174 ATOMS, IONS, SALTS, AND SURFACES 



THE DETERMINATION OF SURFACE AND INTERFACIAL TENSION 



The use of what is often called the "drop-number method" of determining the 

 surface or interfacial tension is not only a waste of time but fills the literature of bi- 

 ology with worse than useless data, since they are so extremely deceptive. 



Drop-weight method. — The capillary-height and the drop-weight methods are the 

 most accurate now known. The drop- weight method is in general much the more 

 suitable of the two for biological investigations. It depends upon the fact that if a 

 drop of liquid is allowed to fall from a horizontal circular disk (end of a glass capillary 

 tube) with sufficient slowness, the weight of the drop is a definite function of the surface 

 tension, the radius of the tip, and the volume of the drop which falls, so 



W = Mg = 2 -nryf , 

 where/ stands for the value of the function. Thus 



2-Kr] 



or if 



V— 



2 7r/' 



F=-' 



7 = M^F. 

 r 



To calculate the surface tension: (i) The volume (F) of the drop is its mass in 

 grams divided by its density ( ^ = ^ ) • (2) Multiply F by - , in which r is the radius 



V . 



of the circular tip. (3) Find — in Table VIII, and note the corresponding value of F. 



(4) Divide g (in dynes per sec.^ = 980.3 at Chicago) by r^ {r in centimeters), and multiply 

 by M in grams, and by F. The result gives the surface tension in dynes per centimeter. 



In the determination of interfacial tension the volume of the drop is measured di- 

 rectly. The weight of the drop as it hangs in a second liquid is V {d^—d^, in which d^ 

 is the density of the heavier and d^ that of the lighter liquid. The liquid which is 

 dropped should always be the aqueous phase, so if the oil is the heavier the tip should 

 face upward. 



Ring method for determination 0] surface tension. — The pull on a ring just being de- 

 tached from the surface of a liquid does not give the surface tension, but the pull must 

 be multiplied by a factor F, as given by Young and Cheng and the writer in a recent 

 paper in Science. The outline of the method is as follows: 



1. Determine the total pull in grams (M) necessary just to detach a circular ring 

 of diameter R, of circular wire of diameter r, from the surface of the liquid by the use 

 of a chainomatic balance or a torsion balance, such as that of Du Nouy. 



2. The surface tension is given by the equation 



3. Obtain the value of F, the correction factor, from Figure 23. F is plotted on the 



Ri 

 F-axis. To do this find the value of vf > the cube of the radius of the ring divided by 



