Long-chain fatty acid monolayers

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. 

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 ... 

Electron Microscopic Investigation of the Adsorption of Long-Chain Fatty Acid Monolayers on Glass... 

Recently, it was also shown that the adsorption forces could be adjusted by covering HOPG with a long-chain fatty acid monolayer prior to sample deposition [40], as this led to improvements in the dragging and imaging of the dendronized polymers. 

Johann R, Vollhardt D (1999) Texture features of long-chain fatty acid monolayers at high pH of the aqueous subphase. Mater Sci Eng C 8-9 35 2... 

The first work published in this area was that of Bigelow mentioned above [116], In 1957, monolayers of long-chain fatty acids were fonned on thin films of silver, copper, iron and cadmium deposited on glass microscope slides [43],... 

Another interesting class of phase transitions is that of internal transitions within amphiphilic monolayers or bilayers. In particular, monolayers of amphiphiles at the air/water interface (Langmuir monolayers) have been intensively studied in the past as experimentally fairly accessible model systems [16,17]. A schematic phase diagram for long chain fatty acids, alcohols, or lipids is shown in Fig. 4. On increasing the area per molecule, one observes two distinct coexistence regions between fluid phases a transition from a highly diluted, gas -like phase into a more condensed liquid expanded phase, and a second transition into an even denser... 

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,... 

We Umit this section to a discussion of stereochemical studies that sought to demonstrate discriminating enantiomeric interactions in monolayers of simple surfactants having one hydrophobic chain of methylenes and, generally, a single chiral center. Work in this area includes derivatives of long chain fatty acids, alcohols, or esters whose chiral center is included in the methylene chain. 

Amplified photochemical quenching of carbazolyl fluorescence was observed in mixed LB films containing pure CUA and long chain fatty acids [12,14], A pure CUA was synthesized from 2-nitrobiphenyl and 11-bromoundecanoic acid methyl ester [12,13], Two monolayers of mixtures of CUA (fc = 0.02 to 0.50) and PA were deposited on five monolayers of cadmium arachidate at 15°C and 20 mN m 1 at pH 6.3. 

In addition to the long-chain fatty acid molecules described above, a large number of studies have appeared that use IRRAS to study phospholipid monolayers as models of biomembrane interfaces. Mitchell and Dluhy [26] reported the first IRRAS spectra of 1,2-distearoyl-5 -glycero-3-phosphocholine (DSPC), l,2-dimyristoyl-i -glycero-3-phosphocholine (DMPC), and 1,2-di-palmitoyl-,v/ -glycero-3-phosphocholine (DPPC) monolayers at the air-water... 

Table 3.7a. Normal long chain fatty acid and fatty alcohols, used in monolayer studies.

Organic adsorbates that are more hydrophobic exhibit different adsorption behavior, particularly at higher concentrations. Long-chain fatty acids adsorb to oxide surfaces in part through surface complexation, as shown by electron spin resonance spectroscopy (32). At higher concentrations at the surface, however, favorable interactions between sorbed molecules (hemimicelle formation) appear to dominate and result in greater than monolayer adsorption (40, 41). Because humic substances (like the fatty acids) are amphiphilic, both surface complexation and hydrophobic interactions may be involved in the adsorption of humic substances on oxide surfaces. 

Studies of the physical properties of UC, reviewed in Chapter 6 of this volume, have contributed much to our understanding of the role of this Upid in membranes and lipoprotein surfaces. The shape and polarity of UC promote its association with the phosphoUpids of membranes and Upoproteins, and this association has important effects on membrane fluidity and permeability. The physical properties of long-chain fatty acid esters of cholesterol, on the other hand, differ strikingly from those of UC, and cause these esters to be largely excluded from phospholipid bilayers and monolayers and to aggregate instead in oil droplets. 

This paper describes a systematic study by electron microscopy of adsorbed monolayers of long-chain fatty acids ranging from C14 to C26 Improvements in replication techniques and the electron microscopes themselves since the time of Epstein s original work, as well as the successful applications of electron microscopy cited above, gave reason to believe that such an investigation would be a useful supplement to the previous experiments of Bigelow and Brockway in elucidating the structure of the adsorbed films. 

The LB technique may also be combined with solid-state chemistry methods to produce novel molecular architectures. For example, a network of conductive polypyrrole (molecular wires ) may be obtained in a fatty acid matrix [37, 38]. First, monolayers of the iron salt of a long-chain fatty acid (e.g., ferric pahnitate) are assembled on an appropriate substrate. The multilayer film is then exposed to saturated HCl vapor at room temperature for several minutes. During this process, a chemical reaction transforms the fatty acid salt into layers of ferric chloride separated by layers of fatty acid. In the third and final step, the film is exposed to pyrrole vapor and a reaction occurs between the pyrrole and the ferric chloride, producing polypyrrole distributed within the multilayer assembly. 

Many insoluble substances such as long chain fatty acids, alcohols and surfactants can be spread from a solvent on to water to form a film that is one molecule thick, called a monolayer. The hydrophilic groups (for example —COOH or —OH) point into the water, whereas the hydrophobic tails avoid it. 

The different kinds of molecules can be incorporated in an LB multilayer. The resultant LB film has the structure of a organic superlattice if each kind of molecule is positioned within different molecular layers. It is also possible to incorporate different kinds of molecule within the same layer by forming mixed monolayers at the air-water interface (Figure 14.4). In the case of mixed monolayers, poorly surface active or even non-surface active molecules can be incorporated into films with the aid of highly surface active molecules such as long-chain fatty acids. 

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