Fatty acids melting temperatures

Fatty acids (the carboxylic acids produced by saponification of fats) usually have an even number of carbons and may also have one or more cis double bonds. The fatty acid that has 18 carbons with a cis double bond at the 9 position, (Z)-9-octadecenoic acid, is called oleic acid. Catalytic hydrogenation of oleic acid produces stearic acid, or octadecanoic acid, another fatty acid. One of these fatty acids melts at 13.4°C and the other melts at 69.6°C. Explain which fatty acid melts at the higher temperature. 

Fig. 9. Phase equilibria for the bile salt (bile acid)-fatty acid-water system at constant water concentration in relation to temperature (see Fig. 5). Six mixtures varying in molar ratios of bile salt (bile acid) and palmitic acid with total concentration of micellar bile acid plus palmitic acid equal to 40 mM were examined. Fatty acid has a finite solubility in the micellar bile acid solution, the excess being crystalline at body temperature. At 50-60 C, there is a marked increase in micellar solubility, and the fatty acid melts. At higher fatty acid/bile acid ratios, the micellar solubility is exceeded, and an immiscible oil phase occurs. The melting point of fatty acid in the presence of water is nearly identical to that in the anhydrous state (38), in contrast to the behavior of monoglyceride (Table I). As shown in Fig. 3, the size of the micellar area decreases with increasing chain length. Unsaturated fatty acids (not shown) behave similarly to saturated fatty acids, but their micellar solubility is greater, and at most experimental temperatures a crystalline phase will not occur.

Mixed Soap Crystals in Super-Fatted Formulations A small quantity of fatty acid, usually less than 10%, is added to normal soap to produce a super fatted soap. The fatty acid is added to the soap phase at temperatures above the fatty acid melting point and new solid crystalline and liquid crystalline phases are formed on cooling. 

In the area of moleculady designed hot-melt adhesives, the most widely used resins are the polyamides (qv), formed upon reaction of a diamine and a dimer acid. Dimer acids (qv) are obtained from the Diels-Alder reaction of unsaturated fatty acids. Linoleic acid is an example. Judicious selection of diamine and diacid leads to a wide range of adhesive properties. Typical shear characteristics are in the range of thousands of kilopascals and are dependent upon temperature. Although hot-melt adhesives normally become quite brittle below the glass-transition temperature, these materials can often attain physical properties that approach those of a stmctural adhesive. These properties severely degrade as the material becomes Hquid above the melt temperature. 

Recently, the use of Hpase enzymes to iateresterify oils has been described (23). In principle, if a 1,3-speciftc Hpase is used, the fatty acid ia the 2 position should remain unchanged and the randomization occur at the terminal positions. However, higher temperatures, needed to melt soHd fats, may cause a 1,2-acyl shift and fatty acids are scrambled over all positions. 

Sohd fats may show drastically different melting behavior. Animal fats such as tallow have fatty acids distributed almost randomly over all positions on the glycerol chain. These fats melt over a fairly broad temperature range. Conversely, cocoa has unsaturated fatty acids predominantly in the 2 position and saturated acids in the 1 and 3 positions. Cocoa butter is a brittle sohd at ambient temperature but melts rapidly just below body temperature. 

Ethoxylation of alkyl amine ethoxylates is an economical route to obtain the variety of properties required by numerous and sometimes smaH-volume industrial uses of cationic surfactants. Commercial amine ethoxylates shown in Tables 27 and 28 are derived from linear alkyl amines, ahphatic /-alkyl amines, and rosin (dehydroabietyl) amines. Despite the variety of chemical stmctures, the amine ethoxylates tend to have similar properties. In general, they are yellow or amber Hquids or yellowish low melting soHds. Specific gravity at room temperature ranges from 0.9 to 1.15, and they are soluble in acidic media. Higher ethoxylation promotes solubiUty in neutral and alkaline media. The lower ethoxylates form insoluble salts with fatty acids and other anionic surfactants. Salts of higher ethoxylates are soluble, however. Oil solubiUty decreases with increasing ethylene oxide content but many ethoxylates with a fairly even hydrophilic—hydrophobic balance show appreciable oil solubiUty and are used as solutes in the oil phase. 

It is unlikely in real tribological events that adsorbed mono-layers work solely to provide lubrication. Instead, adsorption and chemical reactions may occur simultaneously in most cases of boundary lubrication. For example, fatty acid is usually regarded as a friction modiher due to good adsorp-tivity, meanwhile its molecules can react with metal or a metal oxide surface to form metallic soap which provides protection to the surface at the temperature that is higher than its own melting point. 

The melting points of even-numbered-carbon fatty acids increase with chain length and decrease according to unsaturation. A triacylglycerol containing three sam-rated fatty acids of 12 carbons or more is solid at body temperature, whereas if the fatty acid residues are 18 2, it is liquid to below 0 °C. In practice, natural acylglyc-erols contain a mixture of fatty acids tailored to suit their functional roles. The membrane lipids, which must be fluid at all environmental temperatures, are... 

Organic materials, as Figure 103 shows, cover a smaller temperature range from about 0 °C to 150 °C. They include mainly paraffins, fatty acids and sugar alcohols. In most cases, their density is smaller than 1 g/cm3. Thus paraffins and fatty acids usually have smaller melting enthalpies per volume than inorganic materials. They tend to be more expensive, but usually do... 

Although the true waxes, such as beeswax, are esters of fatty acids with alcohols, the term is often applied to certain solid substances which melt at fairly low temperatures, e.g., paraffin wax. Paraffin waxes (crystalline and microcrystalline) are obtained from the refining of petroleum, and are used in rubber compounding as protective agents. They have the property of blooming to the surface of the product, forming a thin film which replenishes itself if removed in service. 

Fatty acids have also been converted to difunctional monomers for polyanhydride synthesis by dimerizing the unsaturated erucic or oleic acid to form branched monomers. These monomers are collectively referred to as fatty acid dimers and the polymers are referred to as poly(fatty acid dimer) (PFAD). PFAD (erucic acid dimer) was synthesized by Domb and Maniar (1993) via melt polycondensation and was a liquid at room temperature. Desiring to increase the hydrophobicity of aliphatic polyanhydrides such as PSA without adding aromaticity to the monomers (and thereby increasing the melting point), Teomim and Domb (1999) and Krasko et al. (2002) have synthesized fatty acid terminated PSA. Octanoic, lauric, myristic, stearic, ricinoleic, oleic, linoleic, and lithocholic acid acetate anhydrides were added to the melt polycondensation reactions to obtain the desired terminations. As desired, a dramatic reduction in the erosion rate was obtained (Krasko et al., 2002 Teomim and Domb, 1999). 

EVALUATION OF MELTING AND CRYSTALLINE RELAXATION TEMPERATURES OF FATTY ACID MONOLAYERS ON THE WATER SURFACE... 

Table 1. Melting temperature, Tm, and crystalline relaxation temperature, Tctc> of fatty acid monolayers on the water surface. 

Therefore, the following melting model of a fatty acid monolayer could be suggested during increase of the temperature Solid domains — Soft domains — Two co-existed phases between domains and molten monolayer — completely molten monolayer, as shown in Figure 2. 

Fats are solid triacylglycerols, oils are liquid triacylglycerols it is the fatty acid composition of the triacylglycerol that determines its physical properties. Triacylglycerols with a high proportion of unsaturated fatty acids have lower melting points than those rich in sam-rated fatty acids, so the former are oils - that is, they are liquid at room temperature. The unsaturated fatty acids are identified by the number of carbon atoms and the position of the double bonds (see below). 

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