Some Applications of Osmotic Phenomena

Since the membrane is also permeable to the solvent, the solvent simultaneously diffuses into the bag, diluting the colloid. Ample air space must be present in the bag at the beginning, otherwise it will rupture owing to the pressure developed by the solvent imbibed. There is also a danger that the porosity of the membrane will increase if the bag is stretched as a result of internal pressure buildup. Cellophane tubing is most commonly used as the membrane material. It is sold in rolls for this purpose and may be cut to length and tied at the ends to make the required bags. 

Finally, it might be noted that colloids may be concentrated by a slight modification of the dialysis procedure. The liquid against which the colloid of interest is being dialyzed may itself be a concentrated colloid. With aqueous dispersions, for example, polyethylene oxide solutions may be used as the second colloid. 

The second colloid is prepared at higher activity therefore the solvent is drawn toward the more concentrated phase. This increases the concentration of the colloid of interest. Alternatively, the concentration increase may be accomplished by allowing the solvent to evaporate from the outer surface of the bag. 

Just as with osmotic pressure, the membranes in dialysis must be carefully selected to be compatible with the system under study. Specifically, this amounts to impermeability with respect to the colloid(s) involved and permeability with respect to low molecular weight components. 

As we discussed in Section 3.2, samples of solution and solvent separated by a semipermeable membrane will be at equilibrium only when the solution is at a greater pressure than the solvent. This is the osmotic pressure. If the solution is under less pressure than the equilibrium osmotic pressure, solvent will flow from the pure phase into the solution. If, on the other hand, the solution is under a pressure greater than the equilibrium osmotic pressure, the pure solvent will flow in the reverse direction, from the solution to the solvent phase. In the last case, the semipermeable membrane functions like a filter that separates solvent from solute molecules. In fact, the process is referred to in the literature by the terms hyperfiltration and ultrafiltration, as well as reverse osmosis (Sourirajan 1970) however, the last term is enjoying common use these days. 

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We conclude the chapter with a discussion of the Donnan equilibrium and the thermodynamic behavior of charged colloids, particularly with respect to osmotic pressure and molecular weight determination (Section 3.5), and some applications of osmotic phenomena (Section 3.6). 

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