LIQUID-GAS INTERFACE 173 



If, as in the structure of a soap bubble, two surfaces of external radius 

 R 2 and internal radius R\ are involved, then the internal pressure is 



p = 2T 



\R 2 + Rj 



or for very thin soap bubbles where R\ may be considered equal to R 2 



AT 



t 



Temperature Effects on Surface Energy 



As the temperature of a liquid rises, the kinetic agitations of the mole- 

 cules increase. It has been found experimentally that the surface 

 energy decreases with rising temperature. Table V-l shows that the 

 reduction in surface-energy measurements with rise in temperature, 



TABLE V-l 

 Temperature Effects on Surface Energy 



A liquid-air interface. Changes in T indicated in ergs per square centimeter. 

 Standard T for water-air interface at 20° C is 72.75 ± 0.05 erg/cm 2 . Air pressure, 

 standard conditions. 



* — 



t°C 10 15 20 25 30 35 40 50 100 



Water T y4 22 ?3 4Q ?2 ?5 ?1 g7 ?1 lg 7Q 3g 6g 56 6? gl 5g g5 



(°C 20 50 100 



Benzene T 1? Q1 13A7 ? ^ 7 



over long ranges of temperature, is practically linear. The tempera- 

 ture effect is caused by a decrease in the inward pull on the surface mole- 

 cules which results in a decrease in surface energy. 



Liquid-Gas Interface 



Under ideal conditions surface-energy measurements are made on an 

 uncontaminated liquid surface subjected to a gas pressure of 1 

 atmosphere at 20° C. The surface is then referred to as a liquid-gas 

 interface. The accepted reference standard of interface-energy measure- 

 ments is water in the presence of air at 20° C and 760 mm pressure. Its 

 magnitude is 72.75 ± 0.05 erg/cm 2 . Benzene, which is sometimes used 

 as a reference standard, has under the same conditions an interface 

 surface energy equal to 28.88 ± 0.03 erg/cm 2 . Water has the highest 

 interface surface energy of the liquids except mercury, which may pos- 



