HYDROPHILIC SOLS AND GELS 147 



the sign of the protein particle (Fig. 177). The adsorption of 

 water in this manner is known as solvation. The Dutch chemists 

 Kruyt and de Jong believe that proteins and lyophilic colloids 

 in general are kept in suspension by both adsorbed water and 

 adsorbed ions. When more water molecules than ions surround 

 the protein particle, it is less sensitive to electrolytes (salts) and 

 is precipitated best by ''desolvating" agents such as alcohol. 

 When less water is present, the system is more sensitive to 

 electrolytes. It is generally true that solutions of lyophihc 

 organic matter are less susceptible to the effects of electrolytes 

 than are suspensions of metals. Freundlich believes these 

 electrical properties to be a natural basis for classification and 

 has called the lyophobic colloids eledrocratic (electrosensitive) in 

 contrast to the electroresistant lyophilic ones. 



An adsorbed envelope of ions or a water mantle or both are 

 apparently responsible for the stability of lyophihc colloidal sys- 

 tems (gelatin, albumin, and soap). When both ions and water 

 stabilize, precipitation is accomplished only by a substantial 

 reduction in both charge (removal of the ionic envelope) and 

 hydration (removal of the water mantle). 



Gortner says that the salting out of lyophilic colloids (a process 

 common in the manufacture of soap) by the addition of 

 ammonium sulphate is not an electrokinetic phenomenon, 

 whereby the electrical charge of the protein in solution is reduced 

 to zero, but a dehydration of electrically neutral micelles. 



Protective Colloids. — The greater resistance of the lyophilic 

 colloidal systems (gelatin) to electrolytes can be transferred to 

 the lyophobes (gold) by giving the solid particles of the latter a 

 coating of the former. Most organic colloids, such as gelatin 

 and albumin, will function in this way and are known as protective 

 colloids. Gelatin when added in hot solution to a colloidal 

 dispersion of a metal forms a thin membrane around each metal 

 particle and thus protects it from the action of an electrolyte. 

 (Organic substances function as protective colloids only when 

 in proper concentration. They may, when in slight concentra- 

 tion, actually make a lyophobic colloidal suspension more sensi- 

 tive to electrolytes.) Zsigmondy has made a special study of the 

 protective efficiency of organic substances and has given a value 

 to each, the so-called gold number, which indicates their relative 

 protective powers. The gold number is that weight in milli- 



