Membrane concentration gradient

Membrane concentration gradients Concentration Concentration Concentration... 

Researchers and engineers did a lot of works on the water transfer in membrane. Water transfer in membrane has two main aspects. First, because of electric drag effect, water passes through the electrolyte to the cathode with protons. Thus, electromigration of water is related with the current density and the number of proton hydration. Second, if the water content of the cathode side is high, the water diffusion reversely occurs from the cathode to the anode. The quantity of water is proportional to the diffusion coefficient and membrane concentration gradient. It is inversely proportional to the thickness of the film. Based on this, different dynamic models related with water transmission in film are established, for example, the model of Shen and Li [20], describing the water molar quantity produced as follows ... 

When a fluid with one or more components is transported across a selective membrane, concentration gradients arise in the layer immediately outside the membrane. Likewise, due to negative or positive enthalpies of adsorption in the membrane, temperature gradients will arise, depending on the ratio of the relative conductivity of the membrane to its surroundings (Inzoli et al, 2009). In a pure gas, there is obviously no composition gradient, only... 

Transporter. A transporter is a protein transporting molecules or ions through the cell membrane against a concentration gradient. 

The performance of reverse osmosis membranes is generaUy described by the water and salt fluxes (74,75). The water flux,/ is linked to the pressure and concentration gradients across the membrane by equation 4 ... 

This deposit is composed of suspended particles similar to conventional filter cakes, and more importantly, a slime that forms as retained solutes exceed their solubility. The gel concentration 6 is a function of the feed composition and the membrane-pore size. The gel usually has a much lower hydrauHc permeabihty and smaller apparent pore size than the underlying membrane (27). The gel layer and the concentration gradient between the gel layer and the bulk concentration are called the gel-polarization layer. 

The concentration boundary layer forms because of the convective transport of solutes toward the membrane due to the viscous drag exerted by the flux. A diffusive back-transport is produced by the concentration gradient between the membranes surface and the bulk. At equiUbrium the two transport mechanisms are equal to each other. Solving the equations leads to an expression of the flux ... 

Membra.ne Diffusiona.1 Systems. Membrane diffusional systems are not as simple to formulate as matrix systems, but they offer much more precisely controlled and uniform dmg release. In membrane-controlled dmg deUvery, the dmg reservoir is intimately surrounded by a polymeric membrane that controls the dmg release rate. Dmg release is governed by the thermodynamic energy derived from the concentration gradient between the saturated dmg solution in the system s reservoir and the lower concentration in the receptor. The dmg moves toward the lower concentration at a nearly constant rate determined by the concentration gradient and diffusivity in the membrane (33). 

Electrically assisted transdermal dmg deflvery, ie, electrotransport or iontophoresis, involves the three key transport processes of passive diffusion, electromigration, and electro osmosis. In passive diffusion, which plays a relatively small role in the transport of ionic compounds, the permeation rate of a compound is deterrnined by its diffusion coefficient and the concentration gradient. Electromigration is the transport of electrically charged ions in an electrical field, that is, the movement of anions and cations toward the anode and cathode, respectively. Electro osmosis is the volume flow of solvent through an electrically charged membrane or tissue in the presence of an appHed electrical field. As the solvent moves, it carries dissolved solutes. 

Back-diffusion is the transport of co-ions, and an equivalent number of counterions, under the influence of the concentration gradients developed between enriched and depleted compartments during ED. Such back-diffusion counteracts the electrical transport of ions and hence causes a decrease in process efficiency. Back-diffusion depends on the concentration difference across the membrane and the selectivity of the membrane the greater the concentration difference and the lower the selectivity, the greater the back-diffusion. Designers of ED apparatus, therefore, try to minimize concentration differences across membranes and utilize highly selective membranes. Back-diffusion between sodium chloride solutions of zero and one normal is generally [Pg.173]

FIG. 22-81 Permeant -concentration profile in a pervaporation membrane. 1— Upstream side (swollen). 2—Convex curvature due to concentration-dependent permeant diffiisivity. 3—Downstream concentration gradient. 4—Exit surface of membrane, depleted of permeant, thus unswollen. (Couttesy Elseoier )... 

When the Donnan equilibrium is operative the entry of ions into the membrane is restricted. Consequently as the concentration of ions in the solution increases the resistance of the membrane remains constant until the concentration of ions in the solution reaches that of the fixed ions attached to the polymer network. At this point their effect will be swamped and the movement of ions will be controlled by the concentration gradient. 

To achieve the transport of ions against their concentration gradients, the reversible change in the nature of ionophores at the both interfaces of a membrane is necessary, and for this object, many ingenious devices in the structure of ionophores and the transport systems have recently been developed. 

The transmembrane potential derived from a concentration gradient is calculable by means of the Nemst equation. If K+ were the only permeable ion then the membrane potential would be given by Eq. 1. With an ion activity (concentration) gradient for K+ of 10 1 from one side to the other of the membrane at 20 °C, the membrane potential that develops on addition of Valinomycin approaches a limiting value of 58 mV87). This is what is calculated from Eq. 1 and indicates that cation over anion selectivity is essentially total. As the conformation of Valinomycin in nonpolar solvents in the absence of cation is similar to that of the cation complex 105), it is quite understandable that anions have no location for interaction. One could with the Valinomycin structure construct a conformation in which a polar core were formed with six peptide N—H moieties directed inward in place of the C—O moieties but... 

Active Transporters use the energy of ATP for vectorial transport through a biological membrane against concentration gradient of the transported substrate. 

The net electrochemical driving force is determined by two factors, the electrical potential difference across the cell membrane and the concentration gradient of the permeant ion across the membrane. Changing either one can change the net driving force. The membrane potential of a cell is defined as the inside potential minus the outside, i.e. the potential difference across the cell membrane. It results from the separation of charge across the cell membrane. 

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