THE STABILITY OF PROTEIN SOLUTIONS 273 



(Chap. XI). As soon as the osmotic pressure in the particle 

 exceeds the forces of cohesion between the casein ions of the 

 particle, the casein ions constituting the particle are separated. 



The question then arises, How can the Donnan effect stabilize 

 the particles of casein chloride in solution, and how can we explain 

 the precipitating effect of low concentrations of neutral salts? 

 Let us assume that the ultimate particles in a solution of casein 

 chloride of pH 2.2 are, (a) isolated casein ions, (6) isolated casein 

 molecules, and (c) small casein aggregates or micellae. The 

 Donnan equilibrium furnishes two kinds of forces preventing 

 that degree of coalescence of these particles which is required for 

 precipitation; namely, the osmotic pressure and the membrane 

 potentials. When isolated protein ions collide and remain 

 attached to form a micella, a Donnan equilibrium is established 

 between the nascent micella and the surrounding solution. 

 The Donnan equilibrium demands that there be a higher con- 

 centration of electrolytes inside than outside and this difference 

 in osmotic pressure leads to water being attracted into the mi- 

 cella. The increase in hydrostatic pressure will force the protein 

 molecules apart again and thus tends to prevent the formation 

 of the micellae. Moreover, if micellae exist in the casein chloride 

 solution (aside from isolated casein ions and molecules) the coales- 

 cence of different micellae into larger aggregates must be prevented 

 by the potential difference between the micellae and the surround- 

 ing solution. As a consequence of this P.D., the micellae must 

 repel each other. This charge as well as the osmotic pressure 

 caused by the Donnan equilibrium is a minimum at the isoelectric 

 point, rises rapidly with increasing hydrogen ion concentration, 

 reaching a maximum, and diminishes again with a further increase 

 in hydrogen ion concentration as shown in a preceding chapter. 

 The osmotic pressure and charge are also diminished by the 

 addition of salt. In this case, both the osmotic pressure as well 

 as the P.D. are depressed, in accordance with Donnan's theory, 

 and when this depression reaches a certain degree the casein 

 particles coalesce. They will also aggregate without the addition 

 of salt at or near the isoelectric point where these forces due to 

 the Donnan equilibrium are also zero or sufficiently low. 



These conclusions were supported by experiments on the pre- 

 cipitation of casein chloride solutions by salts. The concentra- 



18 



