FACTORS AFFECTING SURFACE TENSION 



25 



surface boundaries, do not. In addition to the unbalanced molecular attrac- 

 tions which the molecules in the surface layer of a liquid sustain, such mole- 

 cules are usually definitely oriented. This results in a greater concentration 

 of molecules in the surface layer than in the body of the liquid, and contrib- 

 utes toward maintenance of rigidity of the surface film. Surface tension 

 values are generally stated in terms of dynes per centimeter, that is, as the 

 force in dynes which is exerted by the liquid along a line i cm. in length. 

 The surface tension values of some common liquids are given in Table 5. 



TABLE 5 SURFACE TENSIONS OF SOME COMMON LIQUIDS AT 20 C. 



Acetic Acid. . . . 



Benzene 



Chloroform. . . . 

 Ethyl alcohol . . 

 Ethyl ether. . . . 



Mercury 



Methyl alcohol. 

 Water 



27.6 dynes/cm. 

 28.9 

 27. 1 

 22.3 

 17.0 

 465.0 

 22.6 

 72.8 



Factors Affecting Surface Tension. — i. Temperature. — Since the 

 phenomenon of surface tension is one manifestation of the cohesive forces 

 existing among molecules, the greater these cohesive forces, the greater the 

 surface tension. Temperature is a measure of the kinetic energy of the mole- 

 cules, hence an increase in temperature means an increase in their kinetic 

 energy and vice versa. As the kinetic activity of water molecules increases, 

 the effectiveness of the cohesive forces existing between them decreases. Hence 

 as the temperature of water increases, its surface tension decreases. The 

 decrease in surface tension of any liquid is, in fact, directly proportional to 

 increase in temperature, except when a liquid approaches its critical tempera- 

 ture. The variation in the surface tension of water within the temperature 

 range at which most physiological processes occur is not sufficient, however, 

 to be of any very great significance. 



2. Solutes. — As shown in Table 6, the surface tension of a liquid is 

 influenced by the presence of the molecules or ions of a solute. Some solutes 

 increase the surface tension of water; others decrease it. If alcohol, for 

 example, be added to water the surface tension of the resulting solution will 

 be less than that of pure water. The attraction between an alcohol molecule 

 and a water molecule is less than that between two water molecules. Since 

 the alcohol molecules are distributed among the water molecules, the mean 

 cohesive force in the solution is less than that in pure water. The reduction 



