CHAPTER VII 



THE INADEQUACY OF THE PRESENT THEORIES OF 

 COLLOIDAL BEHAVIOR 



We have given a survey of the influence of electrolytes on the 

 behavior of proteins and we may now single out those character- 

 istics which are specifically colloidal, i.e., which do not seem to 

 occur in crystalloids. These characteristics are: 



1. The addition of little acid (or alkali) to an isoelectric protein 

 (crystalline egg albumin, gelatin, and casein) increases, and the 

 addition of more acid (or alkali) diminishes the osmotic pressure, 

 the viscosity (and also, as will be seen, the potential differences) of 

 solutions of these proteins; and the same is true for the swelling 

 of gelatin. 



2. This effect of acids and alkalies depends only on the sign 

 and the valency of the ions in combination with the proteins; 

 ions of the same sign and valency, e.g., Cl, NO 3 , CH 3 COO, 

 H 2 PO 4 HC 2 O4 etc., influence the properties in the same way, 

 provided that th,e properties of the protein solutions are com- 

 pared for the same pH and the same concentration of originally 

 isoelectric protein, and provided that no constitutional changes 

 occur in the protein molecule or ion. 



3. When the ion in combination with a protein is bivalent 

 (e.g., 864, Ca, Ba) the osmotic pressure, viscosity, and swelling 

 of the protein are considerably less than when the ion is monova- 

 lent (e.g., Cl, Br, NO 3 , H 2 PO 4 , HC 2 O 4 , Na, K, etc.). 



4. The addition of a neutral salt to a protein solution (which is 

 not at the isoelectric point) depresses the osmotic pressure, 

 viscosity (and P.D.) of the solutions and the degree of swelling 

 of gels, and this effect increases with the valency of that ion of 

 the salt which has the opposite sign of charge to that of the 

 protein ion. 



Any theory which claims to be able to explain colloidal behavior 

 must account quantitatively for these four results. As a matter 



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