Fatty acids monolayers

An essential component of cell membranes are the lipids, lecithins, or phosphatidylcholines (PC). The typical ir-a behavior shown in Fig. XV-6 is similar to that for the simple fatty-acid monolayers (see Fig. IV-16) and has been modeled theoretically [36]. Branched hydrocarbons tails tend to expand the mono-layer [38], but generally the phase behavior is described by a fluid-gel transition at the plateau [39] and a semicrystalline phase at low a. As illustrated in Fig. XV-7, the areas of the dense phase may initially be highly branched, but they anneal to a circular shape on recompression [40]. The theoretical evaluation of these shape transitions is discussed in Section IV-4F. 

Because of the charged nature of many Langmuir films, fairly marked effects of changing the pH of the substrate phase are often observed. An obvious case is that of the fatty-acid monolayers these will be ionized on alkaline substrates, and as a result of the repulsion between the charged polar groups, the film reverts to a gaseous or liquid expanded state at a much lower temperature than does the acid form [121]. Also, the surface potential drops since, as illustrated in Fig. XV-13, the presence of nearby counterions introduces a dipole opposite in orientation to that previously present. A similar situation is found with long-chain amines on acid substrates [122]. 

The effect is more than just a matter of pH. As shown in Fig. XV-14, phospholipid monolayers can be expanded at low pH values by the presence of phosphotungstate ions [123], which disrupt the stmctival order in the lipid film [124]. Uranyl ions, by contrast, contract the low-pH expanded phase presumably because of a type of counterion condensation [123]. These effects caution against using these ions as stains in electron microscopy. Clearly the nature of the counterion is very important. It is dramatically so with fatty acids that form an insoluble salt with the ion here quite low concentrations (10 M) of divalent ions lead to the formation of the metal salt unless the pH is quite low. Such films are much more condensed than the fatty-acid monolayers themselves [125-127]. 

Gundlach K H and Kadlech J 1974 The influence of the oxide film on the current in AI-AI oxide-fatty acid monolayer-metal functions Chem. Phys. Lett. 25 293-5... 

Polymeropoulos E E 1977 Electron tunneling through fatty-acid monolayers J. Appl. Phys. 48 2404-7... 

Fig. 8 (a) The I-V curve measured for a junction consisting of an LB monolayer of margaric acid formed on an Al bottom electrode and a top Au electrode fabricated via the PALO technique, (b) Semilogarithmic plot of current density vs the chain-length of fatty-acid monolayers. The fit to a straight line gives a slope of fl = 0.86 A-1. (Reprinted with permission from [78])... 

Novel concepts of aggregation structure of fatty acid monolayers on the water surface... 

EVALUATION OF MELTING AND CRYSTALLINE RELAXATION TEMPERATURES OF FATTY ACID MONOLAYERS ON THE WATER SURFACE... 

It is, therefore, clearly concluded from Figures 9-11 that in the case of conventional fatty acids such as myristic, palmitic, stearic and so on, the crystalline or amorphous phase of monolayer completely depends on the relative magnitude of Tsp to Tm of the monolayer, being independent of the magnitude of surface pressure. The fatty acid monolayers do not show any pressure-induced crystallization during compression of the monolayer on the water surface. The crystalline and amorphous monolayers are schematically summarized in Figure 12. 

DIRECT OBSERVATION OF MOLECULAR ARRANGEMENTS IN FATTY ACID MONOLAYERS WITH AN ATOMIC FORCE MICROSCOPE. 

Therefore, the following melting model of a fatty acid monolayer could be suggested during increase of the temperature Solid domains — Soft domains — Two co-existed phases between domains and molten monolayer — completely molten monolayer, as shown in Figure 2. 

The dielectric anisotropy of long-chain fatty acid monolayers was analyzed. These fatty acids were considered as being oriented in a cylinder cavity with length (L) diameter (D). Considering each bond in these molecules as a polarization ellipsoid with axial symmetry about the -C-C- bonds, the mean polarizibtlity of the bonds was calculated. 

The use of a fatty acid monolayer on a subphase of divalent metal ions compromises the archetypal preparation of LB films containing metal ions. Complexation of subphase metal ions to the fatty acid monolayer occurs through fatty acid proton/metal ion exchange and is pH dependent. For C,d2+ ions, at 2 X 10 4 M. for example, the onset of the exchange is at approximately pH 5 and stoichiometric exchange occurs at pH 6. This stoichiometric reaction is depicted for the general case for divalent metal ions and fatty acids in Eq. (2) ... 

AVmolecule.2 Similar studies using hydrocarbon fatty acid monolayers provided molecular areas of about 20 AVmoleculc." " " ... 

Tphe force-area (tt-A), surface potential-area (AV-A), and surface vis-cosity-area (-q-A) characteristics of long-chain fatty acid monolayers at the air-water interface have been extensively investigated over acid and neutral aqueous solutions (17). Studies of fatty acid monolayer isotherms at high pH and the specific cation effect on the isotherms are less numerous. The difficulties inherent in working at high pH are ... 

The fatty acid monolayer becomes increasingly solubilized into the aqueous substrate. 

The AV-A relationships for fatty acid monolayers were first investigated over a wide range of pH by Schulman and Hughes (49), who found that for myristic acid (Ci4) films over substrates of pH 0 to 13, AV decreased with pH HC1 or NaOH was added to vary pH. Similar results were reported for stearic acid films over substrates whose pH was varied from 2 to 10 with HC1 and NH4OH (48), and over substrates ranging from pH 2 to 12, containing 0.01M sodium ions (5, 18, 20, 53). 

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) ... 

Wolstenholme, G. A., and Schulman, J. H. Metal-monolayer interactions in aqueous systems. Part III. Steric effects with branched chain fatty acid monolayers. Trans. Faraday Soc. 47, 788-794 (1951). 

The situation is different when donor and acceptor molecules are located at different interfaces, that are separated by a fatty acid monolayer of well defined thickness. There are no longer close pairs of donor and acceptor with a high probability of electron transfer as in the "contact" case. Consequently, no change in relative fluorescence intensity with increasing donor density is expected, contrary to the former case. Indeed, in systems with a spacer monolayer of... 

Molecular self-assembly is a technique to form highly ordered, closely packed mono-layers on various substrates via a spontaneous chemisorption process at the interface.11,12 Earlier research done in this field includes the self-assembly of fatty acids monolayers on metal oxides,14,15 SAMs of organosilicon derivatives on metal and semiconductor oxides,16,17 and organosulfur SAMs on metal and semiconductor surfaces.18,19 Among the organosulfur SAMs, the most thoroughly investigated and characterized one is alkanethiol SAM formed on Au(l 11) surfaces.12... 

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