150 FINE-STRUCTURE OF PROTOPLASM II 



Li > Na > K > Rb > Cs 



Using only potassium salts and varying the anion in the halogen 



series, one finds : 



I > Br > CI, 



i.e., the more strongly hydrated CI causes less swelling than the lesser 

 hydrated I. This inversion of the influence of ion hydration shows 

 that the influence of the ions on swelling phenomena is determined 

 primarily by their charge. Biogels, such as agar in the present case, 

 usually possess a weakly negatively charged gel frame. For this reason 

 the discharging effect of cations of equal valency is inversely pro- 

 portional to their hydration. The effect of the anions is due to the 

 fact that the discharging cations are accompanied by their gegenions. 

 These lay greater claim to the charge of the cations, in inverse ratio 

 to their hydration. For this reason, the discharge of the gel framework 

 by a given cation accompanied by I ions is less than if it were ac- 

 companied by CI ions. In other words, for a given cation, the less 

 hydrated the anion of the salt is, the greater will be the water absorp- 

 tion of the gel. 



In many cases, however, gels swell not less but more strongly in 

 salt solutions than in water. This occurs if the gel framework possesses 

 ionogenic groups, as is the case with proteins. For example, the gel frame 



of gelatin, when imbibed with 

 a neutral salt solution, shows a 

 considerable negative charge as 

 a result of the dissociation of 

 COOH-groups. For this reason 

 cations can be retained by ad- 

 sorption ; their hydration is great- 

 er than the dehydration of the gel 

 framework, caused by the adsorp- 

 ^^-r^' tion of the cations. It is therefore 



Fig. 100. Hydration. Influence of ions on possible for thedegreeof swelling 



the hydration of polypeptide chains; A = ^gached in salt solutions tO be 

 difference in swelhng. 



higher than that m water. 

 Fig. 100 indicates how ions of equal valency can cause different 

 degrees of swelling. Consider an anionic side chain and a hydrophilic 

 OH-group of a neighbouring polypeptide chain. Both possess a 





1{0H\^ 





