20 



THEORY OF COLLOIDAL BEHAVIOR 



and osmotic forces, and we intend to show that these forces 

 furnish the explanation of colloidal behavior. 



It may be best to quote Donnan's theory in his own words : 



"We suppose that the membrane (indicated in the following diagram 

 by a vertical line) be impermeable for the anion R of a salt NaR (and 

 also for the non-dissociated part of the salt NaR), but permeable for all 

 the other ions and salts to be considered in this connection . . . 



"Suppose that in the beginning we have a solution of NaR on one side 

 of the membrane (indicated by a vertical line) and of NaCl on the other 

 side 



Na 



R 



(1) 



Na 

 Cl 



In this case NaCl will diffuse from (2) to (1). In the end the following 

 equilibrium will result: 



Na 



R 



Cl 



(1) 



Na 



Cl 



(2) 



"When this equilibrium is established the energy required to transport 



+ 



reversibly and iso thermally 1 grammolecule Na from (2) to (1) equals 

 the energy which can be gained by the corresponding reversible and 



isothermal transport of a grammolecule Cl. In other words, we con- 

 sider the following infinitely small isothermal and reversible change of 

 the system: 



SnMolNa (2) -> (1) 



UnMolCl (2) - (1) J 



"The energy which can be gained in this way (i.e., the diminution of 

 free energy) is zero, hence: 



log 



log -- 2 = 



or 



[Nak[Cl] 2 = [Nh-lCl]! 

 where the brackets signify molar concentrations." 



(1) 



