Fatty acids hydrophilization

Apart from fatty acids, straight-chain molecules containing other hydrophilic end groups have been employed in numerous studies. In order to stabilize LB films chemical entities such as tlie alcohol group and tlie metliyl ester group have been introduced, botli of which are less hydrophilic tlian carboxylic acids and are largely unaffected by tlie pH of tlie subphase. 

A typical biomembrane consists largely of amphiphilic lipids with small hydrophilic head groups and long hydrophobic fatty acid tails. These amphiphiles are insoluble in water (<10 ° mol L ) and capable of self-organization into uitrathin bilaycr lipid membranes (BLMs). Until 1977 only natural lipids, in particular phospholipids like lecithins, were believed to form spherical and related vesicular membrane structures. Intricate interactions of the head groups were supposed to be necessary for the self-organization of several ten thousands of... 

Methyl group (Section 2 7) The group —CH3 Mevalonic acid (Section 26 10) An intermediate in the biosyn thesis of steroids from acetyl coenzyme A Micelle (Section 19 5) A sphencal aggregate of species such as carboxylate salts of fatty acids that contain a lipophilic end and a hydrophilic end Micelles containing 50-100 car boxylate salts of fatty acids are soaps Michael addition (Sections 18 13 and 21 9) The conjugate ad dition of a carbanion (usually an enolate) to an a 3 unsatu rated carbonyl compound... 

Emulsifiers. Removing the remover is just as important as removing the finish. For water rinse removers, a detergent that is compatible with the remover formula must be selected. Many organic solvents used in removers are not water soluble, so emulsifiers are often added (see Emulsions). Anionic types such as alkyl aryl sulfonates or tolyl fatty acid salts are used. In other appHcations, nonionic surfactants are preferred and hydrophilic—lipophilic balance is an important consideration. 

Ethoxylated Anhydrosorbitol Esters. Ethoxylation of sorbitan fatty acid esters leads to a series of more hydrophilic surfactants (Table 19). All hydroxyl groups of sorbitan can react with ethylene oxide. The stmcture of the principal component of a nominal polyoxyethylene (20) sorbitan monostearate illustrates the composition of these products, where w x y z = 20. 

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. 

A series of sorbitol-based nonionic surfactants are used ia foods as water-ia-oil emulsifiers and defoamers. They are produced by reaction of fatty acids with sorbitol. During reaction, cycHc dehydration as well as esterification (primary hydroxyl group) occurs so that the hydrophilic portion is not only sorbitol but also its mono- and dianhydride. The product known as sorbitan monostearate [1338-41 -6] for example, is a mixture of partial stearic and palmitic acid esters (sorbitan monopalmitate [26266-57-9]) of sorbitol, 1,5-anhydro-D-glucitol [154-58-8] 1,4-sorbitan [27299-12-3] and isosorbide [652-67-5]. Sorbitan esters, such as the foregoing and also sorbitan monolaurate [1338-39-2] and sorbitan monooleate [1338-43-8], can be further modified by reaction with ethylene oxide to produce ethoxylated sorbitan esters, also nonionic detergents FDA approved for food use. 

The hydrophilic nature of the carboxyl group balanced against the hydrophobic nature of the hydrocarbon chain allows long-chain fatty acids to form monomolecular films at aqueous Hquid-gas, Hquid—Hquid, or Hquid—soHd interfaces (18). 

Micelle (Section 19.5) A spherical aggregate of species such as carboxylate salts of fatty acids that contain a lipophilic end and a hydrophilic end. Micelles containing 50-100 carboxylate salts of fatty acids are soaps. 

Based on ether carboxylic acids it is possible in principle to make the same derivatives as with fatty acids, such as esters, amides, and acid chlorides. The hydrophilic chain in the molecule may confer special properties in comparison with the fatty acid derivatives. 

Heimann and Vogtle [38] synthesized triesters of glycerol with different ether carboxylic acids with a short alkyl chain. They have found that these hydrophilic lipids, in contrast with the fatty acid glycerol triesters, give complex-ation with alkali and alkali earth metal cations in an analogy of crown ethers. 

Sorbitan esters of fatty acids are well known. Similar products can be made from ether carboxylic acids and sorbitol without an acid catalyst with a good color [39]. The advantage of these products is that the hydrophilicity can be adjusted by the polyethylene glycol content in the ether carboxylic acid. 

Sodium a-sulfonated fatty acid esters of long-chain alcohols have a structural effect on the Krafft point different from that of amphiphiles with short alkyl chains [60]. In a series of homologs with the same total carbon number the Krafft points are highest when the hydrophilic alkyl chain lengths in the a-sulfonated fatty acid and the alcohol are fairly long and equal. In this case the packing of the molecules becomes close and tight. 

The alkaline product from the wood ash was a crude solution of sodium and potassium carbonates called "lye". On boiling the vegetable oil with the lye, the soap (sodium and potassium salts of long chained fatty acids) separated from the lye due to the dispersive interactions between the of the fatty acid alkane chains and were thus, called "lyophobic". It follows that "lyophobic", from a physical chemical point of view, would be the same as "hydrophobic", and interactions between hydrophobic and lyophobic materials are dominantly dispersive. The other product of the soap making industry was glycerol which remained in the lye and was consequently, termed "lyophilic". Thus, glycerol mixes with water because of its many hydroxyl groups and is very polar and hence a "hydrophilic" or "lyophilic" substance. 

Applications of peroxide formation are underrepresented in chiral synthetic chemistry, most likely owing to the limited stability of such intermediates. Lipoxygenases, as prototype biocatalysts for such reactions, display rather limited substrate specificity. However, interesting functionalizations at allylic positions of unsaturated fatty acids can be realized in high regio- and stereoselectivity, when the enzymatic oxidation is coupled to a chemical or enzymatic reduction process. While early work focused on derivatives of arachidonic acid chemical modifications to the carboxylate moiety are possible, provided that a sufficiently hydrophilic functionality remained. By means of this strategy, chiral diendiols are accessible after hydroperoxide reduction (Scheme 9.12) [103,104]. 

Salts of fatty acids are classic objects of LB technique. Being placed at the air/water interface, these molecules arrange themselves in such a way that its hydrophilic part (COOH) penetrates water due to its electrostatic interactions with water molecnles, which can be considered electric dipoles. The hydrophobic part (aliphatic chain) orients itself to air, because it cannot penetrate water for entropy reasons. Therefore, if a few molecnles of snch type were placed at the water surface, they would form a two-dimensional system at the air/water interface. A compression isotherm of the stearic acid monolayer is presented in Figure 1. This curve shows the dependence of surface pressure upon area per molecnle, obtained at constant temperature. Usually, this dependence is called a rr-A isotherm. 

Figure 41-3. Diagrammatic representation of a phospholipid or other membrane lipid. The polar head group is hydrophilic, and the hydrocarbon tails are hydrophobic or lipophilic. The fatty acids in the tails are saturated (S) or unsaturated (U) the former are usually attached to carbon 1 of glycerol and the latter to carbon 2. Note the kink in the tail of the unsaturated fatty acid (U), which is important in conferring increased membrane fluidity.

RH above can represent a very wide variety of xenobi-otics, including drugs, carcinogens, pesticides, petroleum products, and pollutants (such as a mixture of PCBs). In addition, endogenous compounds, such as certain steroids, eicosanoids, fatty acids, and retinoids, are also substrates. The substrates are generally lipophilic and are rendered more hydrophilic by hydroxy-lation. 

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