Osmotic pressure vapor phase osmometry

Colligative properties of dilute solutions—polymer solutions particularly—directly result from the variation of the chemical potential of the solvent into which a solute is added. Such properties can be assessed by measuring the osmotic pressure (membrane osmometry), the decrease of the vapor pressure (vapor phase osmometry) or of the freezing point (cryometry). Contrary to the titration of the terminal functional groups, colligative methods do not require a prior knowledge of the polymer structure and depend exclusively on the number of solute molecules. 

The practical range of molecular weights that can be measured by membrane osmometry is approximately 30,000 to one million. The upper limit is set by the smallest osmotic pressure that can be measured at the concentrations that can be used with polymer solutions. The lower limit, on the other hand, depends on the permeability of the membrane toward low-molecular-weight polymers. For measurements of Mn less than 30,000 another technique known as vapor-phase osmometry described next is more suitable. 

Vapor-phase osmometry is also a colligative method and is based on the decrease of the vapor pressure, due to the addition of a solute into a pure solvent (Figure 6.4). Like osmotic pressure this property depends exclusively on the number of molecules of solute introduced and thus gives access to the number average molar mass. The disadvantage of VPO is its lack of sensitivity, due to the very small variations of the vapor pressure exhibited by dilute polymer solutions. It is, in fact, well suited to the analysis of low molar mass samples (<2 x 10 g mol ) and is thus complementary to membrane osmometry. Because it is easier to measure small variations in temperature than those in vapor pressure, a thermoelectric device is used to transform the increase of vapor pressure in a VPO experiment into a variation of temperature. Thus, two thermistors are placed in a closed chamber containing a pure solvent at a given temperature. If a pure solvent drop is placed on each of the two thermistors, they will indicate the same temperature. On the other hand, if a drop of a dilute polymer solution is placed on one of the two thermistors, a variation in temperature will result, caused by the condensation of pure solvent on this thermistor due to the difference in chemical potential between the drops. 

Because polyelectrolytes are nonvolatile, the most important thermodynamic property for vapor + liquid phase equilibrium considerations is the vapor pressure of water above the aqueous solution. Instead of the vapor pressure, some directly related other properties are used, e.g., the activity of water a, the osmotic pressure 71, and the osmotic coefficient < . These properties are defined and discussed in Sect. 4. Membrane osmometry, vapor pressure osmometry, and isopiestic experiments are common methods for measuring the osmotic pressure and/or the osmotic coefficient. A few authors also reported experimental results for the activity coefficient y i of the counterions (usually determined using ion-selective electrodes) and for the freezing-point depression of water AT p. The activity coefficient is the ratio of activity to COTicentration ... 

---