a physico-chemical model for mediated transport is developed and solved for the case of polar non-electrolyte migration through the cell membrane. The model considers the interstitial space defined by the transport protein subunits to be the migration pathway for polar solutes. A Langmuir-type adsorption equilibrium is assumed at the interfaces and a multicomponent diffusion mechanism of solute and water is postulated within the migration pathway, where the polar residues of the transport protein represent another component of the system. Membrane selectivity is governed by the adsorption constants, which are shown to affect strongly the kinetics of transport. Isosmotic transport and the volume change of the cell are important features incorporated in the model, which is shown to fulfill the peculiar properties of facilitated diffusion systems. It is concluded that the same type of pathway can be used for the transport of other polar solutes through existing or induced hydrophilic channels, for which a similar approach is suggested.

On the use of Polyethylene Insulation in the Manufacture of Submarine Cables

In the manufacturing process of a submarine coaxial cable, a multitrough water cooling system is used to avoid formation of voids and to strengthen the gripping force between the insulation polyethylene and the metallic center conductor. In submarine coaxial cable systems, the standards for dielectric properties of the insulation, polyethylene, are very strict. The adverse effect of small amounts of water in the polyethylene can not be neglected. In the cable manufacturing process quench cooling is favorable to the decrease of the water content in polyethylene. On the other hand, gradual cooling is better to avoid formation of voids. To find the optimum manufacturing process under these two conflicting conditions, computor simulations of the diffusion mechanism of water in polyethylene and the void forming mechanism have been made using available experimental data.

Volume increase of kernels of corn and sorghum accompanying absorption of liquid water

AbstractThe linearity between weight and volume increases, which takes place when kernels of corn and sorghum are immersed in water, was established experimentally. The rate of volume increase was correlated according to an equation analogous to the diffusion equation. The coefficients of volume increase obeyed the Arrehenius equation. The results would be of practical value in designing steeping vessels, as well as of theoretical interest in elucidating the mechanism of water diffusion in cereal grains.