Selective semipermeable membranes

For example, the measurements of solution osmotic pressure made with membranes by Traube and Pfeffer were used by van t Hoff in 1887 to develop his limit law, which explains the behavior of ideal dilute solutions. This work led direcdy to the van t Hoff equation. At about the same time, the concept of a perfectly selective semipermeable membrane was used by MaxweU and others in developing the kinetic theory of gases. 

This is a process mainly used in power plants for removal of dissolved polar compounds. Solute is exchanged between two liquids through a selective semipermeable membrane in response to differences in chemical potential between the two liquids. The process removal efficiency for TDS is about 62-96%. 

Reverse osmosis (RO) is a separation technology that uses selective semipermeable membranes to remove dissolved solids, such as metal salts, from water. In the RO process, the solution containing the contaminant(s) is applied under pressure to one side of a membrane. The water passes through the membrane, leaving behind a solution with a smaller volume and a higher concentration of solutes. 

The PbS04 Layer as a Selective Semipermeable Membrane Electrode Potential... 

It has been established that after a series of galvanostatic polarization runs with low currents, the Pb/PbS04 electrode is passivated permanently and its open-circuit potential remains within the range from —0.40 to —0.60 V [24]. It has been suggested that this behaviour is due to the transformation of the PbS04 layer into a selective semipermeable membrane by the process presented in Fig. 2.12 [24]. 

The relationship between electrical potential and ionic concentrations or, more precisely, ionic activities is frequently considered in electrochemistry. Most commonly, two ionic solutions of different activity are separated by an ion-selective semipermeable membrane that allows one type of ion to pass freely through the membrane. It can be shown that an electric potential E exist between the solutions on either... 

In addition to metal-electrolyte interfaces, electrode potentials can be produced by ion transport through an ion-selective semipermeable membrane. In this case, the membrane is interposed between two liquid phases. Reversible transfer of a selected ion occurs through the membrane. For an ideal membrane, the developed electrode potential E is given by the Nernst equation... 

Membrane separator. A separator that passes gas or vapor to the mass spectrometer through a semipermeable (e.g., silicon) membrane that selectively transmits organic compounds in preference to carrier gas. Membrane separator, membrane enricher, semipermeable membrane separator, and semipermeable membrane enricher are synonymous terms. 

Adsorption systems employing molecular sieves are available for feed gases having low acid gas concentrations. Another option is based on the use of polymeric, semipermeable membranes which rely on the higher solubiHties and diffusion rates of carbon dioxide and hydrogen sulfide in the polymeric material relative to methane for membrane selectivity and separation of the various constituents. Membrane units have been designed that are effective at small and medium flow rates for the bulk removal of carbon dioxide. 

Membrane Sep r tion. The separation of components ofhquid milk products can be accompHshed with semipermeable membranes by either ultrafiltration (qv) or hyperfiltration, also called reverse osmosis (qv) (30). With ultrafiltration (UF) the membrane selectively prevents the passage of large molecules such as protein. In reverse osmosis (RO) different small, low molecular weight molecules are separated. Both procedures require that pressure be maintained and that the energy needed is a cost item. The materials from which the membranes are made are similar for both processes and include cellulose acetate, poly(vinyl chloride), poly(vinyHdene diduoride), nylon, and polyamide (see AFembrane technology). Membranes are commonly used for the concentration of whey and milk for cheesemaking (31). For example, membranes with 100 and 200 p.m are used to obtain a 4 1 reduction of skimmed milk. 

An ion-selective electrode contains a semipermeable membrane in contact with a reference solution on one side and a sample solution on the other side. The membrane will be permeable to either cations or anions and the transport of counter ions will be restricted by the membrane, and thus a separation of charge occurs at the interface. This is the Donnan potential (Fig. 5 a) and contains the analytically useful information. A concentration gradient will promote diffusion of ions within the membrane. If the ionic mobilities vary greatly, a charge separation occurs (Fig. 5 b) giving rise to what is called a diffusion potential. 

In the following sections we highlight only selected works that have contributed toward the further development of passive samplers for SVOCs and/or HOCs. The literature related to the development and use of passive samplers for monitoring gases or VOCs in occupational environments is large. However, these publications are discussed only briefly, because lipid-containing semipermeable membrane devices (SPMDs) are primarily designed for SVOCs. 

Smer-Lauridsen, F. and Kjlholt, J. 2000, Identification of selected hydrophobic organic contaminants in wastewater with semipermeable membrane devices (SPMDs). Water Res. 34 3478—3482. 

An overview of the decomposition and dehydrogenation reactions that have been investigated using semipermeable membranes for selective permeation of one of the reaction products is given in Tables 7.1 and 7.3, respectively. An overview of the most interesting studies is given in Tables 7.2 and 7.4. 

Membrane electrochemistry proper began in 1890 with the work of W. Ostwald [79], who formulated the concept of a semipermeable membrane that selectively affects the membrane transport of ions. According to Ostwald, not only electric currents in muscles and nerves, but also the mysterious phenomenon of electric fishes will finally find an explanation in the properties of semipermeable membranes . 

Membranes are semipermeable barriers that permit the separation of two compartments of different composition or even condition, with the transport of components from one compartment to another being controlled by the membrane barrier. Ideally, this barrier is designed to let pass selectively only certain target compounds, while retaining all others—hence the denotation semipermeable . Membrane separations are particularly suitable for food applications because (1) they do not require any extraction aids such as solvents, which avoids secondary contamination and, hence, the necessity for subsequent purification (2) transfer of components from one matrix to another is possible without direct contact and the risk of cross-contamination (3) membrane processes can, in general, be operated under smooth conditions and therefore maintaining in principle the properties and quality of delicate foodstuff. 

Osmosis, water movement across a semipermeable membrane driven by differences in osmotic pressure, is an important factor in the life of most cells. Plasma membranes are more permeable to water than to most other small molecules, ions, and macromolecules. This permeability is due partly to simple diffusion of water through the lipid bilayer and partly to protein channels (aquaporins see Fig. 11-XX) in the membrane that selectively permit the passage of water. Solutions of equal osmolarity are said to be isotonic. Surrounded by an isotonic solution, a cell neither gains nor loses water (Fig. 2-13). In a hypertonic solution, one with higher... 

Central to the osmosis phenomenon is the semipermeable membrane (SPM), whose physical properties and species-selectivity directly govern the kinetics and thermodynamics of osmotic flow. Naturally occurring biomembranes of high selectivity, permeable to water but not to other solutes, are ubiquitous, for example, in macroscopic stomach linings and blood vessels, as well as in the microscopic cell membranes that encapsulate all known cell types. Some common synthetic membranes, such as Gore-Tex and cellophane, also exhibit selective permeability and osmotic activity. 

Electrodialysis (ED) is a membrane process based on the ability of semipermeable membranes to pass select ions in feedwater. A direct ctinem electrical field transports the ions through the membranes, in the basic system, alternating cation- and anion-selcctivc membranes arc placed in an electrical field. An eleclrodialysis stack schematic is shown in Fig. 3. The cation selective membranes permit only the transport of cations anion-selective membranes allow only the transport of anions. 

Containment Behind a Barrier Cells can be immobilized within micro-capsules that have either a permanent or nonpermanent semipermeable membrane. The advantage of encapsulation techniques is the large surface area for contact of substrate and cells. The semipermeable membrane also selectively passes only low-molecular weight components. 

It is recognized that filtration is operational, that colloidal-bound PCB congeners are not retained by the filter, and that operational dissolved measurements may be biased positively by colloidal material. Techniques to measure truly dissolved PCBs include gas sparging, differential diffusion into membrane-bound lipids (e.g., semipermeable membrane devices, [230]), and selective adsorption (e.g. non-equilibrium solid phase microextraction [231, 232]). Unfortunately, none of these techniques has sufficient sensitivity to reliably and unambiguously measure truly dissolved PCB congeners at the levels present in the Great Lakes. 

The nasal epithelium possesses selective absorption characteristics similar to those of a semipermeable membrane, i.e., it allows a rapid passage of some compounds while preventing the passage of others. The process of transportation across the nasal mucosa involves either passive diffusion, via paracellular or transcellular mechanisms, or occurs via active processes mediated by membrane-bound carriers or membrane-derived vesicles involving endo- or transcytosis. 

Additional semipermeable membrane—forming polymers are selected from the group consisting of acetaldehyde dimethyl cellulose acetate, cellulose acetate ethyl carbamate, cellulose dimethylamino acetate, semipermeable polyamides, semipermeable polyurethanes, or semipermeable sulfonated polystyrenes. Semipermeable cross-linked selectively permeable polymers formed by coprecipitation of a polyanion and a polycation also can be used for this purpose.22 23 Other polymer materials such as lightly cross-linked polystyrene derivatives, semipermeable cross-linked poly(sodium styrene sulfonate), and semipermeable poly (vinylbenzyltrimethyl ammonium chloride) may be considered.24,25... 

Figure 2.6(a) shows a semipermeable membrane separating a salt solution from the pure solvent. The pressure is the same on both sides of the membrane. For simplicity, the gradient of salt (component j) is not shown in this figure, but the membrane is assumed to be very selective, so the concentration of salt within the membrane is small. The difference in concentration across the membrane results in a continuous, smooth gradient in the chemical potential of the water (component i) across the membrane, from //, on the water side to plo on the salt side. The pressure within and across the membrane is constant (that is, pa = pm= pi) and the solvent activity gradient (y,Imj ,Imj) falls continuously from the pure water (solvent) side to the saline (solution) side of the membrane. Consequently, water passes across the membrane from right to left. 

Theory of Ultrafiltration. Ultrafiltration is a membrane process with the ability to separate molecules in solution on the basis of size (Ghosh, 2003). Particles are separated with the use of pressure and specially designed semipermeable membranes (Figure 13.5). An ultrafiltration membrane acts as a selective barrier. It... 

Harman, C., O. Bpyum, K.E. Tollefsen, K. Thomas, and M. Grung. 2008. Uptake of some selected aquatic pollutants in semipermeable membrane devices (SPMDs) and the polar organic chemical integrative sampler (POCIS). J. Environ. Monit. 10 239-247. 

Various analytical techniques make use of both porous and nonporous (semipermeable) membranes. For porous membranes, components are separated as a result of a sieving effect (size exclusion), that is, the membrane is permeable to molecules with diameters smaller than the membrane pore diameter. The selectivity of such a membrane is thus dependent on its pore diameter. The operation of nonporous membranes is based on differences in solubility and the diffusion coefficients of individual analytes in the membrane material. A porous membrane impregnated with a liquid or a membrane made of a monolithic material, such as silicone rubber, can be used as nonporous membranes. 

---