26 



ACTIONS OF DRUGS ON SURFACES 



its physical properties may be profoundly changed. For 

 example, there may be large changes in viscosity. A 

 monolayer of palmitic acid is a liquid, or even a gaseous 

 film upon an alkaline solution of sodium chloride. But 

 when the underlying cation is calcium, the monolayer is 

 an extremely viscous liquid or a solid. The effects of 

 ions upon monolayers can be readily studied by foU- 



-O 



■O 

 ■O 

 -O 

 -O 

 -O 



10- 20 A. U. 



Fig. 4. Structure and thickness of an oil-water interface. The layer of 

 oil molecules oriented at the interface is 10-20 A.U. thick, and (in solu- 

 tions of uni-univalent electrolyte) the thickness of the electrical double 

 layer 6 is 3.i/>'c A.U., where c is the electrolyte concentration. The 

 lower part of the diagram indicates the variation in electrostatic potential 

 V due to the charge on the surface, at various distances from the surface. 

 At the point x=«5, xp=y)ole where e is the base of natural logarithms. The 

 excess of ions at any point x due to the charge on the surface is propor- 

 tional to ev: the greater part of the excess lies within the double layer 



of thickness 6. 



