EFFECTS OF SUBSTANCES IN SOLUTION ON SURFACE ENERGY 175 



salts, such as sodium and potassium chloride, are dissolved in water, 

 the interface energy is raised to a marked degree. This rise is shown in 

 Table V-3. On the other hand, bile salts, tannic acid, starch, and 

 lecithin reduce the surface energy of water. Kopaczewski's [1933] 

 experiments show that " human serum at 20° C has a tension of 67.7 

 dynes per cm," which increases to " 68.3 dynes per cm on dilution with 

 water to form a 50 per cent concentration." If a dissolved substance 

 lowers the surface energy of the solvent, it will tend to become concen- 

 trated in the surface layer. It has been observed that the surface energy 

 of stored samples of serum falls slowly with time. This decrease in 

 surface energy may be due to adsorption of air or other gases at the 

 liquid-gas interface. Substances which cause a decrease in surface 

 energy and are as a result concentrated at the surface of the solvent can 

 produce very large changes in surface energy for very minute quantities 

 dissolved. 



TABLE V-3 



Surface Energy of Aqueous Solutions 



Interface, liquid-air. For M moles per kilogram of solvent an increase in 

 T is shown. Units of AT, ergs per square centimeter. Temperature 20° C. 



NaCl KC1 



M AT AT 



0.025 0.055 



A substance which increases the surface energy of the solvent, as for 

 instance sodium chloride, will tend to flow out of the surface layer, 

 leaving it less concentrated than the solution in the interior of the solvent. 



The Hay test for the presence of bile in urine takes advantage of the 

 fact that the surface energy is decreased because of the concentration of 

 bile salts in the surface layer. If sublimed sulphur is sprinkled on the 

 surface of normal urine or pure water, the sulphur is not wetted but 

 floats on the surface. If the sublimed sulphur is sprinkled on icteric 

 urine, from patients suffering from jaundice, the sulphur sinks to the 



