Fatty acids interactions with water

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. 

The intracellular and plasma membranes have a complex structure. The main components of a membrane are lipids (or phospholipids) and different proteins. Lipids are fatlike substances representing the esters of one di- or trivalent alcohol and two aliphatic fatty acid molecules (with 14 to 24 carbon atoms). In phospholipids, phosphoric acid residues, -0-P0(0 )-O-, are located close to the ester links, -C0-0-. The lipid or phospholipid molecules have the form of a compact polar head (the ester and phosphate groups) and two parallel, long nonpolar tails (the hydrocarbon chains of the fatty acids). The polar head is hydrophihc and readily interacts with water the hydrocarbon tails to the... 

Interaction with water, and dispersion into large aggregates. In samples containing appreciable sodium oleate, a turbid dispersion was present. Microscopically, this phase contained isotropic oil droplets, and the phase should represent emulsified fatty acid and monoglyceride droplets stabilized by acid soap. Unfortunately, x-ray diffraction facilities were not available to characterize this phase properly. 

Fig. 4.10 Emulsified particle in an aqueous medium consisting of a fatty acid in a water insoluble medium, oleic acid, and showing the interaction with water at the surface of the particle...

The cleaning action of soap is due to the dual nature of the conjugate base of the fatty acid molecule. On one end is the ionic carboxylate anion group the rest of the molecule consists of a nonpolar hydrocarbon chain. The tonic part, called the head is attracted to a polar solvent such as water and is hydrophilic, whereas the long hydrocarbon tail is hydrophobic. In water, soap molecules tend to group together in clusters called micelles, with their ionic heads oriented toward the water molecules and their hydrocarbon tails in the interior of the cluster so that unfavorable interactions with water are avoided. Non-... 

Cis double bonds produce a kink, or a bend, of about 30 degrees for each double bond into the backbone, and these can flip over to the trans form under high temperatures. Trans double bonds allow the molecule to lie in a straight line however, the human body cannot convert the trans form into nutrients and so prevents the metabolic activities from converting it to the active cis forms. This can lead to a deficiency in essential fatty acids. The more double bonds, and therefore more kinks, the more beneficial it is to human health. By completely changing the physical and chemical properties, the kinks allow essential protein associations to form more easily, thus permitting more saturated fatty acids to disperse and interact with water or blood. 

Carbon dioxide is continually produced by cellular aerobic metabolism of glucose and fatty acids. Carbon dioxide diffuses down its concentration gradient from the cell to the blood, which carries it to the lungs. It can interact with water to form carbonic acid (H2C03), a process catalyzed by carbonic anhydrase, an enzyme present in erythrocytes. Carbonic acid can then dissociate to liberate bicarbonate ion (HCOJ) and hydrogen ion (H+) as follows ... 

Insoluble nonswelling amphiphiles (H20). Lipids in this group have sufficient polarity to orient themselves at an air-water or air-oil interface, but do not interact with water sufficiently to dissolve in aqueous solutions. Examples of interest are cholesterol, triglyceride, the fat-soluble vitamins, and un-ionized fatty acids. 

Fig. 32.8. Example of the structure of a blood hpoprotein. VLDL is depicted. Lipoproteins contain phospholipids and proteins on the surface, with their hydrophilic regions interacting with water. Hydrophobic molecules are in the interior of the lipoprotein. The hydroxyl group of cholesterol is near the surface. In cholesterol esters, the hydroxyl group is esterified to a fatty acid. Cholesterol esters are found in the interior of lipoproteins and are synthesized by reaction of cholesterol with an activated fatty acid (see Chapter 33).

The transition temperature, i.e., the melting point, of fatty acids when suspended in water is very close to their anhydrogenous melting point (38). Fatty acids do not interact with water and thus behave as di- and triglycerides, i.e., as insoluble amphiphiles irrespective of experimental temperature. 

When placed in aqueous solution, phospholipids spontaneously form a lipid bilayer (Figure 26.13) in which polar head groups lie on the surface, giving the bilayer an ionic coating. Nonpolar hydrocarbon chains of fatty acids lie buried within the bilayer. This self-assembly of phospholipids into a bilayer is a spontaneous process driven by two types of noncovalent forces (1) hydrophobic effects, which result when nonpolar hydrocarbon chains cluster together and exclude water molecules and (2) electrostatic interactions, which result when polar head groups interact with water and other polar molecules in the aqueous environment. 

FIGURE 2.4 Organization of a lipid monolayer at the air-water interface. All fatty acid molecules are oriented with respect to water so that polar head groups interact with water and apolar chains are rejected in the air. The limit between air and water in such systems is referred to as an air-water interface. 

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... 

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. 

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