Permeability to Water

The permeability of concrete and the rates at which ions and gases diffuse in it are of major importance for durability. We shall consider only the behaviour of cement paste. 

Under certain experimental conditions permeability to water obeys d Arcy s law  

Powers and co-workers (P20,P34,P43) determined values of K of hep for various combinations of w/c and age. They assumed that the capillary pores in the paste were initially continuous, but that at a certain stage in hydration they became segmented into isolated cavities, so that the water could only 

Subsequent studies have shown that K is not related to the total water porosity but depends on the volume and connectivity of the larger pores. Mehta and Manmohan (M75) and Nyame and Illston (N16) found linear relations between log and estimates of the maximum continuous pore radius, obtained from MIP, and other quantities derived from the pore size distribution and degree of reaction. A high proportion of the flow appears to be through pores wider than about 100 nm. Typical values of log K for mature pastes cured at ordinary temperatures range from around - 13.4 at w c 0.3 to around — 11.8 at w/c 0.7 (M75,N16,G67,M76,H47). K increases with temperature (G67,M76). 

Hooton (H48) noted some pitfalls in the experimental determination of permeabilities and in the practical utilization of the results. In an experimental determination, accurate results will not be obtained unless the sample has been vacuum saturated and, even if it is, equilibrium flow in accordance with d Arcy s law may not occur because water is being used up to continue hydration. In practice, a concrete sample is probably not often saturated throughout if it is not, capillary forces, as well as the pressure difference. alTect the rate of flow. Applications of results obtained with pastes to concrete are further complicated by the presence in the latter of cracks, poorly compacted areas and other inhomogeneities. 

Polyurethane, like all other plastics, consists of chains that are always moving on the molecular scale. The polyurethane has molecular-size voids in which it will allow small molecules to enter the material. The material breathes and allows moisture to enter. 

This moisture has several effects. Initially, it will react with any remnants of isocyanates and curatives. Moisture acts like a plasticizer, helping to give the final properties of the polyurethane. Under the influence of heat, it will cause some breakdown of the material, especially in the polyesters. 

In all such applications, the bonding surface must be correctly prepared, and the correct bonding agents must be used, allowed to dry, and, if needed, activated properly. 

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Because it is a saturated aUphatic hydrocarbon, LLDPE does not conduct electricity, and so is widely used for wire and cable insulation. LLDPE is poorly permeable to water and inorganic gases and only slightly more so to organic compounds, whether Hquid or gas. 

BiaxiaHy orieated PPS film is transpareat and nearly colorless. It has low permeability to water vapor, carbon dioxide, and oxygen. PPS film has a low coefficient of hygroscopic expansion and a low dissipation factor, making it a candidate material for information storage devices and for thin-film capacitors. Chemical and thermal stability of PPS film derives from inherent resia properties. PPS films exposed to tolueae or chloroform for 8 weeks retaia 75% of theh original streagth. The UL temperature iadex rating of PPS film is 160°C for mechanical appHcatioas and 180°C for electrical appHcations. Table 9 summarizes the properties of PPS film. 

The general conclusion drawn from these considerations is that paint films are so permeable to water and oxygen that they cannot inhibit corrosion by preventing water and oxygen from reaching the surface of the metal, that is to say they cannot inhibit the cathodic reaction. 

Atmospheric exposure trials, carried out in Cambridge, established the fact that when rusty specimens were painted in the summer, their condition, after some years exposure, was very much better than that of similar specimens painted in the winter It was found that steel weathered in Cambridge carried spots of ferrous sulphate, deeply imbedded in the rust, and that the quantity of ferrous sulphate/unit area was very much greater in the winter than in the summer this seasonal variation was attributed to the increased sulphur dioxide pollution of the atmosphere in the winter, caused by the combustion of coal in open grates. It was concluded that there was a causal relationship between the quantity of ferrous sulphate and the effective life of the paint. It was suggested that these soluble deposits of ferrous sulphate short-circuit the resistance of the paint film and, since paint films are very permeable to water and oxygen, the ferrous sulphate will become oxidised and hydrolysed with the production of voluminous rust, which will rupture the film at numerous points, thus giving rise to the characteristic type of failure seen on painted rusty surfaces. 

Makower and Nielsen 21) have described another method that can be used to alter the texture of the dehydrated material in order to increase its permeability to water vapor and to shorten the time required for removal of water. The method, called the lyophilization procedure, involves the following steps ... 

AQP6 is expressed in the intercalated cells of the kidney collecting duct. This channel is hardly permeable to water, but capable of transporting anions, including chloride, and is therefore thought to play a role in maintenance of body acid-base balance or in intracellular vesicle acidification. 

The blood-brain barrier (BBB) forms a physiological barrier between the central nervous system and the blood circulation. It consists of glial cells and a special species of endothelial cells, which form tight junctions between each other thereby inhibiting paracellular transport. In addition, the endothelial cells of the BBB express a variety of ABC-transporters to protect the brain tissue against toxic metabolites and xenobiotics. The BBB is permeable to water, glucose, sodium chloride and non-ionised lipid-soluble molecules but large molecules such as peptides as well as many polar substances do not readily permeate the battier. 

There are aspects of cell membranes other than their permeability to water and solutes that also play a critical role in the responses of cells to freezing. The structure of the plasma membrane allows cells to supercool and probably determines their ice-nucleation temperature. The nucleation temperature along with the permeability of membranes to water are the chief determinants of whether cells cooled at... 

Examples of preservatives are phenylmercuric nitrate or acetate (0.002% w/v), chlorhexidine acetate (0.01 % w/v), thiomersal (0.01 % w/v) and benzalkorrium chloride (0.01 % w/v). Chlorocresol is too toxic to the comeal epithehum, but 8-hydroxyquinoline and thiomersal may be used in specific instances. The principal considerahon in relation to antimicrobial properties is the activity of the bactericide against Pseudomonas aeruginosa, a major source of serious nosocomial eye infections. Although benzal-konium chloride is probably the most active of the recommended preservatives, it cannot always be used because of its incompatibility with many compounds commonly used to treat eye diseases, nor should it be used to preserve eye-drops containing anaesthetics. Since benzalkonium chloride reacts with natural mbber, silicone or butyl rabber teats should be substituted. Since silicone mbber is permeable to water vapour, products should not be stored for more than 3 months after manufacture. As with all mbber components, the mbber teat should be pre-equilibrated with the preservative prior to... 

Key mechanisms important for improved oil mobilization by microbial formulations have been identified, including wettability alteration, emulsification, oil solubilization, alteration in interfacial forces, lowering of mobility ratio, and permeability modification. Aggregation of the bacteria at the oil-water-rock interface may produce localized high concentrations of metabolic chemical products that result in oil mobilization. A decrease in relative permeability to water and an increase in relative permeability to oil was usually observed in microbial-flooded cores, causing an apparent curve shift toward a more water-wet condition. Cores preflushed with sodium bicarbonate showed increased oil-recovery efficiency [355]. 

In these systems, osmotic pressure provides the driving force to generate controlled release of drug. Consider a semipermeable membrane that is permeable to water, but not to drug. A tablet containing a core of drug surrounded by such a membrane is shown in Fig. 9. When this device is exposed to water or any body fluid, water will flow into the tablet owing to the osmotic pressure difference. The rate of flow, dV/dt, of water into the device can be represented as... 

Wettability is defined as "the tendency of one fluid to spread on or adhere to a solid surface in the presence of other immiscible fluids" (145). Rock wettability can strongly affect its relative permeability to water and oil (145,172). Wettability can affect the initial distribution of fluids in a formation and their subsequent flow behavior. When rock is water-wet, water occupies most of the small flow channels and is in contact with most of the rock surfaces. The converse is true in oil-wet rock. When the rock surface does not have a strong preference for either water or oil, it is termed to be of intermediate or neutral wettability. Inadvertent alteration of rock wettability can strong alter its behavior in laboratory core floods (172). 

Changing the wettability of reservoir rock surfaces from oil-wet to water-wet, increases the permeability of the formation to oil, decreases the permeability to water, decreases mobility ratio, increases sweep efficiency, increases the flowing fraction of oil at every saturation, and increases oil recovery at the economic limit of the waterflood. 

Atwood, D.K. "Restoration of Permeability to Water-Damaged Cores," SPE paper 905, 1964 SPE Annual Fall Meeting, Houston, October 11-14. 

Capillaries are the site of exchange between blood and the interstitial fluid surrounding tissue cells. Tissues with a higher metabolic rate have a more extensive capillary network, that is, a greater number of capillaries per unit area. Because of extensive branching of these vessels, the cells of the body are typically within 20 pm of the nearest capillary. Consequently, the distance that substances must travel between blood and the cells is minimized. Capillaries are permeable to water and small water-soluble substances, such as glucose, amino acids, lactic acid, and urea, and impermeable to proteins. 

The descending limb of the Loop of Henle is permeable to water only. As this region of the tubule passes deeper into the medulla, water leaves the filtrate down its osmotic gradient until it equilibrates with the increasingly concentrated interstitial fluid (see Figure 19.5). As a result, the filtrate also... 

Osmotic diuretics such as mannitol act on the proximal tubule and, in particular, the descending limb of the Loop of Henle — portions of the tubule permeable to water. These drugs are freely filtered at the glomerulus, but not reabsorbed therefore, the drug remains in the tubular filtrate, increasing the osmolarity of this fluid. This increase in osmolarity keeps the water within the tubule, causing water diuresis. Because they primarily affect water and not sodium, the net effect is a reduction in total body water content more than cation content. Osmotic diuretics are poorly absorbed and must be administered intravenously. These drugs may be used to treat patients in acute renal failure and with dialysis disequilibrium syndrome. The latter disorder is caused by the excessively rapid removal of solutes from the extracellular fluid by hemodialysis. 

Zeidel, M. L., The relationship between membrane fluidity and permeabilities to water, solutes, ammonia, and protons, J. Gen. Physiol. 1995, 306, 67-84. 

Cell membranes or synthetic lipid vesicles with normal low permeability to water will, if reconstituted with AQP1, absorb water, swell and burst upon exposure to hypo-osmotic solutions. The water permeability of membranes containing AQP 1 can be about 100 times greater than that of membranes without aquaporins. The water permeability conferred by AQP1 (about 3 billion water molecules per subunit per second) is reversibly inhibited by Hg2+, exhibits low activation energy and is not accompanied by ionic currents or translocation of any other solutes, ions or protons. Thus, the movement of water through aquaporins is an example of facilitated diffusion, in this case driven by osmotic gradients. 

Furthermore, the transplants are often packed in aluminum boxes, which are sealed by sterile filters permeable to water vapor. Even if the box can be designed with a negligible resistance to water vapor flow, the heat transfer is substantially reduced. 

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