PHYSICAL PROPERTIES OF FATTY ACIDS 107 



which has the smallest volume. The drops therefore assume a spherical 

 shape, which has the minimum surface area. Surface tension is thus re- 

 sponsible for the formation of raindrops, the production of round lead 

 shot ^^■hen molten lead is dropped through air, for the rounding of a glass 

 tube on fusion, and for many similar phenomena. 



The surface tension of liquids can be easily measured by such standard 

 procedures as the capillary tube method, the determination of drop weight, 

 and the method of maximum pull on a ring. The latter procedure can be 

 carried out quite simply by the use of the du Nouy tensiometer, which 

 permits the determination of surface tension at a liquid-air interface or of 

 interfacial tension at a liquid-liquid junction. The surface tension, in- 

 terfacial tension, and parachors of some acids and esters are reported in 

 Table 38. 



Surface tension decreases with increasing temperature, reaching a value 

 of zero at the critical temperature. Through a large part of the tempera- 

 ture range, it has been found that the relation between surface tension 

 and temperature is a linear one. The experimentally determined values of 

 surface tension, and especially of interfacial tension, are markedly influenced 

 by the presence of impurities. Data on the effect of fatty acids and soaps 

 on surface tension have been assembled by McBain.*^^ The reader is re- 

 ferred to Sugden^^^ for a more detailed discussion of surface tension, es- 

 pecially with regard to the use of the parachor. 



d. Viscosity. Viscosity is the resistance which a liquid offers to change 

 in shape. While a liquid offers no permanent resistance to forces tending 

 to change its shape, the rates at which the change is accomplished vary 

 and are, in fact, measures of its internal friction or viscosity. 



Some liquids, like glycerol or oleic acid, have a higher viscosity than 

 water. They flow down an inclined surface or through a tube at a much 

 slower rate than is the case with water. Such a movement consists in a 

 continuous change in shape for each part of the liquid. 



The unit for expressing viscosity is the poise. When one layer of a 

 fluid exerts a tangential force upon another layer equal to one dyne, and 

 causes a tangential velocity of one centimeter per second, the viscosity is one 

 poise. In general, viscosity is determined by comparison with water. 

 The absolute viscosity of w^ater at 20.20°C. is 0.01 poise or 1.000 centi- 

 poise.* The latter unit is more frequently used in expressing the viscosities 

 of fatty acids. 



The viscosities of the fatty acids have been determined by Dunstan^^^ 



and Deffet.^^" Some of these values are recorded in Table 39. 



^" J. W. McBain, "Properties of Soaps and Their Aqueous Solutions," in International 

 Critical Tables, Vol. V, McGraw-Hill, New York, 1929, pp. 446-460. 

 '^ S. Sugden, The Parachor and Valency, Routledge, London, 1930. 

 '«» A. E. Dunstan, /. Chem. Soc, 107, 667-672 (1915). 

 i"" L. Deffet, Bull. soc. chim. Belg., 40, 385-402 (1931). 



