Fatty acid monolayers Film pressure

As the counterion penetrates the plane of the interfacial head groups, the surface pressure will be affected as a first-order effect thus, the expansion of the 7r-A isotherms for the fatty acid monolayers is in the same sequence as the cation sizes noted above. The penetrated counterions must be held with an energy at least comparable to KT since they are not expelled during the kinetic movement of the film molecules, but remain in place and increase the surface pressure. To penetrate the plane of the head groups in the monolayer, the counterions must possess sufficient adsorption energy to overcome the work against the kinetic surface pressure 7tK, such that, according to Davies and Rideal (10) ... 

A mixed monolayer consisting of stearic acid (9.9%), palmitic acid (36.8%), myristic acid (3.8%), oleic acid (33.1%), linoleic acid (12.5%), and palmitoleic acid (3.6%) produces an expanded area/pressure isotherm on which Azone has no apparent effect in terms of either expansion or compressibility (Schuckler and Lee, 1991). Squeeze-out of Azone from such films was not reported, but the surface pressures measured were not high enough for this to occur. The addition of cholesterol (to produce a 50 50 mixture) to this type of fatty acid monolayer results in a reduction of compressibility. However, the addition of ceramide has a much smaller condensing effect on the combined fatty acids (ratio 55 45), and the combination of all three components (free fatty acids/cholesterol/ceramide, 31 31 38) produces a liquid condensed film of moderate compressibility. The condensed nature of this film therefore results primarily from the presence of the membrane-stiffening cholesterol. In the presence of only small quantities of Azone (X = 0.025), the mixed film becomes liquid expanded in nature, and there is also evidence of Azone squeeze-out at approximately 32 mN m. ... 

The surface shear viscosity of a monolayer is a valuable tool in that it reflects the intermolecular associations within the film at a given thermodynamic state as defined by the surface pressure and average molecular area. These data may be Used in conjunction with II/A isotherms and thermodynamic analyses of equilibrium spreading to determine the phase of a monolayer at a given surface pressure. This has been demonstrated in the shear viscosities of long-chain fatty acids, esters, amides, and amines (Jarvis, 1965). In addition,... 

Figure 8-8 (A) The Langmuir-Adam film balance. Tension on the moveable barrier is recorded for different areas of the surface between the barriers. This gives the surface pressure jt, which is the difference between the surface tension (y0) of a clean aqueous surface and that of a spread monolayer (y) K = ya-y. Courtesy of Jones and Chapman.81 (B) Surface pressure (7t)-area per molecule isotherm for a typical fatty acid (e.g., pentadecanoic acid C14H29C02H) at the aqueous-air interface. From Knobler.81a...

Techniques for spreading monolayers of polar long chain compounds on mercury in a Langmuir type film balance, and for measuring their surface area-pressure properties, have been described by one of the present authors (3). Using these techniques, it has proved possible to measure continuously the change in contact angle of a water droplet superposed on the monolayer, as the film pressure is controllably varied. This has now been done for monolayers of the normal C12-C20 fatty acids and the normal primary Ci4-Ci8 alcohols on the mercury substrate. 

The study of monolayers formed on a wafer surface has also provided imporfanf informahon. A fhin film of an amphiphilic (confaining both polar and nonpolar groups) compormd such as a fatty acid is prepared. This is done by depositing a small quantity of the compound dissolved in a volatile solvent on a clean aqueous surface befween fhe barriers of a Langmuir trough (Fig. 8-8). The difference in surface fension (n) across the barriers is measured with a suitable device for differenf areas of the monolayer, i.e., for differenf positions of the moveable barrier. The value of n is low for expanded monolayers and falls to nearly zero when fhe surface is no longer completely covered. The pressure reaches a plateau when a compact mono-layer is formed, after which if rises again (Fig. 8-8B). 

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. 

Porphyrins with small substituents, e.g., four me o-phenyl or eight p-ethyl groups, do not form stable films on water in pure form. Such porphyrins have, however, been dissolved either in surface monolayers of fatty acids or similar lipids at low surface pressure they are often perfectly integrated. When the domains of the lipid monolayers merge to form a solid film, the porphyrins are squeezed out of the monolayer and form microcrystallites at the edges of the old domains. 

The lowering of surface tension of lipids, which form insoluble monolayers, is identical to the spreading pressure of the monomolecular surface (see Section 8.4), and some surface film data of n-fatty acids are given in Table 8.14. 

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. 

We have studied the viscoelastic behavior of lipid thin films at an air/water interface from surface pressure (tt) vs. surface area (A) curves, since an observation of the k-A curve of a monolayer is one of the most convenient methods to elucidate the viscoelastic behavior of the monolayer. Recently, in spite of compression process, we observed an overshoot-hump, a zero surface pressure, and a flat plateau in the rt-A curve of a synthetic fatty acid [3]. These characteristic features in the curve were explained by using a kinetic model representing the formation of aggregates of the fatty-adds molecules at an air/water interface. 

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