204 SCIENCE PROGRESS 



at the liquid-air interface, and further, that these substances 

 possess the property of lowering the surface tension of the 

 solution. Milner found, however, that the quantity adsorbed, 

 P, exceeded that calculated, approximately ten times. Milner's 

 measurements, therefore, only qualitatively bear out the 

 validity of Gibbs' generalisation. Prior to the publication of 

 Milner's measurements, an investigation of Gibbs' expression 

 had been begun in Donnan's laboratory, and a short account 

 will be given of the main results obtained by Lewis {Phil. 

 Mag., April 1908 and April 1909), and by Donnan and Barker 

 (Proc. Roy. Soc, A. 85, 557, (191 1 )). Lewis's measurements 

 deal with adsorption at a liquid / liquid interface ; those of 

 Donnan and Barker at a liquid / air interface. 



Capillary Adsorption at a Liquid/Liquid Interface 



The systems investigated consisted of a series of aqueous 

 solutions in contact with a large surface of hydrocarbon oil. 

 The solutes employed were sodium glycocholate, congo red, 

 methyl-orange, caustic soda, silver nitrate, potassium chloride, 

 barium chloride, copper chloride, and caffeine. All these 

 substances were found to lower the interfacial tension oil/ 

 water, and all should therefore be positively adsorbed. The 

 first four of these substances exerted a very marked depressing 

 effect upon the tension compared with the others. By way 

 of illustrating the depression of interfacial tension and the 

 mode of calculating the theoretical adsorption, some details 

 will be given of the behaviour of sodium glycocholate. The 

 following table contains some of the values of the interfacial 

 tension : 



Table i 



