Solution and Suspension Colligative Properties

The information in this section was taken fiiom Physiccd Chemistry by Castellan [21]. In a solution where the solute is not volatile (e.g., salts, polymers, and surfactants), the vapor pressure of the solvent is limited by the mole fraction of the solvent at the interface. Several other solution properties are also dependent on the mole fraction of the solute, x, only and not on the chemical nature of the solute. These properties are referred to as coUigative properties (fram the latin Un-gare, to bind, and co, together which include vapor pressure lowering, frezing point depression, boiling point elevation, and osmotic pressure. In each case, two phases are in equilibrium—one of which is the solution. 

For equilibrium, the chemical potentials of each phase must be equal. The chemical potential of the solvent, sub-1, in a solution, fii, is given by 

Vapor Pressure Lowering [21] For a pure vapor (considered to be ideal) in equilibrium with a solution, we have 

Freezing Pbint Depression For a pure solid (i.e., solvent) in equilibriiun with a solution, we have 

From this expresison, we can see that the fieezing point of the solution is lowered in accordance with the activity,, or concentration of solute in solution because the enthalpy of fusion, AH, is positive (i.e., endothermic). 

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Barton [41] has assembled a well-referenced source book for the derivation and use of x and cohesion parameters for various polymer solvent pairs. There are many ways to measure solvent activity, the simplest being boiling point elevation, freezing point depression, and osmotic pressure discussed in Section 11.5, Solution and Suspension Colligative Properties. ... 

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