210 SCIENCE PROGRESS 



can hardly be the sum of the two separate effects, since that 

 substance which is most active in a capillary sense will tend 

 to displace the other to a certain extent, and thus be prefer- 

 entially adsorbed. It is to be expected that a similar state 

 of things will exist when a partially dissociated salt is adsorbed. 

 Qualitatively speaking we are familiar with this phenomenon 

 in the removal of certain adsorbed materials by the addition 

 of other substances, but no quantitative measurements have 

 been carried out up to the present time. 



Electro-capillary Adsorption 



As already mentioned, the existence of a potential difference 

 across the interface will necessarily modify the adsorption of 

 electrically charged material, e.g. ions. This electrical modifica- 

 tion of the adsorption process has been dealt with by Gibbs. 

 His result may be reached by a simpler method. Let us 

 consider a simple binary salt such as sodium chloride in aqueous 

 solution, in contact with a second liquid, say, oil, which is 

 electrically charged with respect to the solution. The result 

 is, that one of the ions is attracted towards the surface, the 

 corresponding ion following the first, so to speak, both together 

 producing a small double-layer condenser at the interface. 

 If a and b are the electro- chemical equivalents of the anion and 

 cation respectively, whilst it is the potential difference at the 

 surface, and a is the surface tension, it can be shown by thermo- 

 dynamic means that the adsorption of the ions is given by 

 the expression : 



•cation "• l^anion = ~ \ a ~T 0) ^. 



If, further, allowance is made for the ordinary capillary 

 adsorption of the salt, we obtain : 



n„ + Pc,t.„„ + n» Jon = - \ m Tc + (a + b) s | 



This expression is supposed to take account of the capillary 

 and electro-capillary effects together. It is doubtful, however, 

 whether this is a complete statement of the problem. No 

 experimental test of this expression has yet been undertaken. 

 The difficulty is to separate out the purely capillary part of the 



