PHYSICAL PROPERTIES OF FATTY ACIDS 57 



rangement of the several double bonds in a conjugate position, or the change 

 of a cis to a trans linkage, tends to bring about a marked elevation in the 

 temperature at which such fatty acids melt. No less important than the 

 several factors already mentioned is the presence of a methyl group as a 

 side chain. The extent to which the resultant melting point is depressed is 

 related to the position of substitution; it is greater when two methyl 

 groups are introduced than when a single one is present. 



The melting points of the triglycerides closely approximate those of the 

 fatty acids (see page 251). Since the digestibility of a fat is decreased 

 markedly when the melting points exceed 50°C., it is evident that some 

 types of isomerism may be related to an alteration of the nutritional value 

 of the fat. 



{3) Solubility of Fatty Acids 



In order for biochemical changes to occur, it is usually necessary for the 

 reacting substances to be in solution in the same medium. Most biological 

 reactions occur in aqueous media, since carbohydrates, proteins, amino 

 acids, their decomposition products, and most inorganic constituents are 

 soluble in water. Although the sodium and potassium soaps of the fatty 

 acids are on the whole quite soluble in water, the fatty acids, as well as 

 their monatomic and polyhydric esters, are quite immiscible with this sol- 

 ^'ent. Nevertheless, the short-chain fatty acids are very soluble in water. 

 Even the higher fatty acids dissolve to some extent in water, and this 

 solubility may be sufficient to enable biochemical changes to take place. 

 Precise data on the solubility of vaony of the saturated fatty acids in water 

 and in a large number of other solvents have been obtained through the re- 

 cent comprehensive studies of Ralston, Hoerr, and their collaborators. 

 Although space is not available to include the results on all of the solvents 

 employed by these workers, it would seem to be of sufficient interest and 

 importance to list the solubilities of the fatty acids in several of the specific 

 lipid solvents. 



a. Solubility of Saturated Fatty Acids in Water and of Water in 

 Fatty Acids. The solubility of the saturated fatty acids from caproic 

 (Ce) to stearic (Cis) has been reported by Ralston and Hoerr^"^^ for tempera- 

 tures from 0° to 60°C. These results are included in Table 20. No data 

 are given for acetic, propionic, or butyric acids, but these are known to be 

 miscible with water in all proportions. Valeric acid is soluble to the extent 

 of 3.7 g. per 100 grams of water. However, with the exception of butyric 

 acid, which occurs as a constituent of cow milk fat, none of these other 

 short-chain acids is important, inasmuch as the acids are not present in 

 naturally occurring fats. 



220 A. W. Ralston and C. W. Hoerr, /. Or,j. Chem., 7, 546-555 (1942). 



