760 TRIVALENT AND TETRAVALENT IONS 



It seemed of interest to find out whether experiments on the sta- 

 bility of aqueous solutions and suspensions of proteins at the 

 isoelectric point are also in harmony with the results of the direct 

 measurements of the membrane potentials. The reason why experi- 

 ments on stability and flocculation were selected was that it is often 

 stated that the flocculation of colloids is influenced in an opposite 

 sense by the two oppositely charged ions of a salt, the ion with the 

 same sign of charge as the colloid increasing the stability, the salt 

 ion with the opposite sign of charge to that of the colloidal particle 

 diminishing the stability of the suspension. 



If we wish to use observations on the influence of salts on the 

 stability of protein solutions at the isoelectric point for conclusions 

 concerning the influence of ions on the electrical charges of particles, 

 we are confronted with the difficulty that the electrical charges of 

 particles are not the only forces which keep proteins in solution. 

 There are two different kinds of forces determining the stability of 

 solutions or suspensions of proteins, namely; first, the attraction 

 between the molecules of the protein and the solvent, and second, 

 forces of electrostatic repulsion between micellae. When the forces 

 of attraction between molecules of the solvent and molecules of the 

 solute (which may be secondary valency forces) are greater than the 

 forces of attraction between the molecules of the solute for each 

 other, the solution will be stable. This type of forces acts in the 

 general case of solutions of crystalloids. 



When the forces of attraction between the molecules of solute and 

 solvent are weak, the molecules of the solute upon colliding may 

 adhere to each other and aggregates will be formed. This aggre- 

 gate formation will lead to a flocculation or coagulation of the whole 

 mass unless new forces originate in the small nascent aggregates 

 (or micellae) which prevent their coalescence into larger aggregates. 

 These forces may be electrical charges whereby the nascent micellae 

 repel each other. The writer has investigated the origin of these 

 charges in the case of protein micellae and has found that they are 

 due to the establishment of a Donnan equilibrium between particles 

 and solution.^ A membrane equilibrium between particles and 



* Loeb, J., Proteins and the theory of colloidal behavior, New York and London, 

 1922, 120; /. Gen. Physiol, 1920-21, iii, 667; 1921-22, iv, 351. 



