480 Mr. W. C. M. Lewis : Experimental Investigation 



The Adsorption of Non-Electrolytes. 



The chief difficulty at the outset with regard to almost all 

 non-electrolytes is their small solubility in water. It is not 

 easy, therefore, to draw a comparison between the tension 

 effects produced by these bodies and the effects produced 

 by electrolytes. Altogether seventeen non-electrolytes were 

 examined as regards their influence on the interfacial tension. 

 Two of these — aniline and p-nitrosodimethylaniline — were 

 found to be soluble in the oil, and to this is probably due 

 their comparatively marked effects on the tension *. 



The following table (p. 481) contains the values obtained 

 from the various substances examined. 



The value obtained for cane-sugar is worthy of note. 

 The lowering effect is small, although a more concentrated 

 solution was employed than in the case of any other substance. 

 The result is in agreement with that observed at the air- 

 surface, in which case the presence of cane-sugar has no 

 appreciable effect. 



In the case of all these substances (except iodine which 

 was practically without effect) there is a lowering of tension 

 at the interface which on Gibbs's theory must be accom- 

 panied by adsorption. The same general rule, therefore, as 

 regards the effect at the oil-surface seems to hold for non- 

 electrolytes as for electrolytes. 



* The Tension between tico partially miscible phases. 



The Laplace theory of capillarity shows that the work required to 

 form unit area of interface between two liquids A and B is given by the 

 expression 



n 00 



where p A and p B =the densities of the liquids A and B respectively and 

 the integral is supposed constant for all substances. This expression 

 leads to the following relationship : — 



which has not been verified by experiment, but shows in a general way 

 what takes place at the surface. Lord Rayleigh has on this hypothesis 

 calculated the effect of making the transition gradual between A and B 

 by the interposition of n liquids whose densities are in arithmetical 

 series, in which case it is shown that the energy due to surface-tension 

 is reduced to (w+1) of its original value. Hence any diminution in the 

 abruptness will diminish the energy due to surface-tension. The solu- 

 bility of the aniline in the oil causes the diminution in abruptness, and 

 consequently we find very marked lowering of tension. 



