## Osmotic pressure

Whenever a solution is separated from a solvent by a membrane that is permeable only to solvent molecules (referred to as a semipermeable membrane), there is a passage of solvent across the membrane into the solution. This is the phenomenon of osmosis. If the solution is totally confined by a semipermeable membrane and immersed in the solvent, then a pressure differential develops across the membrane, which is referred to as the osmotic pressure. Solvent passes through the membrane because of the inequality of the chemical potentials on either side of the membrane. Since the chemical potential of a solvent molecule in solution is less than that in pure solvent, solvent will spontaneously enter the solution until this inequality is removed. The equation which relates the osmotic pressure of the solution, n, to the solution concentration is the van't Hoff equation:

On application of the van't Hoff equation to the drug molecules in solution, consideration must be made of any ionisation of the molecules, since osmotic pressure, being a colli-gative property, will be dependent on the total number of particles in solution (including the free counterions). To allow for what was at the time considered to be anomalous behaviour of electrolyte solutions, van't Hoff introduced a correction factor, i. The value of this factor approaches a number equal to that of the number of ions, v, into which each molecule dissociates as the solution is progressively diluted. The ratio ilv is termed the practical osmotic coefficient,

For nonideal solutions, the activity and osmotic pressure are related by the expression

1 1000

where M1 is the molecular weight of the solvent and m is the molality of the solution. The relationship between the osmotic pressure and the osmotic coefficient is thus lRT\ vmM, n = |—| ^r1 é

vJ 1000

where V x is the partial molal volume of the solvent.

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