Fatty surface layers, structure

Glycerophospholipids are used for membrane synthesis and for producing a hydrophilic surface layer on lipoproteins such as VLDL. In cell membranes, they also serve as a reservoir of second messengers such as diacylglycerol, inositol 1,4,5-triphosphate, and arachidonic acid. Their structure is similar to triglycerides, except that the last fatty acid is replaced by phosphate and a water-soluble group such as choline (phosphatidylcholine, lecithin) or inositol (phosphatidyl-inositol). 

Structure of Surface Layers The adsorption of fatty adds onto polar surfaces has been widely studied, and it usually results in a layer with the carboxyl group at the surface and the hydrocarbon chains oriented vertically to the surface. If the surface is microscopically smooth enough, and the chains sufficiently long, then the layer can be considered as semicrystalline. When packed in this way, the area occupied at the surface by one molecule of a saturated, linear, fatty add is about 0.21 nm. Fatty acids with branched chains, and those containing unsaturation, do not allow such close packing and hence occupy larger areas. Similar layers are believed to form on... 

The structure of the surface layers of fatty acid-treated fillers can be quite complex. In the ideal case, one would expect a single layer with all of the acid groups in the carboxylate form and interacting with a basic surface site (i.e., the coating to be in the form of a partial or half-salt, such as M"(0H)C00R). In reality, a variety of other species can be present, including unreacted acid and fatty acid salt not attached to a surface site. In addition, there may be a separate fatty acid/salt phase in the bulk polymer, not associated with the filler surface. The relative importance of these will vary with the application. When multilayers are present, they will be only weakly attached to the surface monolayer. 

Plant lipids are of two main types structural and storage. The structiu lipids are present as constituents of various membranes and protective surface layers and make up about 7 per cent of the leaves of higher plants. The surface lipids are mainly waxes, with relatively minor contributions from long-chain hydrocarbons, fatty acids and cutin. The membrane lipids, present in mitochondria, the endoplasmic reticulmn and the plasma membranes, are mainly glycolipids (40-50 per cent) and phosphoglyc-erides. Plant storage lipids occur in fruits and seeds and are, predominantly, triacyl-glycerols. Over 300 different fatty acids have been isolated from plant tissues, but only about seven are of common occurrence. The most abundant is a-linolenic acid the most common saturated acid is palmitic acid and the most common monounsaturated acid is oleic acid. 

The polymer-surfactant complex has high surfeice viscosity and elasticity (i.e. surfeice viscoelasticity), both will enhance the foam stability (see below). The amphoteric surfactants such as betaines and the phospholipid surfeictants when used in conjunction with alkyl sulfeites or alkyl ether sulfeites can also enhance the foam stability. All these molecules strengthen the film of surfactant at the air/water interface, thus modifying the lather from a loose lacy structure to a rich, dense, small bubble size, luxurious foam. Several foam boosters have been suggested and these include fatty acid alkanolamide, amine oxides. Fatty alcohol and fatty acids can also act as foam boosters when used at levels of 0.25-0.5 %. Several approaches have been considered to explain foam stability (a) Surface viscosity and elasticity theory The adsorbed surfeictant film is assumed to control the mechanical-dynamical properties of the surface layers by virtue of its surface viscosity and elasticity. This may be true for thick films (> 100 nm) whereby intermolecular forces are less dominant. Some correlations... 

Stems and leaves of plants are covered with a layer of fatty material. The structural component is an hydroxy fatty acid polymer-cutin. Underground parts and wound surfaces are covered in another type of lipid-derived polymeric material - suberin. Both these polymers are associated with or embedded in a complex mixture of lipids imprecisely called waxes. (Strictly speaking a wax is an ester between a long chain fatty acid and a fatty alcohol section 4.7.)... 

Amphipathic lipids spontaneously form a variety of structures when added to aqueous solution. All these structures form in ways that minimize contact between the hydrophobic lipid chains and the aqueous milieu. For example, when small amounts of a fatty acid are added to an aqueous solution, a mono-layer is formed at the air-water interface, with the polar head groups in contact with the water surface and the hydrophobic tails in contact with the air (Figure 9.2). Few lipid molecules are found as monomers in solution. 

It has been shown (Friberg, 2003 Birdi, 2002, 2008) that there exists a correlation between foam stability and the elasticity [E] of the film (i.e., the monolayer). In order for E to be large, surface excess must be large. Maximum foam stability has been reported in systems with fatty acid and alcohol concentrations well below the minimum in y. Similar conclusions have been observed with -C12H25S04Na [SDS] + -C12H25OH systems that give minimum in y versus concentration with maximum foam at the minimum point (Chattoraj and Birdi, 1984). Because of mixed mono-layer formation it has been found that SDS + C12H25OH (and some other additives) make liquid-crystalline structures at the surface. This leads to a stable foam (and liq-... 

We have noted that the adhesion of the polar groups to water and to one another is much greater than the weak adhesion of hydrocarbon chains either to water or to one another. It is thus reasonable to assume, an anticipation verified by Perrin Ann. Rhys. X. 160, 1918), that soap films may be made up of composite surfaces each of which consists of two layers of orientated molecules of soap the outer surface of each side consisting of hydrocarbon chains and the polar groups held together with water as a sandwich between the orientated hydrocarbon chains. These elementary leaflets which will possess but little adhesion for one another may be built up to form thick films similar in structure to the crystalline fatty acids examined by Shearer (see p. 73). The leaflets may slip over one another with great ease, thus providing the play of interference colours noticed in soap films. The elementary leaflet has in fact been shown by Perrin and others to be two molecules in thickness. 

---