250 THEORY OF COLLOIDAL BEHAVIOR 



These experiments also show that the solution of solid gelatin 

 does not depend upon swelling (while the solution of casein 

 chloride is, as we shall see, determined by swelling). The swelling 

 of gelatin in acid reaches a maximum at pH of about 2.8 and then 

 diminishes upon further increase in hydrogen ion concentration, 

 while the rate of solution of solid gelatin granules continues to 

 increase steadily when the hydrogen ion concentration increases 

 beyond pH of 2.8 down to pH 1.0 (and possibly less). The 

 mechanism of swelling and the mechanism of solution of solid 

 gelatin in solutions of acid or alkali are determined by forces of 

 an entirely different character; the swelling by osmotic pressure, 

 and the solution in all probability by those forces which are 

 responsible for the solution of crystalloids. The role of secondary 

 valency forces in the process of solution is suggested by the follow- 

 ing quotation from Langmuir. 



"Acetic acid is readily soluble in water because the COOH group 

 has a strong secondary valency by which it combines with water. 

 Oleic acid is not soluble because the affinity of the hydrocarbon chains 

 for water is less than their affinity for each other. When oleic acid 

 is placed on water the acid spreads upon the water because by so doing 

 the COOH can dissolve in the water without separating the hydrocarbon 

 chains from each other. 



"When the surface on which the acid spreads is sufficiently large the 

 double bond in the hydrocarbon chain is also drawn onto the water 

 surface, so that the area occupied is much greater than in the case of 

 the saturated fatty acids. 



"Oils which do not contain active groups, as for example pure paraffin 

 oil, do not spread upon the surface of water." 1 



It should be added that if we replace the H in the carboxyl 

 group of oleic acid by K the very soluble potassium oleate is 

 formed, so that the whole molecule is now dragged into the water. 

 The Na oleate is less soluble than K oleate. Ca oleate is again 

 sparingly soluble. 



In the case of proteins we have to deal with hydrocarbon 

 groups possessing more affinity for each other than for water, 

 and with COOH and NH 2 groups (or COO and NH 3 + groups) with 

 a strong affinity for water. It is probable that the NH 2 or 



1 LANGMUIR, I., J. Am. Chem. Soc., vol. 39, p. 1850, 1917. 



