Rate of permeation

Neutralization to terrninate processing was effected by the polymeric acid layer of the covet sheet the onset of this reaction was controlled by the rate of permeation of the overlying polymeric timing layers. MobiUty of the transferred dyes was also reduced by reaction with a mordant contained in the image-receiving layer. A development inhibitor released from one of the timing layers by the alkaline hydrolysis of its precursor assisted in restraining further development and consequent additional dye release. 

In general, the rate of permeation of the permeating species is difficult to calculate. It is a complex matter which intimately involves a knowledge of the structure and dynamics of the membrane and the structure and dynamics of the complex fluid mixture in contact with it on one side and the solvent on the other side. Realistic membranes with realistic fluids are beyond the possibihties of theoretical treatment at this time. The only way of dealing with anything at all reahstic is by computer simulation. Even then one is restricted to rather simplified models for the membrane. 

Rate of permeation relative to tliat of maltose. Data adjusted to 100 s for maltose. The LamB-containing liposomes were added to buffer solutions containing 40 mM of die corresponding test sugars. 

This equilibrium is of importance in providing diagnostic criteria for the mechanism of the h.e.r., since the rate of permeation of hydrogen through a thin iron membrane can provide information on the coverage of the surface with adsorbed hydrogen. 

Peivaporation is a relatively new process that has elements in common with reverse osmosis and gas separation. In peivaporation, a liquid mixture contacts one side of a membrane, and the driving force for the process is low vapour pressure on the permeate side of the membrane generated by cooling and condensing the permeate vapour. The attraction of peivaporation is that the separation obtained is proportional to the rate of permeation of the components of the liquid mixture through the selective membrane. Therefore, peivaporation offers the possibility of separating closely boiling mixtures or azeotropes that are difficult to separate by distillation... 

NOTE Generally speaking, the higher the TDS of the RW source to the RO, the higher the applied pressure required to produce a constant permeate water TDS. Also, for practical reasons, the rate of permeate recovery tends to decrease with increase in source water TDS. 

Dynamic osmotic methods have been used occasionally, the rate of permeation being measured as a function of the pressure difference. The interpolated pressure for zero rate should equal the osmotic pressure. In practice, however, more reliable results usually have been secured by establishing an equilibrium pressure which remains unchanged over an extended period of time. 

The high specificity required for the analysis of physiological fluids often necessitates the incorporation of permselective membranes between the sample and the sensor. A typical configuration is presented in Fig. 7, where the membrane system comprises three distinct layers. The outer membrane. A, which encounters the sample solution is indicated by the dashed lines. It most commonly serves to eliminate high molecular weight interferences, such as other enzymes and proteins. The substrate, S, and other small molecules are allowed to enter the enzyme layer, B, which typically consist of a gelatinous material or a porous solid support. The immobilized enzyme catalyzes the conversion of substrate, S, to product, P. The substrate, product or a cofactor may be the species detected electrochemically. In many cases the electrochemical sensor may be prone to interferences and a permselective membrane, C, is required. The response time and sensitivity of the enzyme electrode will depend on the rate of permeation through layers A, B and C the kinetics of enzymatic conversion as well as the charac-... 

This equation teaches us that the total stead-state flux (total rate of permeation across a membrane in the steady state of permeation), dM/dt, is proportional to the involved area (A) and the concentration differential expressed across the membrane, AC. In an experiment, flux is the experimentally measured parameter while A and AC are fixed in value when setting up an experiment. The value of the permeability coefficient, Ptotai, is what is calculated upon completion of an experiment using Eq. (8). The permeability coefficient, besides having the specific attributes ascribed to it, is... 

Diffusion of a gas or liquid through a semi-permeable material. The permeability of elastomers to gases varies with the elastomer type and with the gas. Butyl rubber is much less permeable to air than is natural rubber hence its use in tyre inner tubes and similar apphcations. The rate of permeation is generally related to the size of gas molecule, i.e., the smaller the molecule the higher the rate. The exception is C02 which has a rate 10 to 100 times greater than that of nitrogen. 

If the pore-mechanism applies, the rate of permeation should be related to the probability at which pores of sufficient size and depth appear in the bilayer. The correlation is given by the semi-empirical model of Hamilton and Kaler [150], which predicts a much stronger dependence on the thickness d of the membrane than the solubility-diffusion model (proportional to exp(-d) instead of the 1 Id dependence given in equation (14)). This has been confirmed for potassium by experiments with bilayers composed of lipids with different hydrocarbon chain lengths [148], The sensitivity to the solute size, however, is in the model of Hamilton and Kaler much less pronounced than in the solubility-diffusion model. 

Applying this information to a typical diaphragm-cell tail gas, Fig. 7.4 shows the logarithm of the amount of unrecovered chlorine versus the relative membrane area required. Recovery of chlorine is not far from a first-order process. As chlorine selectively passes through the membrane, the partial pressures of the impurities increase in the remaining gas. This causes their rates of permeation to increase. The membrane area required for permeation of, say, 30% of the nitrogen is less than twice that required for 15%. 

The rate of permeation was found to depend on a number of parameters. In the separation of toluene from toluene/cyclohexane mixtures, with a polystyrene in polyacrylamide membrane, the rate increased with increasing temperature and toluene concentration, and decreasing polyacrylamide content [182], Selectivity, on the other hand, increased with decreasing temperature and increasing polyacrylamide content therefore selectivity is inversely related to permeation rate. Similar results were found in experiments on the separation of water from water/ethanol mixtures by hydrophilic-hydrophobic membranes [183],... 

The rate of filtration - that is, the rate of permeation of a liquid through a filtering medium - depends on the area of the filtering medium, the viscosity of the liquid, the pressure difference across the filter, and the resistances of the filtering medium and the cake. 

Table IV presents the comparative data on the permeation rates of the three types of membranes with two different porosities for various aqueous organic solutions and for pure water as measured over the duration of the study. The data shown here represent the relative chronological order in which the samples were tested. In the beginning, even though the percent rejection of NaCl is high for PA and CAc (indicating small-size pores), the rates of permeation of pure water are higher for denser membranes than for membranes having lower percent rejection of NaCl. In the case of the PBI membrane, the reverse of this phenomenon is observed.

Polyimide adhesion to the substrate surface is not only important initially after the interface preparation, but also after exposure for extended times to elevated temperature and humidity (T H) conditions. As Plueddemann [7] states Water molecules diffuse through any plastic and thus will reach the interface in composites exposed to humid environment. Individual water molecules, however, are relatively harmless at the interface unless they are capable of clustering into a liquid phase. The concentration of water at the interface is not determined by the rate of permeation of the water through the polymer matrix (silicones and... 

The layer of water adjacent to the absorptive membrane of the enterocyte is essentially unstirred. It can be visualized as a series of water lamellas, each progressively more stirred from the gut wall toward the lumen bulk. For BCS class 2 compounds the rate of permeation through the brush border is fast and the diffusion across the unstirred water layer (UWL) is the rate-limiting step in the permeation process. The thickness of the UWL in human jejunum was measured and found to be over 500 pm [3]. Owing to its thickness and hydrophilicity, the UWL may represent a major permeability barrier to the absorption of lipophilic compounds. The second mechanism by which the UWL functions act as a barrier to drug absorption is its effective surface area. The ratio of the surface area of the UWL to that of the underlying brush border membrane is at least 1 500 [4], i.e., this layer reduces the effective surface area available for the absorption of lipophilic compounds and hence impairs its bioavailability. 

The rate of permeation for the case shown schematically in Fig. 2.57 is defined as the mass of penetrating gas or liquid that passes through a polymer membrane per unit time. The rate of permeation, m, can be defined using Fick s first law of diffusion as... 

One of the newest commercial methods of sweetening gas is the use of membranes. This separation works on the principle that there are different rates of permeation through a membrane for different gases. 

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