i68 



ATOMS, IONS, SALTS, AND SURFACES 



a In Cj 

 , but the correction is less than lo per 



The difficulty in the application of this equation is that activities in solution are 

 known for almost no organic substances. Figure 20 shows how the surface tension of 



solutions of organic acids varies with the logarithm of the concentration 



The slope of k-t — is slightly greater than ^r-. — - 

 o In a o-'o d In C 



cent in the case of the condensed monomolecular film of butyric acid. The nearly iden- 

 tical slopes of the 7, log C, curves indicate that the adsorption is nearly independent 



of the number of carbon 

 atoms in the molecule for 

 the tightly packed mono- 

 molecular films. 



The calculations made 

 thus far indicate that the 

 area per molecule in the 

 film for these soluble sub- 

 stances is of the same order 

 as that for the longer mole- 

 cules which are insoluble. 

 Data on the surface and 

 interfacial tensions indi- 

 cate that between water 

 and benzene, or between 

 water and hexane, the num- 

 ber of butyric acid mole- 

 cules in unit area of the film is exactly the same, within the limits of experimental 

 error, as between water and vapor. This is additional evidence that the number of 

 molecules in a condensed film depends mainly on the size of the molecules and their 

 orientation. 



MEAN THICKNESS OF THE WATER FILM ON SALT SOLUTIONS 



Water is positively absorbed on aqueous salt solutions, i.e., the surface contains 

 much less salt than the solution. It is possible to calculate a mean thickness for the 

 water film if it is assumed that the film consists of pure water, and that just below the 

 film the concentration of the solution is the same as farther inside. 



The remarkable relation which emerges from such calculations is that in very dilute 

 solutions the calculated film thickness is almost exactly the cube root of the volume occupied 

 in water by a water molecule (3.1 A). Figure 21 plots the thickness of the water film as 

 a function of the concentration of the salt. 



The water film between water and benzene is found also to have practically the 

 same thickness at the same concentration as between water and air. 



Thus, either between aqueous solution and vapors, or between the aqueous phase 

 and a non-polar liquid, the water film is monomolecular in dilute solutions, and even 

 thinner in concentrated solutions. Both between salt solution and vapor and between 

 salt solution and benzene the surface tension increases as a linear function of the mo- 

 lality of the salt in the solution. 



Fig. 21.— 

 Strom units). 



Mols of salt per i,ooo-gm. water 

 ■Thickness of the water film on salt solutions (Ang- 



