Osmotic pressure difference

It was pointed out at the beginning of this section that ir could be viewed as arising from an osmotic pressure difference between a surface region comprising an adsorbed film and that of the pure solvent. It is instructive to develop... 

Feed High pressure feed side AP = Transmembrane pressure difference An = Osmotic pressure difference Membrane Concentrate... 

The pressure difference between the high and low pressure sides of the membrane is denoted as AP the osmotic pressure difference across the membrane is defined as Att the net driving force for water transport across the membrane is AP — (tAtt, where O is the Staverman reflection coefficient and a = 1 means 100% solute rejection. The standardized terminology recommended for use to describe pressure-driven membrane processes, including that for reverse osmosis, has been reviewed (24). 

Concentration of Seawater by ED. In terms of membrane area, concentration of seawater is the second largest use. Warm seawater is concentrated by ED to 18 to 20% dissolved soHds using membranes with monovalent-ion-selective skins. The EDR process is not used. The osmotic pressure difference between about 19% NaCl solution and partially depleted seawater is about 20,000 kPa (200 atm) at 25°C, which is well beyond the range of reverse osmosis. Salt is produced from the brine by evaporation and crystallisa tion at seven plants in Japan and one each in South Korea, Taiwan, and Kuwait. A second plant is soon to be built in South Korea. None of the plants are justified on economic grounds compared to imported solar or mined salt. 

Physical methods such as osmotic shock, in which the cells are exposed to high salt concentrations to generate an osmotic pressure difference across the membrane, can lead to cell-wall disruption. Similar disruption can be obtained by subjecting the cells to freeze/thaw cycles, or by pressuriziug the cells with an inert gas (e.g., nitrogen) followed by a rapid depressurization. These methods are not typically used for large-scale operations. 

The total mobile ion concentration (c++c ) inside the gel at equilibrium will inevitably exceed that in the external solution, c+ +c = vcj where v = v +v-. This must result in an osmotic pressure difference which tends to drive solvent into the gel from the less concentrated external solution. (We neglect for the moment the osmotic effects of polymer itself.) The osmotic pressure arising from the difference in mobile ion concentrations will be given approximately (see Appendix B), assuming the solutions to be dilute, by... 

In these systems, osmotic pressure provides the driving force to generate controlled release of drug. Consider a semipermeable membrane that is permeable to water, but not to drug. A tablet containing a core of drug surrounded by such a membrane is shown in Fig. 9. When this device is exposed to water or any body fluid, water will flow into the tablet owing to the osmotic pressure difference. The rate of flow, dV/dt, of water into the device can be represented as... 

Fig. 6.4 Electroosmotic pressure. Hydrostatic pressure difference Ap compensates the osmotic pressure difference between the compartments 1 and 1 and prevents the solvent from flowing through the membrane 2...

Figure 6 KC1 release rate versus the osmotic pressure difference between the core formulation and the release medium. The osmotic pressure of the release medium was varied by addition of urea. (Redrawn from data in Ref. 16.)... 

When A/zi becomes small, only the osmotic pressure difference An remains. Thus, for constant ui ... 

As a result of Internal concentration polarization, the effective osmotic pressure difference across the membrane can be significantly below the osmotic pressure difference between the bulk solutions. The effective osmotic pressure can be calculated from the salt permeation coefficient and the salt diffusion resistance in the porous membrane substrate. The highest power output for a membrane is obtained at an operating pressure equal to about one half of the effective osmotic pressure. 

The force acting on a particle suspended in a solution will be caused by the osmotic pressure differences in the suspension or... 

The influence of neutral salts as well as of acids and bases on the swelling of gelatine which we have seen can be attributed to an apparent change in the solvation of the gel fibrils and may be interpreted in the light of Donnan s theory of the effect of a non-diffusible ion on the osmotic pressure differences between the two phases, is likewise to be noted in the alteration of the viscosity and alcohol precipitation values of protein solutions. From the considerations already advanced there should exist two well-defined maxima in the viscosity and alcohol precipitation curves when these properties are plotted as functions of the Ph, the maxima coinciding with the points of maximum dissociation of the salts... 

One more boimdary condition is required to specify the problem. Suppose that the rate of myelin growth is directly proportional to the osmotic pressure difference Ap between the bulk aqueous solution and the lamellar phase at the base of the myelins (x = 0) and inversely proportional to myehn length. For example, contact between the myelins could provide resistance to the swelling process proportional to myelin length. Hence... 

For ideal solutions the osmotic pressure is simply given by CkT. Hence, the osmotic pressure difference between the mid-plane region and the... 

In accordance with the Spiegler-Kedem model (Krishna and Wesselingh, 1997), Avd is proportional to the net pressure difference across the membranes. Since the pressure difference (AP) may be regarded as negligible, Ava is mainly controlled by the corresponding instantaneous osmotic pressure difference (Are), this being proportional to the difference in solute concentration across the membranes for a great number of solutes (Lo Presti and Moresi, 2000) ... 

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