Diffusion through membranes

The transport of gas in polymers has been studied for over 150 years (1). Many of the concepts developed in 1866 by Graham (2) are still accepted today. Graham postulated that the mechanism of the permeation process involves the solution of the gas in the upstream surface of the membrane, diffusion through the membrane followed by evaporation from the downstream membrane surface. This is the basis for the "solution-diffusion model which is used even today in analyzing gas transport phenomena in polymeric membranes. 

Passive Transport. Transport by simple diffusion This mode of transport is available for apolar molecules. Permeation is predominantly governed by partitioning of the substrate between the lipid and water. The membrane simply acts as a permeability barrier small molecules pass more easily than large ones. The transport is explained in terms of a simple diffusion model involving three steps passage of the substrate from the exterior into the membrane, diffusion through the membrane, and passage out of the membrane. 

Cold sensitivity. Some antibiotics act as carriers that bind an ion on one side of a membrane, diffuse through the membrane, and release the ion on the other side. The conductance of a lipid-bilayer membrane containing a carrier antibiotic decreased abruptly when the temperature was lowered from 40°C to 36°C. In contrast, there was little change in conductance of the same bilayer membrane when it contained a channel-forming antibiotic. Why ... 

In the applications where high pressure of residual gases (nonpermeate stream) is required, membranes are well suited. The transport steps involved in a membrane system are very similar to the transport steps involved in a heterogeneous reaction system where first the reactant should be adsorbed on the surface, then diffused into the pellet, and finally the products are desorbed. The permeating component of the gas stream is sorbed in the membrane, diffused through the membrane, and then desorbed on the other side of the membrane. Fundamental research in the area of development of suitable membranes for different gas processing and petrochemical-related industry applications is being carried out in various academic institutions. 

POLYMER MEMBRANES. The transport of gases through dense (nonporous) polymer membranes occurs by a solution-diffusion mechanism. The gas dissolves in the polymer at the high-pressure side of the membranes, diffuses through the polymer phase, and desorbs or evaporates at the low-pressure side. The rate of mass transfer depends on the concentration gradient in the membrane, which is proportional to the pressure gradient across the membrane if the solubility is proportional to the pressure. Typical gradients for a binary mixture are shown in Fig. 26.2. Henry s law is assumed to apply for each gas, and equilibrium is assumed... 

Currently, the mainstream of RO membrane transport theory is the solution-diffusion model [50]. According to the model, mass transfer occurs in three steps absorption to the membrane, diffusion through the membrane, and desorption from the membrane. The chemical potential gradient from the feed side of the manbrane to the permeate side of the membrane is the driving force for the mass transfer. When the difference in hydrostatic pressure is greater than the difference in osmotic pressure between the upstream and downstream sides of the membrane, a chemical potential difference of water across the membrane drives water against the natural direction of water flow. 

In the solution-diffusion model the penetrant molecules are absorbed on the feed (or upstream) side of the membrane, diffuse through the active layer, and are finally... 

The processes of delivery of substrates to the cells of an organ begin with the local flows, followed by permeation of the capillary membrane, diffusion through the interstitial fluid space, and transport across the sarcolemma. After this the intercellular events such as reaction, binding, or sequestration can occur. The question to be discussed is whether a specific component of the system, such as transsarcolemmal transport, or an intracellular metabolic rate, can be characterized quantitatively with reasonable accuracy from a particular type of experiment. 

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