CHAPTER V 



SURFACE TENSION AND ADSORPTION 



We are sometimes surprised at the strength of the surface 

 film when we see insects supported on the surface of water, but 

 according to Colton (1910) a small clam suspended itself by 

 threads (byssus) attached at only three points from the surface 

 film in the Naples aquarium. This is more surprising when we 

 realize that the thickness of the film is .00000006 mm. 



Although instances were already known, it was first formu- 

 lated by J. Willard Gibbs (1874) into a general rule that sub- 

 stances which lower the surface tension of a solvent become 

 more concentrated in the surface film than in the interior. Take 

 for example a system of two phases, oil and water. If a substance 

 which lowers the surface tension of water be dissolved in the 

 water it will concentrate in the surface film, next to the oil. 

 If the substance be soluble in oil, it will diffuse into the oil until 

 an equilibrium is reached between the concentration of the sub- 

 stance in the oil and in the surface film of the water. If this 

 substance therefore has the same solubility in water and in oil 

 and does not lower the surface tension of oil it will become more 

 concentrated in the oil than in the water. If the substance be 

 insoluble in the oil it will remain concentrated in the film of water 

 next to the oil and is said to be adsorbed by the oil. Hence, 

 surface tension, adsorption and diffusion are related phenomena. 



In the phase boundary between two fluids it is easier to meas- 

 ure the surface tension than the adsorption. On the contrary 

 it is impossible to measure the surface tension in the phase 

 boundary between a fluid and a solid, but the adsorption may 

 be measured, actually or relatively. 



The most practicable method of measuring surface tension, 

 in biochemistry, is the estimation of the average weight of a 

 drop of the one of the fluids falling from a standardized pipette 

 into the other fluid. In most measurements one of the fluids 



