82 DISPERSE SYSTEMS 



The behaviour of most organic gels is complicated not only by 

 the presence of electrolytes, and by the fact that the content of 

 the intermicellar fluid in electrolytes may be rapidly altered, but 

 also by the fact that the dispersed substance is a mixture of 

 closely related substances. Thus agar-agar, a carbohydrate 

 superficially similar to the protein-hydrate gelatin, consists of at 

 least two substances a and ft agar-agar which are mutually con- 

 vertible under certain conditions. Purified, a agar-agar is 

 practically insoluble in water. The ft form is very soluble in water. 

 On warming some of the former with water it gradually passes 

 into the soluble form and thus goes into solution. Insoluble 

 a particles may be dispersed in larger particles of ft-}- water. 

 They in turn form a true sol with water. Alteration of physical 

 or chemical conditions will therefore alter the relative concen- 

 tration of a and /?. The ft colloid protects its a relative from 

 coagulation by thus forming a pellicle round it. Starch a 

 pseudo-colloid is a mixture of several carbohydrates of high 

 molecular weight, each of which is capable of taking up a different 

 quantity of water. (See under Emulsions.) 



Of great physiological interest are soaps, the alkali salts of the 

 fatty acids. These soaps are found in the body wherever fats 

 are found in bile, blood, faeces, ear wax, sebum, etc., as well as 

 in some pathological fatty secretions. The soaps furnish a series 

 in which the molecular weight regularly increases. Step by step 

 with this increase in molecular weight there is a regular gradation 

 of the properties of the dispersoid from the true solution of the 

 soaps of the lower fatty acids to the colloidal gels of the higher 

 homologues. Still more important physiologically is the effect 

 of altering the anion. Sodium, potassium, ammonium, calcium 

 and magnesium soaps are found in physiological analyses and these 

 differ from one another, especially in their power to hold water. 

 Ammonium and potassium soaps are so hydrophilic that they do 

 not solidify but form jellies (soft soap). Sodium soaps also hold 

 a considerable amount of water, but only about j? of that held by 

 " soft " soaps. So little water is held by the soaps of calcium 

 and magnesium that they do not form a sol to any appreciable 

 extent. The addition of alkali to a sodium soap greatly increases 

 its hydrophilic properties. Soap solutions may be broken up 

 in various ways. 



(a) The addition of an acid stronger than the fatty acid frees 

 the fatty acids, e.g. H 2 SO 4 + 2NaA-Na 2 SO 4 +2HA. 



(6) On adding a powdered neutral salt to a soap solution the 



