576 RICE 



ABT. L 



The data available are not numerous, and concern aqueous 

 solutions of familiar inorganic salts such as the chlorides, 

 nitrates and sulphates of familiar metals. A table of results will 

 be found on page 74 of Rideal's book; these indicate that for a 

 given salt the increase in surface tension above that of water 

 varies in an approximately linear manner with the salt con- 

 centration. Langmuir has considered these results also from 

 the point of view of a unimolecular layer. The quantity of 

 solute which has gone out of the surface film so as to leave it 

 poorer in the solute than a corresponding volume of solution is 

 cf per unit area (where c is bulk concentration and f the film 

 thickness) if a film of pure water one molecule thick exists at the 

 surface. Hence on this hypothesis cf should equal — V obtained 

 by the equation 



da 

 da 



d{kc) 

 dc 



= — kc , 



since, as we have stated, o-q — o- is approximately equal to —kc, 

 where A; is a constant. Hence f can be calculated. This should 

 be the thickness of an adsorbed water layer on the surface. Lang- 



o 



muir found f to be from 3.3 to 4.2 A, which is certainly the right 

 order of magnitude for a water molecule if it is not of an elongated 

 shape. More recent work by Goard, Harkins and others, using 

 the accurate form of Gibbs equation, finds varying values for f 



o 



which decrease from Langmuir's value between 4 and 5 A at 



o 



low bulk concentrations to about 2.5 A at high concentrations. 

 Adam suggests that this diminution may be due to the increas- 

 ing tendency of the solute to diffuse into the surface layer as 

 the bulk concentration increases. 



The evidence for the truth of Gibbs' law in connection with 

 the hypothesis of unimolecular layers is imposing, and one 

 further remark may be made with reference to the cases of 

 apparent failure in the attempts to verify it by direct 

 measurements. 



