114 



IMBIBITION 



associated with individual cations of various species, and partly to the replace- 

 ment of a larger proportion of hydrogen ions in the double layer by some 

 species of cations than others. 



In addition to this lyotropic effect upon swelling, ions often exert an 

 electrostatic effect as well. Solutions of chlorides producing bi- and trivalent 

 cations (CaCl2, AlClo, etc.) result in a great reduction in the swelling of 

 kelp stipe even when the concentration is so dilute that their osmotic effects 

 are negligible. Exchange of the polyvalent cations for the H+ ions in the 

 double layer apparently results in a type of flocculating effect upon the nega- 

 tively charged particles of the kelp. This brings the particles closer together 

 and results in less swelling. Such effects are also exerted when polyvalent 

 cations are associated with other univalent anions, but when paired with poly- 

 valent anions the decreasing effect of salts producing polyvalent cations upon 

 the swelling of negatively charged imbibants is less marked. There is a close 

 similarity between this phenomenon and the effect of electrolytes in flocculat- 

 ing negatively charged sols. 



4. Effects of H-ion Concentration. — Hydrogen ions in any appreciable 

 concentration affect the swelling of a negatively charged imbibant adversely, 



while hydroxyl ions in any appreciable 

 concentration cause greater swelling 

 of such systems than occurs in pure 

 water. Exactly the reverse effects are 

 exerted by hydrogen and hydroxyl 

 ions upon the swelling of positively 

 charged imbibants. The swelling of 

 amphoteric gel-forming substances 

 such as gelatin and certain other pro- 

 teins is influenced in an even more 

 complex manner by the pH of the swelling medium as indicated in Fig. 24. 

 Minimum swelling occurs at the isoelectric point, and with increase or decrease 

 of pH from this point increased swelling occurs. At relatively low or relatively 

 high pH values a decreasing effect upon swelling is again evident. 



Imbibition Pressure. — Pressures, sometimes of an enormous magnitude, 

 develop during the swelling of an imbibing substance. Such pressures only 

 become evident if the imbibant is confined in some way during the process of 

 imbibition. One common method of demonstrating the development of pres- 

 sures during imbibition is as follows: A glass funnel, lined with filter paper, 

 is partly filled with moist plaster of Paris paste. The surface of the plaster 

 of Paris is then strewn with a number of pea seeds, after which more paste 

 is added until the funnel is full. In a few minutes the moist matrix contain- 



4 5 6 



pH VALUE 



Fig. 24. Relation between pH and the 

 swelling of gelatin (diagrammatic). 



