chapter Four 

 SURFACE CHEMISTRY 



The phenomenon of adsorption has been known and has been studied 

 for many years. For example, Sir James Dewar found that charcoal 

 cooled in liquid air was capable of taking up large quantities of such gases 

 as oxygen and nitrogen. This was known to be a surface action depending 

 on the very fine state of division of the charcoal. 



The effect of soap in lowering the surface tension of water depends 

 upon the presence of a higher concentration of soap molecules in the surface 

 layer than in the solution. 



WiLLARD GiBBS provcd thermodyuamically that there is a general 

 relation between the surface adsorption, the lowering of the surface tension 

 and the concentration of the solution. The equation that he deduced can 

 readily be put in the form 



^^ =okT, (I) 



d (In p) 



where p is the partial pressure of the vapor of the adsorbed substance in 

 equilibrium with the surface of the liquid, or it may be the partial osmotic 

 pressure of a substance dissolved in the liquid, o is the number of molecules 

 adsorbed on the surface per unit area, T is the absolute temperature and k 

 is the Boltzmann constant 1.37 X io~^^ erg deg~^, and F, which may be 

 called the spreading force, is given by 



y F = ^„ - 7 , (2) 



where yo is the surface tension of the pure solvent (in dynes cm~^) and 

 Y is the surface tension of the solution. 



The form of Gibbs' equation represented by Eq. (i) is thermo- 

 dyuamically valid if the film is in equilibrium with the 2 volume phases 

 and if the law of ideal gases 



P = nkT (3) 



is applicable in the volume phases. 



Prior to 1910 many different theories of adsorption had been proposed, 

 but none of them had been very successful. In most of these theories the 



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