Diffusion through the membrane

By changing the enzyme and mediator, the amperometric sensor in Figure 11.39 is easily extended to the analysis of other substrates. Other bioselective materials may be incorporated into amperometric sensors. For example, a CO2 sensor has been developed using an amperometric O2 sensor with a two-layer membrane, one of which contains an immobilized preparation of autotrophic bacteria. As CO2 diffuses through the membranes, it is converted to O2 by the bacteria, increasing the concentration of O2 at the Pt cathode. 

Most theoretical studies of osmosis and reverse osmosis have been carried out using macroscopic continuum hydrodynamics [5,8-13]. The models used include those that treat the wall as either nonporous or porous. In the nonporous models the membrane surface is assumed homogeneous and nonporous. Transport occurs by the molecules dissolving in the membrane phase and then diffusing through the membrane. Mass transfer across the membrane in these models is usually described using the solution-diffusion... 

Kittelberger and Elm measured the rate of diffusion of sodium chloride through a number of paint films. Calculations based on their results showed clearly that the rate of diffusion of ions was very much smaller than the rate of diffusion of either water or oxygen. Furthermore, they found that there was a linear relationship between the rate of diffusion and the reciprocal of the resistance of the film. This relationship suggests that the sodium chloride diffused through the membrane as ions and not as ion pairs, since the diffusion through the film of un-ionised material would not affect the resistance, because if a current is to flow, either ions of similar charge... 

Fluidised bed The fluidised bed consists of two boxes on top of one another. The top and larger one contains the powder, and the lower one is separated from it by metal mesh and a semipermeable membrane. Air is pumped under pressure into the lower compartment and then diffuses through the membrane and through the powder. The powder particles are lifted and separated by the air. This results in a considerable reduction in the bulk density so that the item to be coated can easily be submerged in the powder. 

In this application we examine the influence of solute polarity on its partitioning into, and diffusion through the membrane. The concentration of solute is held constant at [S] = 30 cells in the W2 compartment. The dynamics are run 10 times for each value of Pb(WS), and the results averaged. 

In Figure 6.5, the count of the number of solute cells diffusing from the W2 compartment, into the membrane, and the number diffusing through the membrane into the Wj compartment is shown for several values of solute lipophilicity. The permeation of solute through the membrane increases with increasing lipophilicity up to about T b(WS) = 0.50, and then decreases as the lipophilicity increases. The concentration of solute within the membrane is very low, as the / b(WS) value increases from 0.15 to 0.45. At a value of Pb(WS) = 0.50, the count of solute molecules in the membrane exhibits a sharp increase at Pb(WS) = 0.55, as shown in Figure 6.5 [5]. 

The solubility-diffusion theory assumes that solute partitioning from water into and diffusion through the membrane lipid region resembles that which would occur within a homogeneous bulk solvent. Thus, the permeability coefficient, P, can be expressed as... 

Permeability of an FML is evaluated using the Water Vapor Transmission test.28 A sample of the membrane is placed on top of a small aluminum cup containing a small amount of water. The cup is then placed in a controlled humidity and temperature chamber. The humidity in the chamber is typically 20% relative humidity, while the humidity in the cup is 100%. Thus, a concentration gradient is set up across the membrane. Moisture diffuses through the membrane, and with time the liquid level in the cup is reduced. The rate at which moisture is moving through the membrane is measured. From that rate, the permeability of the membrane is calculated with the simple diffusion equation (Fick s first law). It is important to remember that even if a liner is installed correctly with no holes, penetrations, punctures, or defects, liquid will still diffuse through the membrane. 

Most hydrophilic, or water-soluble, substances are repelled by this hydrophobic interior and cannot simply diffuse through the membrane. Instead, these substances must cross the membrane using specialized transport mechanisms. Examples of lipid-insoluble substances that require such mechanisms include nutrient molecules, such as glucose and amino acids, and all species of ions (Na+, Ca++, H+, Cl, and HC03). Therefore, the plasma membrane plays a very important role in determining the composition of the intracellular fluid by selectively permitting substances to move in and out of the cell. 

Fig. 15.2. Physicochemical molecular descriptors affect the transport route utilised across the intestinal epithelium. To passively diffuse through the membrane (1), the compound (here illustrated with testosterone) should preferably be small, with a molecular weight <500 Da, as well as uncharged and fairly lipophilic. However, compounds that are too lipophilic can stick to the membrane and will not pass through the cells. The paracellular route (2), here exemplified with mannitol, is mainly utilised by smaller (Mw < 200 Da)...

Gas-sensing electrodes differ from ion-selective electrodes in that no species in solution can interfere with the electrode response as only gases can diffuse through the membrane. However, it should be noted that any gas which causes a pH change in the internal electrolyte solution will affect electrode response. 

Figure 11. Uptake rates of inorganic Hg (a) and of methylmercury (b) by a marine alga as a function of the octanol-water distribution ratio of the Hg-species under various conditions of pH and chloride concentrations. The neutral species HgCl and CH5HgClH diffuse through the membranes. Reprinted with permission from [79] Mason, R. P. et al. (1996). Uptake, toxicity, and trophic transfer in a coastal diatom , Environ. Sci Technol., 30, 1835-1845 copyright (1996) American Chemical Society...

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