AMPHOTERIC PROPERTIES OF PROTEIN SOLS 53 



strong solution of such a salt to a hydrophilic sol is a two-fold one. A small 

 initial amount of the added electrolyte discharges the micelles. Addition of 

 further increments of an electrolyte eventually brings about dehydration of 

 the micelles due to the great attraction of the added ions for water, or to 

 the effect of the solute in reducing the diffusion pressure of the water 

 (Chap. IX), or to both, and the resultant separation of the dispersed phase 

 out of the system. Salting out, therefore, consists in an electrical discharge of 

 the micelles, followed by their dehydration. It results in the destruction of 

 both of the stability factors of the system. 



Amphoteric Properties of Protein Sols. — Protein sols differ from most 

 others in that the micelles are amphoteric, i.e. they may act either as an acid 

 or as a base. The acid properties of proteins depend upon their — COOH 

 groups; their basic properties upon their — NH2 groups (Chap. XXVI). 

 Whether the proteins will combine with acids or bases depends principally 

 upon the pH of the dispersion medium. In a gelatin sol, for example, in 

 which the pH of the medium is above the isoelectric point ^ the —COOH 

 groups of the molecules react with a base such as sodium hydroxide, forming 

 "sodium gelatinate." This compound then dissociates into sodium ions and 

 negatively charged gelatin micelles. If the pH of the medium is below the 

 isoelectric point the — NHo groups of the gelatin molecules may combine 

 with the molecules of an acid such as HCl forming "gelatin hydrochloride." 

 Dissociation of this compound produces chloride ions and positively charged 

 gelatin micelles. 



Like the micelles of other sols, those of proteins are uncharged with respect 

 to the medium at the isoelectric point. Therefore no migration of the micelles 

 occurs if an electric current is passed through a protein sol at its isoelectric 

 point. At pH values higher than the isoelectric point protein micelles migrate 

 towards the anode, while at values below the isoelectric point they migrate 

 towards the cathode. 



The principles governing the stability and flocculation of a protein sol 

 are similar to those which hold for other hydrophilic sols with the one further 

 complication that protein micelles may be either positively or negatively 

 charged. Protein sols are stable at their isoelectric point because, although 

 uncharged, they possess, like all hydrophilic sols, micelles which are highly 

 hydrated. On either side of the isoelectric point the micelles of a protein sol 

 have the additional stability factor of an electrical charge. Addition of a 

 sufficient quantity of a dehydrating agent such as alcohol to a gelatin sol at its 

 isoelectric point will result, as it does with an uncharged agar sol, in immediate 

 flocculation. If the micelles are charged, however, addition of alcohol will 



^ The pH value of the isoelectric point of gelatin Is about 4.7. 



