Osmometer components

We consider this system in an osmotic pressure experiment based on a membrane which is permeable to all components except the polymeric ion P that is, solvent molecules, M" , and X can pass through the membrane freely to establish the osmotic equilibrium, and only the polymer is restrained. It does not matter whether pure solvent or a salt solution is introduced across the membrane from the polymer solution or whether the latter initially contains salt or not. At equilibrium both sides of the osmometer contain solvent, M , and X in such proportions as to satisfy the constaints imposed by electroneutrality and equilibrium conditions. 

Figure 6.1 Diagram to show the essential components of a vapour phase osmometer...

Figure 4.4 Schematic diagram of essential components of a high-speed membrane osmometer.

Figure 4.4 Schematic diagram of essential components of a high-speed membrane osmometer. (Hewlett-Packard Corp., Avondale, Pa.)...

This colligative property involves the equilibrium of a liquid solution and the pure liquid solvent on opposite sides of a semipermeable membrane that allows only the solvent to equilibrate. The equilibrium is achieved by having different pressures in the two phases. A simple osmometer is shown in Figure 6.26. The left side of this apparatus contains a solution containing a solute (component 2) dissolved in a solvent (component 1), and the right side contains pure solvent. The pressure of the solution is increased above that of the pure solvent by the gravitational (hydrostatic) force on the solution in the left column. 

Up to this point, the discussion has concerned systems with one or more polymer species dissolved in a pure solvent. However, thermodynamic behavior in the more general case of systems with multiple solvents involves additional effects that are of considerable interest. Here, for simplicity, only a three-component system of a polymer (component 2) in two low molecular weight solvents (components 1 and 3) is considered but an exhaustive analysis of the general multicomponent case is available. The classification of solvents and solute is of course arbitrary, but it has utility with respect to specific experimental measurements. For instance, the operative principle in an osmotic measurement is that solvents pass through the membrane that confines a polymeric solute. The obvious effect of the additional thermodynamic degree of freedom in the three-component system is the possibility of selective interaction (preferential solvation or binding ) of the solute with a solvent component. In an osmotic experiment this will be manifested by equilibrium solvent compositions that are different in the solute and solvent compartments of the osmometer. It is possible, though not always very useful, to formally redefine a solute component so as to include in it any excess (or deficiency) of a solvent component required to make the free solvent mixture appear... 

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