ADSORPTION PHENOMENA 207 



value. In the case of inorganic salts there is approximate 

 agreement as regards order of magnitude, but very little 

 reliance can be placed on the result. In the case of such 

 complex bodies as sodium glycocholate, methyl-orange, and 

 congo red, the discrepancy may with some confidence be 

 ascribed to the colloidal nature of the solute, producing at the 

 interface a layer of coagulated material, a process which, of 

 course, is not taken account of in the simple reversible surface- 

 effect postulated by Gibbs. The behaviour of caustic soda is 

 less clear. In some later work by Lewis [Zeitschr. Physik. 

 Chem., 73, 129, (1910)) an adsorbing surface of mercury was 

 employed, the mercury being allowed to fall through the 

 solution in a fine spray. To avoid the disturbance due to 

 dissolved oxygen, the solvent was made up of 80 per cent, 

 water and 20 per cent, alcohol. Employing a very dilute 

 solution of aniline (0*007 P er cent.), the calculated adsorption 

 amounted to 1 x io~ 8 grams per cm. 2 , whilst that found varied 

 from 2 to 3 x io~ 8 gram/cm. 2 ; that is, there is approximate 

 agreement. Measurements were also carried out with sodium 

 glycocholate at the mercury/solution interface, and again, as 

 in the case of the oil/solution interface, the observed adsorp- 

 tion exceeded that calculated about 25 times. 



In general, therefore, the conclusion arrived at as regards 

 the liquid/liquid interface is simply this : certain substances of 

 a colloidal nature exhibit adsorption much in excess of that 

 calculated, whilst simple non-electrolytes such as aniline and 

 caffeine, as well as inorganic salts in aqueous solution, show 

 agreement as regards the order of magnitude. It is evident 

 that much work still remains to be done before the phenomenon 

 can be regarded as properly understood. 



Capillary Adsorption at a Liquid/Air Interface 

 or Surface 



Donnan and Barker (loc. cit.) were the first to investigate 

 quantitatively the adsorption at the free air surface of an aqueous 

 solution. The solute chosen was nonylic acid, a substance 

 which possesses the property of markedly depressing the sur- 

 face tension, and is at the same time sufficiently simple in 

 chemical constitution that abnormal effects due to the colloidal 

 state may be safely assumed to be absent. The value of the 

 surface tension was employed in this case also as the analytical 



