Osmosis concentration equilibrium

A semipermeable membrane is a material that permits some particles to pass through it but not others. The diagram below shows a membrane that permits solvent but not solute to pass through it. When a semipermeable membrane separates a dilute solution from a concentrated solution, the solvent flows from the dilute to the concentrated solution (i.e., from higher solvent to lower solvent concentration) in a process called osmosis until equilibrium is achieved. Notice that there is now more solvent on the side that originally had the higher concentration of solute. 

If a semipermeable membrane separates two identical solutions, solvent molecules move in both directions at the same rate, and there is no net osmosis. The two sides of the membrane are at dynamic equilibrium. The situation changes when the solutions on the two sides of the membrane are different. Consider the membrane in Figure 12-14a. which has pure water on one side and a solution of sugar in water on the other. The sugar molecules reduce the concentration of solvent molecules in the solution. Consequently, fewer solvent molecules pass through the membrane from the solution side than from the pure solvent side. Water flows from the side containing pure solvent to the side containing solution, so there is a net rate of osmosis. 

In the absence of other forces, osmosis continues until the concentration of solvent is the same on both sides of the membrane. However, pressure can be used to stop this process. An increase in pressure on the solution side pushes solvent molecules against the membrane and thereby increases the rate of transfer of water molecules from the solution side to the solvent side. Figure 12-14Z> shows that dynamic equilibrium can be established by increasing the pressure on the solution until the rate of solvent transfer is equal in both directions. 

In general, the flow of water due to a chemical potential gradient is called chemical osmosis. When compacted, clay can act as a semi-permeable membrane due to overlapping diffuse double layers. This means that the movement of solute particles is restricted across the membrane, while solvent is free to flow. To attain chemical equilibrium in case of an initial concentration gradient across the clay, water flows from low to high salt concentration. The degree of semi-permeability is described by the reflection coefficient a, which ranges from 0 (no osmosis) to 1 (no solute transport). 

The experiments demonstrate the development of a streaming potential in consolidated bentonite clay when flushed by a NaCl-solution of either low or high concentration. The streaming potential measured in our experiments is at least 5 to 10 times larger than values reported for bentonite in the literature. Apparently this is caused by a very low electric conductivity of the bentonite samples studied. This low conductivity might be ascribed to overlapping diffuse double layers on the clay particles, caused by the high compaction and the presence of monovalent ions in the equilibrium solution. The bentonite, thus compacted, will be a very effective medium for active application of electroosmosis. Compared with electrically shorted conditions, chemical osmosis will be reduced when the clay is not short-circuited. 

Solvent from a lower concentration solution will move spontaneously to a higher concentration solution across an ideal semipermeable membrane, permeable only to the solvent but impermeable to the solute. Although the solvent flows in both directions, the rate of flow from the dilute concentration (or pure solvent) is much faster than from the concentrated solution. This phenomenon is called osmosis. The flow of the solvent can be reduced by directly applying pressure to the higher concentration side of the membrane as shown in Figure 3.17. At a certain pressure, equilibrium is reached, causing the movement of water to cease. This pressure is called the osmotic pressure and is the sole property of the solution. 

Nanofiltration is a rapidly advancing membrane separation technique for concentration/separation of important fine chemicals as well as treatment of effluents in pharmaceutical industry due to its unique charge-based repulsion property [5]. Nanofiltration, also termed as loose reverse osmosis, is capable of solving a wide variety of separation problems associated with bulk drug industry. It is a pressure-driven membrane process and indicates a specific domain of membrane technology that hes between ultrafiltration and reverse osmosis [6]. The process uses a membrane that selectively restricts flow of solutes while permitting flow of the solvent. It is closely related to reverse osmosis and is called loose RO as the pores in NF are more open than those in RO and compounds with molecular weight 150-300 Da are rejected. NF is a kinetic process and not equilibrium driven [7]. 

The flow of water through a semi-permeable membrane (clay, shale) from water with a small concentration of dissolved solids to water with a greater concentration is called osmosis (e.g. Bredehoeft et al., 1982 Neuzil, 1986). The osmotically-induced flow of water occurs because of a difference in vapour pressure across the membrane (Hinch, 1980). The aqueous activity will be relatively small in water with a relatively large concentration of dissolved solids, because more water molecules are bonded on the dissolved ions (Hinch, 1980). In a sandstone-shale sequence with water of equal chemical concentration, the aqueous activity of the shale water will be less than that of the sandstone-water, because water molecules are adsorped on the large mineral surfaces of the shale (Hinch, 1980). As a consequence, the water salinity differences that may exist in sandstone-shale sequences in the intermediate and deep subsystems of burial-induced groundwater flow may actually be in osmotic equilibrium. 

It should be expected that calculated values of 6jjp correlate better with equilibrium properties of the membranes in aqueous solution than with transport properties. Che of the few such equilibrium measurements that have been published is by Anderson et al (.2J). Their measured partition coefficients (K), diffusion coefficients (D), and reverse osmosis rejection (R) of the organic solutes are shown in Table III for cellulose acetate membranes. Their data for cellulose acetate butyrate was similar and is not shown here. Aqueous solutions of the organic solutes, usually at concentrations of about 10 g 1 , were used in the measurement of partition coefficients by UV absorption. In Table III,... 

Concentration-dependent activity coefficients can be accommodated with relative ease by an added term (e.g., [see Helfferich, 1962a Brooke and Rees, 1968] and variations in diffusivities are easily included in numerical calculations (Helfferich and Petruzzelli, 1985 Hwang and Helfferich, 1986). In both instances, however, a fair amount of additional experimental information is required to establish the dependence on composition. Electro-osmotic solvent transfer and particle-size variations are more difficult to deal with, and no readily manageable models have been developed to date. A subtle difficulty here is that, as a rule, there is not only a variation in equilibrium solvent content with conversion to another ionic form, but that the transient local solvent content is a result of dynamics (electro-osmosis) and so not accessible by thermodynamic considerations (Helfferich, 1962b). Theories based on the Stefan-Maxwell equations or other forms of (hcrniodyiiainics of ir-... 

Reverse osmosis is simply the application of pressure on a solution in excess of the osmotic pressure to create a driving force that reverses the direction of osmotic transfer of the solvent, usually water. The transport behavior can be analyzed elegantly by using general theories of irreversible thermodynamics however, a simplified solution-diffusion model accounts quite well for the actual details and mechanism in most reverse osmosis systems. Most successful membranes for this purpose sorb approximately 5 to 15% water at equilibrium. A thermodynamic analysis shows that the application of a pressure difference, Ap, to the water on the two sides of the membrane induces a differential concentration of water within the membrane at its two faces in accordance with the following (31) ... 

The model that was earlier employed to illustrate dialysis can also be used for understanding ultrafiltration. If the membrane which divides the vessel into the two compartments, of which one contains water while the other one is occupied by the solution to be processed, is impermeable to all solutes, the only diffusion that takes place is that of water into the solution and the process is ordinary osmosis. At osmotic equilibrium, the solution is under a hydrostatic pressure (the osmotic pressure) which causes sufficient flow out of the solution to balance the diffusion of water molecules into it. Now, if the hydrostatic pressure is increased beyond the osmotic pressure, there is a net flow of water out of the solution compartment, resulting in concentrating of the solution with respect to the solute, and the phenomenon is ultrafiltration. Similarly, in dialysis (wherein the membrane used is such that it is permeable to the crystalloidal solutes) when carried out under... 

The process of osmosis occurring between pure water and glucose (sugar) solution is illustrated in Figure 7.3. Note that the "driving force" for the osmotic process is the need to establish an equilibrium between the solutions on either side of the membrane. Pure solvent enters the more concentrated solution in an effort to dilute it. If this process is successful, and concentrations on both sides of the membrane become equal, the "driving force," or concentration difference, disappears. A dynamic equilibrium is established, and the osmotic pressure difference between the two sides is zero. 

Figure 13.13 The development of osmotic pressure. A, In the process of osmosis, a solution and a solvent (or solutions of different concentrations) are separated by a semipermeabie membrane, which allows only solvent molecules to pass through. The molecular-scale view below) shows that more solvent molecules enter the solution than leave it in a given time. B, As a result, the solution volume increases, so its concentration decreases. At equilibrium, the difference in heights in the two compartments reflects the osmotic pressure (11). The greater height in the solution compartment exerts a backward pressure that eventually equalizes the flow of solvent in both directions. C, Osmotic pressure is defined as the applied pressure required to prevent this volume change.

The water transfer due to osmosis and ion hydration can be significant at higher brine salt concentrations. The membrane selectivity also depends on the salt concentration due to a Donnan equilibrium between the salt solution and the membrane as discussed earlier. 

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