l'HV81CAL PUOPEKTIES OF FATTY AC1D8 105 



n^ - \ M 



n- + 1 d 



in which R^ is the molecular refract ivity, ti is the refractive index, M is the 

 molecular weight, and d is the density. 



The relationship of chain length to molecular refractivity is illustrated 

 by the fact that the values obtained by application of the Lorentz-Lorenz 

 formula can be almost exactly duplicated by using the equation: 



R,„ = 4.(354n + 3.83 



where n represents the number of carbon atoms. These data are recorded 

 in Table 37. 



Table 37 



V'alues for Molecular Refractivity (Rm) at 80 °C. Calculated from Refractive 



Index or Theoretical Formula Based on Number of Carbon Atoms" 



No. of R^ calcd. from R^ calcd. from 



Name of acid C Atoms L.-L. formula 4.654n + 3.83 & 



" Adapted from \. W. RaLston, Fatty Acids and Their Derivatives, Wilev, New York, 

 1948, p. 390. 

 *■ n refers to number of carbon atom.s. 



Although the calculated and the measured values of the molecular re- 

 fractivity of the saturated acids are practically identical, this agreement 

 does not obtain in the case of some of the polyethenoid acids, especially 

 the elaeostearic acids. This divergence, although also present in the re- 

 fractive index and density, is magnified in the molecular refractivity. It 

 is referred to as molecular exaltation, or EM, and is believed to be largely 

 attributable to the effect of conjugation. ^••^^^•^^^ 



c. Surface Tension and Interfacial Tension. The surface tension is 

 another property of the fatty acids which is related to the density and 

 molecular weights by the equation: 



P = M y'/*/(D-d) 



'«« W. C. Smit, Rec. trav. chim., 49, 539-551 (1930). 



