Semipermeable membrane interface

Sometimes, the term osmotic dispersion pressure [378, 383] is used instead of capillary rarefaction. The osmotic pressure is defined as the excessive external pressure that must be applied to the semipermeable membrane interface between foam and fluid to stop the flux of the fluid sucked into the foam from the free volume. In this case, it is assumed that foam cell faces are flat, and therefore, the capillary pressure in foam bubbles is zero. 

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. 

The subduction interface can be viewed as a semipermeable membrane whose properties change with depth, allowing element distillation... 

There are other techniques, however, including microbatch crystallization, where the protein and precipitant are just mixed at the final supersaturation concentration. Free interface diffusion is similar to microbatch but the two components have to diffuse toward each other the concentrations of both protein and precipitant therefore vary with distance from the original interface. In microdialysis, the precipitant solution is allowed to equilibrate with the protein solution through a semipermeable membrane, which permits passage of the precipitant but not the protein (Figure 7). Of these techniques, the first two also lend themselves to automation. 

The Donnan membrane theory was developed in 1911 to explain the unequal diflusion of ions across a semipermeable membrane and is accepted as a reasonable explanation of the exchange mechanism for the organic type resins. The solution-bead surface interface is deemed to be similar to the membrane in the Donnan theoiy. 

Dialysis is a diffusion-based separation process that uses a semipermeable membrane to separate species by vittue of their different mobilities in the membrane. A feed solution, containing the solutes to he separated, flows ou one side of the membrane while a solvent stream, die dialysate, flows on die other side (Fig. 21. -1). Solute transport across the membrane occurs by diffusion driven by the difference in solme chemical potential between the two membrane-solution interfaces. In practical dialysis devices, no obligatory transmembrane hydraulic pressure may add an additional component of convective transport. Convective transport also may occur if one stream, usually the feed, is highly concentrated, thus giving rise to a transmembrane osmotic gradient down which solvent will flow. In such circumstances, the description of solute transport becomes more complex since it must incorporate some function of die trans-membrane fluid velocity. 

A semipermeable membrane which is selective for the analyte that travels on to the MS can be used at the interface. This is used in particular for packed column GC where the carrier gas flow rates are higher than those used with capillary columns. 

An ion-selective electrode consists of a semipermeable membrane in contact with a reference solution on one side and the sample solution on the other. The membrane has to be selectively permeable to either a cation or an anion, but the penetration of the related counter ion must be restricted. Thus, charge sepeiration occurs at the interface leading to a potential difference (Donnan potential) which contains the analytically useful information. Within the membrane the diffusion of an ion is promoted by a concentration gradient, and when the mobilities of the cations and anions vary greatly, a diffusion potential is additionally developed by charge separation. The change in the membrane potential predominates, under well-deHned conditions (pH, ionic force, temperature), over changes in the overall cell potential due to concentration differences in the substance in question in the analyte. Hence, the cell potential is proportional to the potential drop over the ion-selective membrane. 

External diffusion limitation by mass transfer through layers in front of the enzyme membrane, eg, a semipermeable membrane or the boundary layer at the solution/biosensor membrane interface. 

In the electrolytic cell of Fig. 6.1.1 the cupric ions and sulfate ions both contribute to the conduction mechanisms, but only the cupric ions enter into the electrode reaction and pass through the electrode-solution interface. The electrode therefore acts like a semipermeable membrane which is permeable to the Cu " ions but impermeable to the ions. Anions accumulate near the anode and become depleted near the cathode, resulting in concentration gradients in the solution near the electrodes of both ions. This is termed concentration polarization, in accord with the meaning of the phrase when applied to neutral species. 

The membrane model, proposed by Arcus, treats the interface between the resin and developer as a partially semipermeable membrane that may differentiate between the ions of aqueous developers due to variations in size, composition, and charge. .. the membrane properties can be modified by chemical treatments, changes in concentration. .. and most importantly by the photochemistry of the included naphthoquinone-diazide. This model appears to account for a great... 

Transport problems in discontinuous (heterogeneous) system discuss the flows of the substance, heat, and electrical energy between two parts of the same system. These parts or phases are uniform and homogeneous. The two parts make up a closed system, although each individual part is an open system, and a substance can be transported from one part to another. There is no chemical reaction taking place in any part. Each part may contain n number of substances. For example, thermal diffusion in a discontinuous system is usually called thermal osmosis. If the parts are in different states of matter, there will be a natural interface. However, if both parts are in liquid or gas phases, then the parts are separated by a porous wall or a semipermeable membrane. 

To facilitate studies of tracheal epithelium from transgenic mouse models of human disease, Davidson et al. (2000) have developed a primary culture model of differentiated mouse tracheal epithelium. When grown on semipermeable membranes at an air interface, dissociated cells formed confluent polarised epithelia with high transepithelial resistances ( 12kQ cm ) that remained viable for up to 80 days. 

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... 

In the electrolytic cell, the cupric ions and sulfate ions both contribute to the conduction mechanisms. But only cupric ions enter into the electrode reaction and pass through the electrode-solution interface. The electrode therefore acts like a semipermeable membrane which is permeable to the Cu ions but impermeable to the 80 ions. Anions accumulate near the anode and become depleted near the cathode, resulting in concentration gradients in the solution near the electrodes of both ions. This is termed as concentration polarization. Let us determine the current-voltage characteristic of the cell, that is, the concentration polarization. To do this, we must calculate the flux of metal ions (cations) arriving at the cathode and depositing on it. We assume that the overall rate of the electrode reaction is determined by this flux. Once the cation distribution is known, the potential drop can be calculated. Note that anions are effectively motionless and do not produce a current. Let us assume that electrodes of the electrolytic cell are infinite planes at the anode (y = 0) and cathode (y = h) (Figure 6.3). The electrolyte velocity is zero. The definition of the current densities is... 

Buck ME, Lynn DM (2010) Reactive layer-by-layer assembly of suspended thin films and semipermeable membranes at interfaces created between aqueous and organic phases. Adv Mater 22 994-998... 

In this Section, it is implicitly assumed that the mass transport resistance at the fluid-membrane interface on either side of the membrane is negligible. Also the following is information that is made available publicly by the membrane manufacturers, when not otherwise noted. As in technical processes, mass transport across semipermeable medical membranes is conveniently related to the concentration and pressme driving forces according to irreversible thermodynamics. Hence, for a two-component mixture the solute and solvent capacity to permeate a semipermeable membrane under an applied pressure and concentration gradient across the membrane can be expressed in terms of the following three parameters Lp, hydraulic permeability Pm, diffusive permeability and a, Staverman reflection coefficient (Kedem and Katchalski, 1958). All of them are more accurately measured experimentally because a limited knowledge of membrane stmcture means that theoretical models provide rather inaccurate predictions. 

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