Osmosis, definition

Electro-osmosis has been defined in the literature in many indirect ways, but the simplest definition comes from the Oxford English Dictionary, which defines it as the effect of an external electric held on a system undergoing osmosis or reverse osmosis. Electro-osmosis is not a well-understood phenomenon, and this especially apphes to polar non-ionic solutions. Recent hterature and many standard text and reference books present a rather confused picture, and some imply directly or indirectly that it cannot take place in uniform electric fields [31-35]. This assumption is perhaps based on the fact that the interaction of an external electric held on a polar molecule can produce only a net torque, but no net force. This therefore appears to be an ideal problem for molecular simulation to address, and we will describe here how molecular simulation has helped to understand this phenomenon [26]. Electro-osmosis has many important applications in both the hfe and physical sciences, including processes as diverse as water desahnation, soil purification, and drug delivery. 

Water for injection (WFI) is the most widely used solvent for parenteral preparations. The USP requirements for WFI and purified water have been recently updated to replace the traditional wet and colorimetric analytical methods with the more modern and cost-effective methods of conductivity and total organic carbon. Water for injection must be prepared and stored in a manner to ensure purity and freedom from pyrogens. The most common means of obtaining WFI is by the distillation of deionized water. This is the only method of preparation permitted by the European Pharmacopoeia (EP). In contrast, the USP and the Japanese Pharmacopeias also permit reverse osmosis to be used. The USP has also recently broadened its definition of source water to include not only the U.S. Environmental Protection Agency National Primary Drinking Water Standards, but also comparable regulations of the European Union or Japan. 

If a solution and the pure solvent are separated by a semipermeable membrane, the solvent tends to pass through the membrane into the solution, and the osmotic pressure is the pressure that must be applied to the latter to keep the solvent from entering into it. The term osmotic pressure of the solution is, therefore, strictly speaking, incorrect, as osmotic pressure is, according to the definition, produced only when the solution is separated from the solvent by a semipermeable membrane. If this is remembered, it disposes of the objection sometimes raised that osmotic pressure works the wrong way, in that it causes motion from places of lower to places of higher osmotic pressure. It is osmosis which causes osmotic pressure, and not osmotic pressure which produces osmosis. 

Assumptions 0 = 02 = 03 = 0 fr(Xj ) X., Xj 3 l D /K6 is independent of X 2 the A value considered and longitudinal diffusion is negligible these assumptions are practically valid for many reverse osmosis systems in water treatment applications. Definitions ... 

On the basis of the above assumptions and definitions, for a reverse osmosis system illustrated in Figure 13, the following relationships have been derived (112,113). 

Polyelectrolyte complexes are very promising materials for preparing semi-permeable membranes of definite permeability and selectivity The methods of preparation and the properties of membranes made of polyelectrolyte complexes based on strong polyelectrolytes, e.g. poly(sodium sterene sulfonate) and poly(vinylbenzyl-trimethyl ammonium chloride) were described These membranes may be applied for reverse osmosis in the desalting of sea-water, for dialysis and ultrafiltration in purifications and concentration of water solutions containing coUoids or micro-and macroparticles ... 

In this chapter, we will introduce fundamental concepts of the membrane and membrane-separation processes, such as membrane definition, membrane classification, membrane formation, module configuration, transport mechanism, system design, and cost evaluation. Four widely used membrane separation processes in water and wastewater treatment, namely, microfiltration (MF), ultrafiltration (UF), nanofiltrafion (NF), and reverse osmosis (RO), will be discussed in detail. The issue of membrane foufing together with its solutions will be addressed. Several examples will be given to illustrate the processes. 

One can actually consider the trapped solution morphology as a functional definition of the asymmetric membranes. It should be emphasized that this viewpoint clearly differentiates asymmetric membranes that have shown the highest reverse osmosis fluxes from membranes with a thin dense layer of normal solid morphology. 

Reverse osmosis, by definition, is the reverse of conventional osmosis, which is familiar from biology. If a dilute solution and a concentrated solution are separated by a semi-permeable membrane (which allows water molecules to pass through it, but not other solutes), water will flow from the dilute solution into the concentrated one. Under these conditions, a substantial difference in height between the two water columns may be established (Eigure 7). 

Reverse osmosis (and nanofiltration) by contrast operate by diffusion from a solution. Under the high differencial pressure across the membrane, the solvent from the solution actually dissolves in the material of the membrane, diffuses across it and transfers out into the clean solvent on the ocher side. It is not a perfect separation, because the dissolved species from the feed solution have a definite ability to diffuse through it as well, but the diffusion coefficient for the solvent is so much higher than that for the solute that the separation is virtually complete. 

Membrane Process for Recovery of Alkanesulfonates. Many attempts have been made over the years to reduce the wastewater load—which represents a loss of product—by a number of different methods. These include evaporation, extraction, reverse osmosis, and ultrafiltration. All of these processes have the disadvantage of high equipment cost and high energy requirements, and the space-time yield is low. The first breakthrough came with the development of new types of membrane with a definite separating efficiency and a large surface area, so-called spiral-wound modules. 

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