of Gibus's Theory of Surface-Concentration. 



487 



Hence 1 gram charcoal adsorbs from a 0*218 per cent, solution 

 '0-324 gram of caffeine. 



It will be noted that the behaviour of caffeine and sodium glycocholate 

 towards charcoal and oil is very different. In contact with charcoal 

 there appears to be no very marked difference as regards the magnitude 

 of the adsorption for both substances. Possibly this may be due to 

 oxidation of the caffeine by means of the oxygen adsorbed on the 

 charcoal surface. 



Calculation of the Range of the Concentration Effects 

 at the Oil-surface. 



Consider the case of an infinite mass of solution (of a non- 

 electrolyte in water) whose bulk-concentration is i\ and 

 surface-concentration c 2 . Suppose one gram-mole of solute 

 transferred from the bulk into the surface-layer. The osmotic 

 •work done is 



RTloo-^. 



There is a corresponding decrease in the surface energy 

 due to adsorption of the solute given by 



da 

 clT ; 



and equating these two expressions, 



RTWi 2 = -t 



-a dr 



Experimental case : — 



If we assume that the solution of caffeine obeys in all 

 probability Gibbs's expression for surface-condensation, from 

 the data previously given we may construct the following- 

 table : — 



Aqueous solution of Caffeine at 15° C. 



Per cent. 



Bulk-concentration 



(anhydrous). 



Tension 

 in dynes cm. 



T calculated from 



Gibbs's equation 



in grin., cm. - 



00 



4S 



00 



0046 



45 8 



l-2xKT s 



0092 



44 4 



1-8 „ 



0114 



435 



2-0 „ 



0-229 



40-9 



2-3 „ 



0-457 



38-1 



2-8 „ 



0-915 



848 



30 „ 



