HYDBOPHILIC SOLS AND GELS 143 



albumin with a molecular weight of 34,000 (hydrogen being 1) 

 diffuses very slowly. Whether or not it is the molecules or the 

 micelles (molecular aggregates) of proteins that diffuse in solu- 

 tion, it is impossible to say with certainty, but whichever they 

 are, they are colloidal in behavior. The diffusion rate of all 

 colloidal particles is very low. 



The following diffusion constants are of several colloidal and 

 crystalloidal substances : 



Diffusion Constants 



(D = cm.Vsec. X 10^) 



Eggalbumin 0.063 Urea 1.01 



Pepsin . 073 Sodium ion 4.51 



Glucose . 57 Hydrogen ion 32 . 50 



Albumin, colloidal in nature and with a molecular weight of 

 34,000, moves slowly; the sugar glucose, which is crystalloidal, 

 with a molecular weight of 180, moves much faster; urea, with 

 a molecular weight of 60, is still faster; sodium, of atomic weight 

 23, is faster yet; and hydrogen, with a weight of 1, is fastest. 



If rate of diffusion is in part dependent upon size and mass, it 

 should be possible to calculate one of the latter two, if the other 

 and the diffusion rate are known. This can be done with the 

 aid of two formulas derived by Einstein. The radius of egg 

 albumin so determined is 2.8 m/i. 



Diffusion studies in colloids concern not only the movement of 

 the colloidal substance itself (albumin in water) but also those 

 of other substances through the colloids (salt through gelatin). 

 Graham suspected that substances diffuse more slowly through 

 gels than in free liquids, but he found common salt to diffuse in 

 a gelatin gel just as quickly as in pure water. His surmise and 

 his experiment were both correct, as many substances diffuse 

 far more slowly in gels of high concentration (10 per cent or 

 more) than in water, but in dilute gels the diffusion rate of a 

 substance is approximately the same as in a free liquid. The 

 rate decreases rapidly with increase in concentration of the gel. 



Retardation in diffusion rate through gels may be caused by 

 adsorption forces operating between the diffusing substance and 

 the inner surfaces of the gel, but it is probably the purely mechan- 

 ical features (structure) of the gel that are primarily responsible. 

 If the structural units are close together, as in a highly concen- 



