PHYSICAL PROPERTIES OF FATTY ACIDS 



93 



parallel, while in the odd-numbered carbon chains the end groups are at 

 an angle to each other. This requii-es a difference in the inclination of the 

 chains in the layers A\hich, in turn, will influence many of the physical 

 properties. In all cases of long-chain aliphatic compounds, irrespective 

 of whether they contain an odd oi' an even number of carbons, the addition 

 of another carbon atom is always accompanied by an increase in the c axis 

 bv 4.6 A. 



n = l 3 5 7 9 10 12 14 16 18 20 22 



Number of 

 Carbon Atoms 



16 

 17 

 18 

 19 

 20 

 21 



23 



n = ] 3 5 7 9 1] 14 16 18' ■ ■ ' 23 ' 25 ■ ■ 28 ' 32 



Fig. 20. The effect of chain length on the diffraction pattern of n-aUphatic acids.^ 



Fig. 21. The x-ray pattern.s of the Co-: acid and correspondmg derivatives.'' 



As has been indicated b}^ Figure 18, the four molecules of stearic acid in a 

 unit cell are so arranged that t\vo molecules are packed side by side in the 

 upper and lower half of the cell, respectively. The second one in the lower 

 half in Figure 18 lies directly behind it and in the same plane. The 

 "packing distance" between these individual chains has been shown to be 

 between 3.7 and 4.0 A., compared to a distance of 1.54 A. betAveen the ad- 



