Semipermeable Interface

In this case, most ions can pass through the interface but a few are excluded. This situation, which is depicted in Fig. 6.4, is common in dialysis when small ions and low molecular weight substrates can move across the dialysis membrane, but large polyelectrolyte ions, such as proteins, cannot. 

This exclusion is based on size of the ion and leads to the formation of the Don-nan potential, first mentioned in Chapter 2. Its origin can be explained using the simplest case, involving a uni-univalent electrolyte (NaCl) and a large polyelectrolyte anion V, which is present only in the left compartment (marked with a in Fig. 6.4) and carries z negative charges. We again recall two conditions that must be satisfied at this membrane equilibrium  

The equality of the electrochemical potential at the two sides of the membrane for those species that can communicate across it. 

The condition of electroneutrality, which must exist in both compartments. 

The first condition leads to the potential difference shown in (6.3) which, when written for the specific ions in question, is 

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Judging from the hydraulic heads, the vertical flow across the semipermeable interface 1 is in a downward direction, whereas across the semipermeable interface 2 it is in an upward direction. Therefore, unit 2 is being fed by fluid from the phraetic aquifer above it and the confined aquifer below. 

Considering mass transfer coefficients p pubhshed in the literature the question is important whether the coefficient is valid for eqttimolar counterdiffiision or for a semipermeable interface or crystal sirrface (no transport from the solid into the fluid phase is assttmed). Frrrthermore the transfer coefficient can be based on diffusion only or on a combined transport by diffusion and convection. The differences can become essential for high mass transfer rates occtrrring in systems with great solubilities. 

Nested wells can also be used to analyze multilayer aquifer flow. There are many situations involving interaquifer transport owing to leaky boundaries between the aquifers. The primary case of interest involves the vertical transport of fluid across a horizontal semipermeable boundary between two or more aquifers. Figure 4 sets out the details of this type of problem. Unit 1 is a phraetic aquifer, bound from below by two confined aquifers, having semipermeable formations at each interface. 

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. 

Encapsulation. Immobilization of enzymes by encapsulation within semipermeable structures dates back to the 1970s. There are three fundamental variations of this approach. In coacervation, aqueous microdroplets containing the enzyme are suspended in a water-immiscible solvent containing a polymer, such as cellulose nitrate, polyvinylacetate, or polyethylene. A solid film of polymer can be induced to form at the interface between the two phases, thereby producing a microcapsule containing the enzyme. A second approach involves interfacial polymerization in which an aqueous solution of the enzyme and a monomer are dispersed in an immiscible solvent with the aid of a surfactant. A second (hydrophobic) monomer is then added to the solvent and condensation polymerization is allowed to proceed. This approach has been used extensively with nylons, but is also applicable to polyurethanes, other polyesters, and polyureas. 

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. 

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... 

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