Fatty acid structure, surface area

A as a Function of Fatty Acid Structure. Unsaturated fatty acids have larger surface areas than saturated fatty acids (9) consequently, the unsaturated fatty acid anions form weaker fields than the saturated ones. Thus the expansion of an oleic acid film, maintained at the same tt as a palmitic acid film, was less abrupt and occurred at a lower pH than the expansion of a palmitic acid film (Figure 4). 

Monolayers of distearoyl lecithin at hydrocarbon/water interfaces undergo temperature and fatty acid chain length dependent phase separation. In addition to these variables, it is shown here that the area and surface pressure at which phase separation begins also depend upon the structure of the hydrocarbon solvent of the hydrocarbon oil/aqueous solution interfacial system. Although the two-dimensional heats of transition for these phase separations depend little on the structure of the hydrocarbon solvent, the work of compression required to bring the monomolecular film to the state at which phase separation begins depends markedly upon the hydrocarbon solvent. Clearly any model for the behavior of phospholipid monolayers at hydrocarbon/water interfaces must account not only for the structure of the phospholipid but also for the influence of the medium in which the phospholipid hydrocarbon chains are immersed. 

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... 

Seidl created a model based on the state of the surface film (e.g. expanded or condensed), the equilibrium spreading pressure, and the area per film molecule to describe organic film formation from fatty acids, then applied it to rainwater and aerosol particles [245]. He concluded that, in most cases, only dilute films (with concentrations below that necessary to form a complete monolayer) would form on aerosols and raindrops, and such films would not affect their physical or chemical properties. However, dense films were predicted to form on aerosols in the western U.S., mainly attributable to biomass burning. Mazurek and coworkers developed a model to describe structural parameters (elastic properties, etc.) of fatty acid films on rainwater without requiring knowledge of the surfactant concentration or composition by using surface pressure-area and surface pressure-temperature isochors and the rain rate and drop diameter distribution [33]. This model can be used to identify the origin of specific compounds and an approximate chemical composition based on the force-area characteristics of collected rainwater films. 

The shape of the surface pressure-area isotherm depends on the lateral interactions between molecules. This in turn depends on molecular packing which is influenced by factors such as the size of head group, the presence of polar groups, the number of hydrocarbon chains and their conformation (straight or bent). Here we focus on two fatty acids with different chain lengths and consider the structures formed in monolayers at different surface pressures as a function of the area per molecule. 

Tlie carboxylate salts of fatty acids have long, nonpolar, hydrocarbon chains. Therefore, they do not form solutions of individual ions, but are dispersed as weakly associated structures called micelles, which are spherical aggregations of molecules or ions. In a micelle of carboxylate salts, the nonpolar hydrocarbon chains occupy the interior of the sphere, and the polar carboxylate heads lie on the surface of the sphere. This spherical arrangement encloses the maximum amount of hydrocarbon material in the smallest surface area. Therefore, a micelle disrupts the hydrogen-bonded structure of water to the smallest extent possible. 

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