Stearic structure, melting point

Melting Points of Lipids The melting points of a series of 18-carbon fatty acids are stearic acid, 69.6 °C oleic acid, 13.4 °C linoleic acid, - 5 °C and linolenic acid, - 11 °C. (a) What structural aspect of these 18-carbon fatty acids... 

Table 10.2 lists the structure and melting point of four fatty acids containing 18 carbon atoms. Stearic acid is one of the two most common saturated fatty acids, and oleic and linoleic acids are the most common unsaturated ones. The data show the effect of Z double bonds on the melting point of fatty acids. 

Saturated FA, such as palmitic and stearic acids, are stable toward oxidation and polymerization. Because they have a high melting point, they add structure to certain products. However, due to this characteristic the appearance of a fried food may be adversely affected (e.g., waxy mouth-feel, dry surface of stored fried food). Monoenoic FA, primarily oleic acid, are considered to be beneficial from a health standpoint. Frying oils rich in such FA do not add to the structure they are stable against oxidation and provide a light taste. Polyenoic FA (PEFA) deteriorate more rapidly that monoenoic FA and the shelf life of products fried in oils rich in these acids is shorter. Oxidation products formed from PEFA vary widely, depending on the structure of the FA and the relative concentration of linoleic and linolenic acids. The percentage of linolenic acid in heated oils should be very low. 

The interaction between zinc oxide and stearic acid in a medium suitable to simulate a vulcanized system has been investigated [65] experimentally using vibrational spectroscopic technique. Confocal Raman micro spectroscopy revealed that at ambient temperature both components are phase-separated in the form of microcrystals. When the reaction temperature (SO C and above) is reached only zinc oxide is present in the form of particles while the stearic acid melts and gets molecularly dispersed within the rahher matrix. The analysis points to a core-shell structure of the reacting system stearic acid diffuses to the surface of zinc oxide domains causing the shrinkage of the zinc oxide core and the formation of a shell of increasing thickness made of zinc stearate. 

The low melting point of stearic acid makes it ideal as a dispersive agent for the production of soaps, shampoos, detergents and shaving creams. They are added to these products melted and allowed to cool down at room temperature with a gentle stirring to recrystallize and form a semisolid and stable structure. Further, due to its hydrophobic properties and the ability to form hard compacts has been used for controlled release of some drugs. 

Table 15.1 shows that the melting points of unsaturated fatty acids are appreciably lower than those of saturated acids. Compare, for example, the melting points of stearic and oleic acids, which differ structurally by only one double bond. The same difference applies to triglycerides The more double bonds in the fatty acid portion of the triester, the lower its melting point. 

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