Membranes, semi-permeable

Fillers influence the performance of semi-permeable membranes. Semi-permeable membranes were obtained by stretching a highly filled film. " In another application, zeolites were used to obtain polymer membranes used in gas... 

The above method could be applied to other biologically important problems like ion-selective membranes, semi-permeable membranes, etc. The above example shows, that constraints (at least when they are linear) do not destroy the symmetry relations. 

A method of separation that uses a semi-permeable membrane. 

Osmotic Control. Several oral osmotic systems (OROS) have been developed by the Alza Corporation to allow controUed deHvery of highly water-soluble dmgs. The elementary osmotic pump (94) consists of an osmotic core containing dmg surrounded by a semi-permeable membrane having a laser-drilled deHvery orifice. The system looks like a conventional tablet, yet the outer layer allows only the diffusion of water into the core of the unit. The rate of water diffusion into the system is controUed by the membrane s permeabUity to water and by the osmotic activity of the core. Because the membrane does not expand as water is absorbed, the dmg solution must leave the interior of the tablet through the smaU orifice at the same rate that water enters by osmosis. The osmotic driving force is constant until aU of the dmg is dissolved thus, the osmotic system maintains a constant deHvery rate of dmg until the time of complete dissolution of the dmg. 

If the dmg itself cannot provide the osmotic driving force, then a push-puU design of the osmotic system is available with other salts as the osmotic force. This system is schematized in Eigure 5. The outer surface is a rigid semi-permeable membrane that surrounds the osmotic layer of salt (propeUant). Inside the osmotic layer is a compressible membrane that surrounds the dmg solution. As the salt layer sweUs with water, the inner membrane compresses and pushes out the dmg solution. 

USE OF SEMI PERMEABLE MEMBRANE DEVICES (SPMDs) TO INDOOR AIR MONITORING OF PYRETHROID INSECTICIDES... 

An easy, rapid and environmentally friendly methodology was developed for the extraetion of pyrethroid inseetieide residues from semi permeable membrane deviees (SPMD), based in a mierowave-assisted extraetion, in front of a dialysis method nowadays widely employed. Several solvent sueh as hexane, toluene, aeetonitrile, eyelohexane and ethyl aeetate were tested as mierowave-assisted extraetion solvent. Mixtures of hexane and toluene with aeetone were also assayed and provide better results than single solvents. 

You should remember that RO uses a semi-permeable membrane. As such, the membrane is permeable to only very light molecules like water. Under atmospheric condirtions the fresh water flows into the solution which is called osmotic flow. But for purification purposes, this is no use, and hence we employ the reverse of osmotic flow. For this to happen, we need to apply external pressure in excess of osmotic pressure. The osmotic pressure is given by ... 

Fluids in Contact with Semi-permeable Membranes... 

Semi-permeable membranes are quasi-two-dimensional barriers which, given a fluid mixture of two or more species of particles (usually two different molecular species, mixed or in solution) on one side, allow the passage either way through the membrane of one or more, but not all, of the molecular species, in either direction. This gives the possibility of separating, at... 

The most important application of semi-permeable membranes is in separations based on reverse osmosis. These membranes generally have pores smaller than 1 nm. The pressure across the semi-permeable membranes for reverse osmosis is generally much larger than those for ultrafiltration, for example. This is because reverse osmosis is usually used for small molecules which have a much higher osmotic pressure, because of the higher number density, than the colloids separated in ultrafiltration. As a result reverse osmosis membranes have to be much more robust than ultrafiltration membranes. Since the focus of our discussion in this chapter will be on reverse osmosis based separations, we will describe these membranes in greater detail. 

The semi-permeable membrane is the heart of the reverse osmosis separation process. Semi-permeable membranes for reverse osmosis are broadly divided into two types. The earhest practical membrane was of the asymmetric type [3-6]. It consisted of an osmotically active surface layer with very small pores (less than 1 nm) with a thickness of 30-100 nm. This layer was physically supported on a porous substructure, whose porosity increased with distance from the surface layer. In such a membrane, the... 

The most significant application of reverse osmosis has been in the field of desalination to produce drinking water. Other important apphcations include the treatment of industrial waste water, concentration of fruit juices, and concentration of weak solutions such as aqueous ethanol [3-6]. The rest of the chapter will focus almost entirely on semi-permeable membranes used for reverse osmosis based applications. We chose this focus in view of the importance of reverse osmosis as a rather efficient separation technique for separating a wide range of solutions, especially very dilute solutions—which are usually notoriously difficult to handle using conventional techniques such as distillation. 

The first Monte Carlo study of osmotic pressure was carried out by Panagiotopoulos et al. [16], and a much more detailed study was subsequently carried out using a modified method by Murad et al. [17]. The technique is based on a generalization of the Gibbs-ensemble Monte Carlo (GEMC) method applied to membrane equihbria. The Gibbs ensemble method has been described in detail in many recent reports so we will only summarize the extension of the method to membrane equilibria here [17]. In the case of two phases separated by semi-permeable membranes... 

The chemical potential of associating systems has also been studied more recently by Bryk et al. [2]. They have extended the usual GEMC method for studying osmotic equihbrium by including four simulation cells in series, rather than the usual two compartments, but with osmotic equilibrium established between only two adjacent compartments (e.g. I and II, II and III, or IV and I). Each semi-permeable membrane was made permeable to only one species as shown and described below ... 

The arrows indicate a semi-permeable membrane and the species allowed to permeate is shown within the arrows. The parentheses show a GEMC phase (or region) and the species it contains. The first and the last region are also connected to each other. Using such a scheme, Bryk et al. showed that osmotic Monte Carlo can be successfully used to study the association of two different molecular species when an associating intermolecular potential is included in the simulation. The results agreed well with the more traditional grand-canonical Monte Carlo methods. 

Another method has also been suggested for tethering [23]. This would require all the molecules designated as the membrane molecules to be tethered to some or all of their neighbors, that are also part of the membrane. Fig. 1 shows the typical structure of a semi-permeable membrane while Fig. 2 shows a typical MD simulation system for osmosis with each membrane one molecular layer thick. In addition, as can be seen from Fig. 2, it is not necessary for the simulation system to be a cube. In fact it is desirable for... 

FIG. 2 The xy and a 3D projection of a typical osmotic MD simulation system. The semi-permeable membrane walls are in the yz plane. Periodic boundary conditions automatically generate an infinite pair of walls, infinite in the yz (transverse) directions, with alternating solution and solvent cells, each of thickness half the system width. 

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