26 



INTERFACIAL PHENOMENA 



in surface tension occasioned by the addition of alcohol to the water is one 

 aspect of the reduction of the cohesive forces between the molecules in the 

 solution. Most organic compounds have a similar and often very marked 

 effect on the surface tension of water when dissolved in it. However a few 

 such as sucrose have a slight increasing effect on surface tension. 



Most inorganic salts when dissolved in water increase its surface tension, 

 as shown in Table 6. The presence of such solutes increases the cohesion 

 within the liquid by attracting the water molecules more strongly than the 

 water molecules attract each other. The mean cohesive force of the molecules 

 in the solution is thus increased. The result of this effect is a greater surface 

 tension in the solution than in pure water. The magnitude of the effect of 

 inorganic salts upon surface tension is never very great, however.^ 



T.A.BLE 6 THE EFFECTS OF VARIOUS SOLUTES UPON THE SURFACE TENSION OF WATER 



As a rule, increasing the concentration of a solute in water increases the 

 magnitude of its effect, whether positive or negative, upon surface tension. 

 The influence of solutes upon surface tension is not proportional to their 

 concentration, however, as low concentrations of any solute produce relatively 

 greater effects upon surface tension than high concentrations. 



Interfacial Tension. — The condition of tension or strain in a limiting 

 layer of molecules is not confined to boundaries between liquids and gases. 

 At a boundary between two immiscible liquids or between a solid and a liquid 

 each of the abutting layers of molecules is subject to similar tensions. Boun- 



1 In general compounds which decrease the surface tension of water are of the 

 type known as "non-polar," while those which increase it are of the type known as 

 "polar" (Chap. X). 



