Films amphipathic

The out-of-plane orientation of chromophores can be more easily controlled in LB films as compared with the in-plane orientation. Many chromophores are known to show anisotropic orientation in the surface normal direction. The molecular structure of chromophores and their position in amphiphile molecules, the surface pressure, the subphase conditions are among those affect their out-of-plane orientation. The out-of-plane orientation has been studied by dichroic ratio at 45° incidence, absorbance ratio at normal and 45° incidence, and incident angle dependence of p-polarized absorption [3,4,27,33-41]. The evaluation of the out-of-plane orientation in LB films is given below using amphipathic porphyrin (AMP) as an example [5,10,12]. 

Because of the near linearity of these portions of the isotherm, it is easy to extrapolate both regions to their value at ir = 0. The intercepts for the solid and liquid-condensed regions, as and o c, respectively, differ only slightly. Values of a°c for alcohols are about 0.22 nm2, and for carboxylic acids about 0.25 nm2, more or less independent of the length of the hydrocarbon chain. The intercept as° has a value of about 0.20 nm2, independent of both the length of the chain and the nature of the head. The film pressures in the condensed states (LC or S) are of the same magnitude as the equilibrium spreading pressure for amphipathic molecules. 

Emulsions and foams are two other areas in which dynamic and equilibrium film properties play a considerable role. Emulsions are colloidal dispersions in which two immiscible liquids constitute the dispersed and continuous phases. Water is almost always one of the liquids, and amphipathic molecules are usually present as emulsifying agents, components that impart some degree of durability to the preparation. Although we have focused attention on the air-water surface in this chapter, amphipathic molecules behave similarly at oil-water interfaces as well. By their adsorption, such molecules lower the interfacial tension and increase the interfacial viscosity. Emulsifying agents may also be ionic compounds, in which case they impart a charge to the surface, which in turn establishes an ion atmosphere of counterions in the adjacent aqueous phase. These concepts affect the formation and stability of emulsions in various ways ... 

M. Fujihira, H. Yamada, Molecular Photodiodes Consisting of Unidirectionally Oriented Amphipathic Acceptor Sensitized Donor Triads , Thin Solid Films, 160, 125 (1988)... 

Hydrophobic interactions (the alleged direct cohesion between the hydrophobic chains of the lipid) may be irrelevant to surface viscosity since cholesterol and dihydrosphingomyelin films are not viscous. Cholesterol, one of the most hydrophobic amphipathic molecules, forms highly incompressible films this indicates small intermolecular distances. Dihydrosphingomyelin has hydrophobic chains that are as long and saturated as those of DPL and also a smaller cross-section than that of DPL. 

When amphipathic molecules are dispersed in water, their hydrophobic parts (i.e., hydrocarbon chains) aggregate and become segregated from the solvent. This is a manifestation of the hydrophobic effect which comes about because of exclusion and hence ordering of water at the interface between these distinct types of molecule. Aggregates of amphipathic molecules can be located at a water-air boimdary (monolayers) (Fig. 3-24) however, only a small quantity of an amphipathic lipid dispersed in water can form a monolayer (unless the water is spread as a very thin film). The bulk of the lipid must then be dispersed in water as micelles (Fig. 3-24). In both of these structures the polar parts, or heads (O), of the lipid make contact with the water, while the nonpolar parts, or tails (=), are as far from the water as possible. Micelles can be spherical as shown in Fig. 3-24, but can also form ellipsoidal, discoidal, and cylindrical stmctures. 

When amphipathic molecules are spread on either an aqueous surface or an aqueous/oil interface, they usually form a thin film composed of a molecular layer at the interface, and it is possible to measure a change in potential across this interface, which is a function of the electrolyte concentration in the aqueous phase. 

For some time there has been considerable interest in the ability to polymerize amphipathic molecules as monomolecular films either at the air/water interface or on solid substrates . The most commonly studied species have incorporated diacetylenic groups in both the polar head groups and the hydrophobic z.cy chains and the simplest of these have been single chain unsubstituted acids or alcohols. 

Emulsions are disperse systems of one or more immiscible liquids. They are stabilized by emulsifiers - compounds which form interface films and thus prevent the disperse phases from flowing together (cf. 8.15). Due to their amphipathic nature, proteins can stabilize o/w emulsions such as milk (cf. 10.1.2.3). This property is made use of on a large scale in the production of food preparations. 

Soap solutions have the remarkable property of forming stable bubbles and films. This property is a consequence of the surface structure of the soap solution and the soap film. The surface of a bath of soap solution and a soap film consists of a monomolecular layer of amphipathic] ions. These are ions... 

The stability of soap films is determined by the amphipathic ions in the surface. If a soap film is perturbed from equilibrium so that the area of an element of film increases, the surface density of amphipathic ions will decrease. That is, the number of ions per unit area will diminish and consequently the surface will behave more like the surface of water. Hence the surface tension of the surface element will increase because the surface tension of water is greater than that of soap solution. This increased force in the region of increased area will restore the surface to its former equilibrium configuration. This stabilizing effect was first observed by Marangoni and is known as the Marangoni effect. 

The typical soap film consists of two surfaces of amphipathic solute ions plus a distribution of solute ions in the water between the surfaces (Fig. 2.4(a)). It will be recalled that these amphipathic ions consist of two dissimilar parts. One part is hydrophilic with an affinity for water and tends to become surrounded by water molecules. This is the polar carboxyl head . The other part... 

Liquid Films and the Electrically Charged Double-Layer Amphipathic Organic Molecules Molecular Interactions The Skip Resistance of Automobile Tyres Conclusions... 

The next stage, by analogy with metals, is the lubrication achieved by thin films of amphipathic molecules such as stearic acid, stearamide, oieamide, etc. These materials may be adsorbed on the surfaces from solution or deposited directly as dry films . They are not as effective on polymers as they are on metals partly because they do not adsorb so well. Recently Senior and West have shown that by suitably treating the polymer surface it can be made to provide more adsorption sites and in that case fairly effective boundary lubrication can be achieved. Of course if the chemically modified surface is worn away the effect is lost. The shear properties of the boundary lubricant are also important. For example in the experiments carried out on glass surfaces (see abovethe shear strength of oieamide was found to be about one half of that of stearamide at the same contact pressure. A similar difference is found in the lubrication of a material such as polythene the friction is considerably lower with oieamide than with stearamide. 

The review also deals with boundary lubrication in terms of decreasing lubricant film thickness from the electrically charged double layer (ca lOoA thick), the monomolecular layer of amphipathic molecules (20-40A thick) to the very thin adsorbed layers of small molecules that may be only 5 10A thick. Reference is also made to chemical modifications of the surface and to the behavior of polymers containing a small amount of dissolved additives. It is evident that, in the boundary lubrication of rubbers and polymers, certain aspects are fairly well understood. On the other hand our knowledge Is rather fragmentary. We need a more comprehensive picture of the physical and chemical reactions of lubricants with polymer surfaces, the properties of the surface films formed, and their mechanical and thermal stability as a function of speed, load and temperature. 

LANGMUIR-BLODGETT DEPOSITION OF AMPHIPATHIC AZOBENZENE COMPOCJNDS FOR SURFACE ACTIVATION AND FABRICATION OF FUNCTIONALISED THIN FILMS... 

We have approached the goal of functionalised monolayer fabrication by synthesis of a group of azobenzene-containing amphipathic molecules. Such molecules can be deposited by either chemisorption or physisorption and should allow hybrid films to be fabricated. The monolayers are reversibly photochromic (which might prove useful in sensor fabrication) and are capable of chemical activation for surface attachment of macromolecules, etc. In addition, the activated surface can be photochemi-cally modified so that photolithography could be used to fabricate surface structures such as danains of particular ccxnposition. With particular regard to conductive polyers, polyanilines could be locally formed as discussed later. 

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