Fatty acid monolayers, adsorbate surface

Fig. 3.5 Representation of a scheme of an experiment (upper set of drawings) and the obtained experimental results presented as AFM images (middle part) and cross-sectional profiles (bottom) that provides evidence of silica nucleation and shell formation on biopolymer macromolecules. Scheme of experiment. This includes the following main steps. 1. Protection of the mica surface against silica precipitation. It was covered with a fatty (ara-chidic) acid monolayer transferred from a water substrate with the Langmuir-Blodgett technique. This made the mica surface hydrophobic because of the orientation of the acid molecules with their hydrocarbon chains pointing outwards. 2. Adsorption of carbohydrate macromolecules. Hydrophobically modified cationic hydroxyethylcellulose was adsorbed from an aqueous solution. Hydrocarbon chains of polysaccharide served as anchors to fix the biomacromolecules firmly onto the acid monolayer. 3. Surface treatment by silica precursor. The mica covered with an acid mono-...

The surface charge density of adsorbed fatty acid monolayer increases proportionally with the increase in their ionization constants, ionic strength of the subphase, pH, and packing density of the surface-active ions in the space of monolayer. Maximum value of the surface charge density corresponds to the case when = I-IO" M and the minimum when = I-IO M. 

The adsorbed layer at G—L or S—L surfaces ia practical surfactant systems may have a complex composition. The adsorbed molecules or ions may be close-packed forming almost a condensed film with solvent molecules virtually excluded from the surface, or widely spaced and behave somewhat like a two-dimensional gas. The adsorbed film may be multilayer rather than monolayer. Counterions are sometimes present with the surfactant ia the adsorbed layer. Mixed moaolayers are known that iavolve molecular complexes, eg, oae-to-oae complexes of fatty alcohol sulfates with fatty alcohols (10), as well as complexes betweea fatty acids and fatty acid soaps (11). Competitive or preferential adsorption between multiple solutes at G—L and L—L iaterfaces is an important effect ia foaming, foam stabiLizatioa, and defoaming (see Defoamers). 

One question addressed in the literature is the relationship between the angle of orientation of the adsorbed species within the monolayer and their amphiphilic character. The case of surfactants like fatty acids or phospholipids is deferred until Section VI, since the technique of choice is SFG in order to perform a surface vibrational study. Phenol deri-... 

A recent study by DePalma and Tillman [ 10] also demonstrates the potential of surface modification by self-assembled monolayers of low surface energy fiuoroalkyl-containing silanes. Fatty acids, amines and alcohols have long been known to adsorb as monomolecular films on metals. Silane coupling agents have featured strongly in new studies to develop more robust films, covalently bound together and to the metal substrate. 

The products of lipid oxidation in monolayers were also studied. Wu and coworkers (41) concluded that epoxides rather than hydroperoxides might be the major intermediates in the oxidation of unsaturated fatty acids adsorbed on silica, presumably because of the proximity of the substrate chains on the silica surface. In our work with ethyl oleate, linoleate and linolenate which were thermally oxidized on silica, the major decomposition products found were those typical of hydroperoxide decomposition (39). However, the decomposition patterns in monolayers were simpler and quantitatively different from those of bulk samples. For example, bulk samples produced significantly more ethyl octanoate than those of silica, whereas silica samples produced more ethyl 9-oxononanoate than those of bulk. This trend was consistent regardless of temperature, heating period or degree of oxidation. The fact that the same pattern of volatiles was found at both 60°C and 180°C implies that the same mode of decomposition occurs over this temperature range. 

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. 

Freshly machined surfaces of ordinary metals adsorb a complete monolayer of fatty acid. On desorption by an effective solvent, the entire layer is removed as metal soap. Gold and platinum adsorb less than a monolayer of fatty acid, and on solvent desorption the fatty acid is removed as such and not as metal soap. 

In the homologous series of fatty acids, only small variations were found in the value of y characteristic of the acid monolayer at temperature T. Whereas an increase in chain length in going from octanoic to octadecanoic acid resulted in a rise in t from 23° to 106°, the corresponding change in y was only to decrease from 28 to 25 dynes per cm. Since these values do not greatly exceed the value of Vc characteristic of hydrocarbon surfaces in closest packing, they indicate that the adsorbed film is wetted by the melt at temperatures at which the adsorbed films are still relatively intact. 

The replicas were stripped from the monolayered glass surfaces by gradual immersion in dilute aqueous solutions of HF with the slide at an angle to the liquid surface. The stripped replicas were subsequently washed with distilled water and mounted on copper grids. Stripping was slower for the surfaces which were more wetted—i.e., the ones with fewer acid molecules adsorbed on their surface. From this, it is assumed that the fluoride attack is primarily upon the fatty acid. 

Table II. Contact Angles and Surface Coverage for Monolayers of Fatty Acids Adsorbed on Glass Surfaces (Representative Data)...

Cottington, R. L., Levine, O., Zisman, W. A., "Effects of Adsorbed Water and Surface Roughness of Glasses of Various Compositions on the Friction and Wear Properties of Adsorbed Fatty Acid and Amine Monolayers," 134th Meeting, ACS, Chicago, III., September 1958. 

The previously reviewed investigations of spreading and wetting make it possible to give a simple and fundamental explanation of the epilame treatment of Woog [27, 28]. Essentially the epilame is a modification of the substrate surface. A close-packed adsorbed monolayer of a higher fatty acid, such as stearic acid, has a value of 24 dynes per cm. Any oil or other liquid having a surface tension, yLv°> 20°C. 

In contrast, there are practically insoluble amphiphiles, for example, a fatty acid molecule that consists of a hydrophilic part (carboxyl group) and a hydrophobic part (a long hydrocarbon chain), which prevents the molecule from dissolving in the aqueous phase. Such a molecule forms an insoluble monolayer on a water surface. Drops of the solution of the fatty acid in a volatile organic solvent are placed on an aqueous surface and after evaporation of the solvent a fatty acid s monolayer remains. This process is called spreading. The adsorbed molecules forming the insoluble monolayer are essentially isolated on the surface therefore, the surface excess F is equal to the added amount of material divided by the surface area. 

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