Permeability to salt

Permeability to salt. PTHF shows a certain resistance to the flow of salt (96) and has been suggested as a possible membrane for desalina- 

Polymer Thickness Water-flux A/hr/cm Salt flux /Kg/hr/cm Salt rejection equivalent % 

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Reverse osmosis and normal osmosis (dialysis) are directly related processes. In simple terms, if a selective membrane (i.e., a membrane freely permeable to water, but much less permeable to salt) separates a salt solution from pure water, water will pass through the membrane from the pure water side of the membrane into the side less concentrated in water (salt side) as shown in Figure 2.8. This process is called normal osmosis. If a hydrostatic pressure is applied to the salt side of the membrane, the flow of water can be retarded and, when the applied pressure is sufficient, the flow ceases. The hydrostatic pressure required to stop... 

Membranes are typically made of cellulose acetate or aromatic polyamides because of their high permeability to water and low permeability to salts. They are normally produced in tubular or spiral-wound modules, which are then packed inside a reaction vessel. Their fouling can be minimized by either pretreating the influent streams or diluting them with the clean water produced. 

The use of interfacially formed thin films for membrane separations was, therefore, obvious. Reference to such membranes in Morgan s book characterizes them as retentive to dyes but permeable to salts. Early attempts to use interfacially formed membranes at the North Star Research Laboratories in the late 1960 s tended to confirm the low salt retention described by Morgan. In these early attempts to prepare composite membranes polysulfone support films were saturated in solutions of various diamines, acid acceptors and surfactants. After draining, the films were contacted with hexane solutions of various diacyl chlorides. In this initial work, salt rejections of these membranes were generally too low for practical use in seawater or brackish water applications. Similarly poor results were described in a patent by Scala and co-workers, dating to that era, wherein interfacial condensation membranes were produced (34). 

Several elastomers were evaluated for permeability to salt and deionized water at several temperatures. A WRT Neoprene (Burke Rubber Co. Type 5109) was tested at 23, 40 and 70°C using both salt water (3.5%) and DI water as permeants. The results for salt water are listed In Table III and for DI water In Table IV. Salt water permeability Increased 24 times with a 47°C temperature rise while the DI water Increase was 33 times. The difference may be due to the much greater solubility of DI water relative to salt water. 

The state of water in a membrane can become important for RO membranes as it is often related to rejection. Murphy and dePinho (1995) studied the state of water in RO and UF membrane active layers using attenuated total reflection fourier transform infrared spectroscopy (ATR-FTIR). Water content increased with porosity, and the strucmre of the water was dependent on average pore size. Large clusters of water were linked to low salt rejection, whereas low association of water in clusters indicated a low permeability to salt. 

When working with a colloidal solution, mention has to be made about the nature of the semipermeable membrane. Protein solutions, in general, are prepared in dilute salt solutions. If the membrane is impermeable to both salt and colloidal particles, we obtain total osmotic pressure. With a membrane impermeable to protein but permeable to salt ions we obtain colloid osmotic pressure. The latter is useful while deducing the molar mass of the colloidal particles. 

In an elegant modeling study of lipid bilayers, Yagisawa et al. showed that oscillations can be induced by a transmembrane pH and salt gradient, with no electrical stimulation or pressure gradients [51]. Briefly, the pH difference leads to a transmembrane dipole and electrical stress on the nonpolar interior of the bilayer, triggering a gel/liquid crystal transition. Following this transition, permeability to salt increases and there is a relaxation of electrical stress, followed by reversal of the lipid transition, restoration of membrane potential, and reinitiation of the cycle. 

We consider this system in an osmotic pressure experiment based on a membrane which is permeable to all components except the polymeric ion P that is, solvent molecules, M" , and X can pass through the membrane freely to establish the osmotic equilibrium, and only the polymer is restrained. It does not matter whether pure solvent or a salt solution is introduced across the membrane from the polymer solution or whether the latter initially contains salt or not. At equilibrium both sides of the osmometer contain solvent, M , and X in such proportions as to satisfy the constaints imposed by electroneutrality and equilibrium conditions. 

AletabolicFunctions. The chlorides are essential in the homeostatic processes maintaining fluid volume, osmotic pressure, and acid—base equihbria (11). Most chloride is present in body fluids a Htde is in bone salts. Chloride is the principal anion accompanying Na" in the extracellular fluid. Less than 15 wt % of the CF is associated with K" in the intracellular fluid. Chloride passively and freely diffuses between intra- and extracellular fluids through the cell membrane. If chloride diffuses freely, but most CF remains in the extracellular fluid, it follows that there is some restriction on the diffusion of phosphate. As of this writing (ca 1994), the nature of this restriction has not been conclusively estabUshed. There may be a transport device (60), or cell membranes may not be very permeable to phosphate ions minimising the loss of HPO from intracellular fluid (61). 

Electrodialysis. In electro dialysis (ED), the saline solution is placed between two membranes, one permeable to cations only and the other to anions only. A direct electrical current is passed across this system by means of two electrodes, causiag the cations ia the saline solution to move toward the cathode, and the anions to the anode. As shown ia Figure 15, the anions can only leave one compartment ia their travel to the anode, because a membrane separating them from the anode is permeable to them. Cations are both excluded from one compartment and concentrated ia the compartment toward the cathode. This reduces the salt concentration ia some compartments, and iacreases it ia others. Tens to hundreds of such compartments are stacked together ia practical ED plants, lea ding to the creation of alternating compartments of fresh and salt-concentrated water. ED is a continuous-flow process, where saline feed is continuously fed iato all compartments and the product water and concentrated brine flow out of alternate compartments. 

Toxicity. The toxicity of barium compounds depends on solubility (47—49). The free ion is readily absorbed from the lung and gastrointestinal tract. The mammalian intestinal mucosa is highly permeable to Ba " ions and is involved in the rapid flow of soluble barium salts into the blood. Barium is also deposited in the muscles where it remains for the first 30 h and then is slowly removed from the site (50). Very Httle is retained by the fiver, kidneys, or spleen and practically none by the brain, heart, and hair. 

With a three-component system, such as a polymer in an aqueous salt solution, preferential adsorption of one component to the polymer can affect the analysis of light-scattering data.199 Such interactions can affect the SRI. Therefore, measurements of the SRI must be made at constant chemical potential. Constant chemical potential is achieved experimentally by dialyzing the solvent and polymer solution to equilibrium through a membrane permeable to the solvent but impermeable to the polymer.199... 

Morimoto et al. [33] demonstrated that the ocular absorption of hydrophilic compounds over a wide range of molecular weights could be increased by 2 and 10 mM sodium taurocholate and sodium taurodeoxycholate in a dose-dependent manner. The compounds were glutathione (307 Da), 6-carboxyfluorescein (376 Da), FTTC-dextran (4 kDa), and insulin (5.7 kDa). Of the two bile salts, sodium taurodeoxycholate was more effective. At 10 mM, this bile salt increased the permeability of 6-carboxyfluorescein from 0.02% to 11%, glutathione from 0.08% to 6%, FITC-dextran from 0% to 0.07%, and insulin from 0.06% to 3.8%. Sodium taurocholate, on the other hand, increased the permeability to 0.13%, 0.38%, 0.0011%, and 0.14%, respectively. Taurodeoxycholate was more effective than taurocholate in the nasal epithelium as well [202], This difference in activities can possibly be attributed to their micelle-forming capability, which is higher for taurodeoxycholate, a dihydroxy bile salt [190],... 

Diffusive Salt Permeability Through PVA Membranes. In order to verify the Interdependence between the water and salt transport through PVA, the diffusive permeability of salt through an un-pressurlzed PVA membrane was measured. The diffusion system employed In this experiment Is described schematically In Figure 8. The diffusion cell consists of an upper cell and a lower cell, separated by the tested membrane, a 150 micron thick, untreated PVA membrane. Initially, the lower cell contalnes distilled water, which Is circulated through a conductivity cell, and the... 

The production of concrete with low water-cement ratios to attain high early and ultimate strengths and much reduced permeability to moisture and salts. 

Polymers. In combination with various metal salts, sorbitol is used as a stabilizer against beat and light in poly(vinyl chloride) (qv) resins and, with a phenolic antioxidant, as a stabilizer in uncured styrene—butadiene mbber (qv) compositions and in polyolefins (see He AT STABILIZERS Olefin polymers Rubber compounding). Heat-sealable films are prepared from a dispersion of sorbitol and starch in water (255). Incorporation of sorbitol in collagen films gready restricts their permeability to carbon dioxide (256). 

They prepared also a similar copolymer membrane having pendant sodium sulfonate groups 37 by using disodium 2,2 -benzidinedisulfonate in addition to the above-described materials 96). The water permeability and salt rejection of the latter mem-... 

In the membrane tlisiilluliim process, combined use of distillation and membranes is made. Salt water is warmed to produce vapor. This vapor passes through porous membranes, which are permeable to vapor but not to the liquid phase. The vapor is condensed on a cooled surface to produce fresh water. The main advantage of this process is jts simplicity and the need for only small temperature differentials to operate. 

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