Solute permeability through membranes

The most common method to measure alcohol permeability through membranes is the diffusion cell method under non-stationary conditions. In this method the membrane separates two reservoirs the receptor reservoir containing pure water, and the donor reservoir containing the alcohol solution of known concentration. Usually the alcohol solution in the donor reservoir is refreshed during the experiment to maintain its concentration, Cj, constant along the time. The non-stationary alcohol concentration in the receptor reservoir, c, is followed as a function of time by in situ or ex situ sensors. By integrating Eq. 6.4 the time dependence of is given by... 

The value of vapour pressure osmometry is that the technique is particularly good for low molecular weight polymers, i.e. for molecular weights less than approximately 20000. Membrane osmometry is subject to error at low molecular weight because of solute diffusion through membranes, which are not perfectly semi-permeable as required by the theory. This makes vapour pressure osmometry a convenient method for obtaining M for low molecular weight polymers. 

Plastic membrane This is done by the use of a water permeable plastic membrane held deep enough under the sea so that the hydrostatic pressure is greater than the osmotic pressure of the seawater. The water distills out of the solution through the membrane and is pumped to the surface. Large areas of the membranes, mechanically supported to withstand the very high pressures are essential to make the process perform rapidly for the most economical production. 

Water Permeation and Solute Separation through the Membrane. The measurements of water permeability of the 67 membranes prepared under different conditions were carried out by using an Amicon Diaflo Cell (effective membrane area, 13.9 cm2) under a pressure of 3 kg/cm2 at 25 °C. Some results are listed in Table 1067. It is apparent that much higher water absorption and permeability than the cellulosic membrane are characteristic of the 67 membranes prepared by both the casting polymerization and conventional casting. 

The permeability tests for alkali metal ions in the aqueous solution were also conducted. When an aqueous salt solution moves to cell 2 through the membrane from cell 1, the apparent diffusion coefficient of the salt D can be deduced from a relationship among the cell volumes Vj and V2, the solution concentration cx and c2, the thickness of membrane, and time t6 . In Table 12, permeabilities of potassium chloride and sodium chloride through the 67 membrane prepared by the casting polymerization technique from the monomer solution in THF or DMSO are compared with each other and with that the permeability through Visking dialyzer tubing. The... 

In accordance with observed data, this model shows that water flux increases linearly with applied pressure AP, decreases with higher salt concentration through its impact on osmotic pressure Jt, increases with a smaller membrane thickness I, and increases with temperature through the temperature dependence of the water permeability P . The model also demonstrates that the solute or salt flux J, increases linearly with applied pressure AP, increases with higher salt concentration c , increases with a smaller membrane thickness I, and increases with temperature through the temperature dependence of the solute permeability Pj. Polarization, as described early in this section, causes the wall concentration c to exceed the bulk concentration ci,. 

Most drugs are ionized in aqueous solution (Table 2.1), and can therefore exist in a neutral or a charged state, depending on the pH of the local environment. Molecules are more lipophilic when neutral than when charged. Ionization is expressed by the aqueous ionization constant, pKa. As pointed out below, log D is a p Independent term for ionizable drugs. Permeability and aqueous solubility are also pKa-dependent. Lipophilicity, pKa, permeability through artificial membranes and... 

In this method the sample is acidified and the inorganic carbon is removed with nitrogen. An aliquot is resampled for analyses. Buffered persulfate is added and the sample is irradiated in the ultraviolet destructor for about 9 min. The hydroxylamine is added and the sample stream passes into the dialysis system. The carbon dioxide generated diffuses through the gas-permeable silicon membrane. A weakly buffered phenolphthalein indicator solution is used as the recipent stream, and the colour intensity of this solution decreases proportionately to the change in pH caused by the absorbed carbon dioxide... 

Cell membranes or synthetic lipid vesicles with normal low permeability to water will, if reconstituted with AQP1, absorb water, swell and burst upon exposure to hypo-osmotic solutions. The water permeability of membranes containing AQP 1 can be about 100 times greater than that of membranes without aquaporins. The water permeability conferred by AQP1 (about 3 billion water molecules per subunit per second) is reversibly inhibited by Hg2+, exhibits low activation energy and is not accompanied by ionic currents or translocation of any other solutes, ions or protons. Thus, the movement of water through aquaporins is an example of facilitated diffusion, in this case driven by osmotic gradients. 

It was postulated that the aqueous pores are available to all molecular species, both ionic and non-ionic, while the lipoidal pathway is accessible only to un-ionised species. In addition, Ho and co-workers introduced the concept of the aqueous boundary layer (ABL) [9, 10], The ABL is considered a stagnant water layer adjacent to the apical membrane surface that is created by incomplete mixing of luminal contents near the intestinal cell surface. The influence of drug structure on permeability in these domains will be different for example ABL permeability (Paq) is inversely related to solute size, whereas membrane permeability (Pm) is dependent on both size and charge. Using this model, the apparent permeability coefficient (Papp) through the biomembrane may therefore be expressed as a function of the resistance of the ABL and... 

Instead of the dilute solution approach above, concentrated solution theory can also be used to model liquid-equilibrated membranes. As done by Weber and Newman, the equations for concentrated solution theory are the same for both the one-phase and two-phase cases (eqs 32 and 33) except that chemical potential is replaced by hydraulic pressure and the transport coefficient is related to the permeability through comparison to Darcy s law. Thus, eq 33 becomes... 

Figure 13.9. Membrane permeability coefficience of solutes. Solute permeabilities across typical lipid bilayers of liposomes or lipid vesicles are presented as their respective coefficients in cm/s. In the absence of other transport processes, it would require 10 s to move Na+ across 1 cm distance. When there is a concentration difference across a membrane, multiplying the concentration difference (mole/ml equivalent to mole/cm ) by the permeability coefficient (cm/s) allows estimation of flow rate (mole/s-cm ). For example, a concentration difference of 1Q- mole/cm Na (or 1 x 10" M Na ) would provide a flow of 10 mole/cm x 10" cm/s = IQ- mole/s through 1 cm or 0.006 mole/s through 1 pm of a membrane bilayer.

Polypeptides obtained by the anionic polymerization of optically active N-carboxy-a-amino acid anhydrides are apt to have such an ordered structures as a-helices, which is useful for investigation on the relationship between the physical structure and the permeability of the membrane. Takizawa et al.44 46) studied the water permeation and solute separation through poly(n-alkyl L-glutamate) membranes 3. It was concluded that water molecules permeate through relatively large free spaces... 

MWCO), usually defined as the molar mass at which the membrane rejects 90% of solute molecules. However, as in microfiltration, the molecular shape can affect permeability through the membrane pores. For example, a membrane with a nominal cut-off of 100 kDa, which does not allow globular molecules with a molar mass of 100 kDa to flow through, may allow fibrous molecules with higher molar masses to flow across the pores. As in microfiltration, the membrane pore size is not uniform, with a normal distribution around an average value. 

Gas sensors usually incorporate a conventional ion-selective electrode surrounded by a thin film of an intermediate electrolyte solution and enclosed by a gas-permeable membrane. An internal reference electrode is usually included, so that the sensor represents a complete electrochemical cell. The gas (of interest) in the sample solution diffuses through the membrane and comes to equilibrium with the internal electrolyte solution. In the internal compartment, between the membrane and the ion-selective electrode, the gas undergoes a chemical reaction, consuming or forming an ion to be detected by the ion-selective electrode. (Protonation equilibria in conjunction with a pH electrode are most common.) Since the local activity of this ion is proportional to the amount of gas dissolved in the sample, the electrode response is directly related to the concentration of the gas in the sample. The response is usually linear over a range of typically four orders of magnitude the upper limit is determined by the concentration of the inner electrolyte solution. The permeable membrane is the key to the electrode s gas selectivity. Two types of polymeric material, microporous and homogeneous, are used to form the... 

The question of the osmotic relationship between a cestode and its environment is complicated by the fact that some substances can pass through membranes by means other than diffusion (p. 42). Hence the actual osmotic pressure of a solution as measured by physico-chemical means may not be as significant to a worm as the actual content to which the tegument of the worm is permeable - using the word in its widest sense as indicated above. The reason for this is that substances in the medium which (theoretically) contribute to the total osmotic pressure of the medium do not actually exert osmotic pressure across the tegument of the worm, which separates the worm from its environment. 

Osmosis is defined as the movement of water through a semi-permeable membrane into a solution. The semipermeable membrane is such that only water molecules can move through it the movement of solutes, including drags, is restricted (although the extent of this restriction depends on the characteristics of the... 

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