Monolayer-forming amphiphile

The adsorption of amphiphilic molecules at the surface of a liquid can be so strong that a compact monomolecular film, abbreviated as monolayer, is formed. There are amphiphiles which, practically, do not dissolve in the liquid. This leads to insoluble monolayers. In this case the surface excess T is equal to the added amount of material divided by the surface area. Examples of monolayer forming amphiphiles are fatty acids (CH3(CH2) c 2COOH) and long chain alcohols (CH3(CH2)nc iOH) (see section 12.1). 

Fukuda K., and Nakahara, H. Electronic Sp tra of Oriented Chromophore Sy ems by Incorporation of Azobenzene in the Polymer Main Chain and in Monolayer Forming Amphiphiles. J. Colloid Interface Sci. 98, 555 (1984). 

Most LB-forming amphiphiles have hydrophobic tails, leaving a very hydrophobic surface. In order to introduce polarity to the final surface, one needs to incorporate bipolar components that would not normally form LB films on their own. Berg and co-workers have partly surmounted this problem with two- and three-component mixtures of fatty acids, amines, and bipolar alcohols [175, 176]. Interestingly, the type of deposition depends on the contact angle of the substrate, and, thus, when relatively polar monolayers are formed, they are deposited as Z-type multilayers. Phase-separated LB films of hydrocarbon-fluorocarbon mixtures provide selective adsorption sites for macromolecules, due to the formation of a step site at the domain boundary [177]. 

The terminology of L-B films originates from the names of two scientists who invented the technique of film preparation, which transfers the monolayer or multilayers from the water-air interface onto a solid substrate. The key of the L-B technique is to use the amphiphih molecule insoluble in water, with one end hydrophilic and the other hydrophobic. When a drop of a dilute solution containing the amphiphilic molecules is spread on the water-air interface, the hydrophilic end of the amphiphile is preferentially immersed in the water and the hydrophobic end remains in the air. After the evaporation of solvent, the solution leaves a monolayer of amphiphilic molecules in the form of two-dimensional gas due to relatively large spacing between the molecules (see Fig. 15 (a)). At this stage, a barrier moves and compresses the molecules on the water-air interface, and as a result the intermolecular distance decreases and the surface pressure increases. As the compression from the barrier proceeds, two successive phase transitions of the monolayer can be observed. First a transition from the gas" to the liquid state. 

Figure 6 (Top) Chemical structure of dequalinium (bottom) possible conformations of single-chain bola amphiphiles. Amphiphiles in a stretched conformation (bola) would form monolayers, while amphiphiles in a bended conformation (horseshoe) would form bilayers.

Figure 11.41 (Left) Structure of amphiphilic dendronized polymer and (right) schematic representations of the Langmuir monolayer formed by the amphiphilic pol)nneric cylinders.

The investigation was then extended to a monolayer formed from dipalmitoyl phosphatidyl choline and the same amphiphilic photochromic polypeptide XXIII.11211 When the monolayer was kept in the dark, the polypeptide molecules arranged themselves perpendicularly to the membrane (the water/air interface) and formed a bundle of helices which could be observed by atomic force microscopy as a transmembranous particle of about 4 nm in diameter. Irradiation with UV light and the consequent trans—>tis isomerization of the azobenzene moiety caused a bending of the molecular main chain, which in turn produced a destabilization and dena-turation of the bundle of helices in the monolayer. After removal of the light, the polypeptide molecules reverted to their original bundle structure. 1211... 

The interfacial properties of an amphiphilic block copolymer have also attracted much attention for potential functions as polymer compatibilizers, adhesives, colloid stabilizers, and so on. However, only a few studies have dealt with the monolayers o well - defined amphiphilic block copolymers formed at the air - water interface. Ikada et al. [124] have studied monolayers of poly(vinyl alcohol)- polystyrene graft and block copolymers at the air - water interface. Bringuier et al. [125] have studied a block copolymer of poly (methyl methacrylate) and poly (vinyl-4-pyridinium bromide) in order to demonstrate the charge effect on the surface monolayer- forming properties. Niwa et al. [126] and Yoshikawa et al. [127] have reported that the poly (styrene-co-oxyethylene) diblock copolymer forms a monolayer at the air - water... 

In the Fangmuir-Blodgett technique, amphiphilic monolayers, formed at a liquid-air interface, are transferred to a solid substrate by horizontal or vertical transfer. The thickness of such monolayers is of the order of a few nanometers, depending on the materials being used. With this method, multilayer structures can also be produced, either by repeated deposition of the same layer or by the deposition of alternate layers. In this manner, multilayers containing several hundred individual components can be obtained. In general, the thermal and mechanical stabilities of such layers are, however, limited. 

For n-A isotherms of insoluble monolayes of amphiphilic molecules, assuming the association or dissociation of these molecules in the surface layer, a generalised Volmer equation was derived (based on Butler s and Gibbs equations33 35), which has the form... 

Typical LB films are constructed through transferring monolayers of amphiphilic molecules at the air-water interface onto solid substrates [8,12,13]. The amphiphilic molecules are first dissolved in an organic solvent that is immiscible with water, spread on a water surface, and compressed by decreasing the area in which the molecules are confined, to form a monolayer at the air-water interface. Then the monolayer is transferred onto a solid substrate by moving the substrate vertically or horizontally. This procedure allows us to obtain ultrathin films with the struc-... 

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). 

In section 6.2.4 we examined the case in which the surface of a solution containing an amphiphile became covered with a monomol-ecular film as a result of spontaneous adsorption from solution. The molecules in such films are in equilibrium with those in the bulk of the solution, i.e. there is a continuous movement of molecules between the surface and the solution below it. If, however, a surfactant has a very long hydrocarbon chain it will be insufficiently water-soluble for a film to be formed in this way. In such cases we can spread a film on the surface of the solution by dissolving the surfactant in a suitable volatile solvent and carefully injecting the solution on to the surface. The insoluble monolayer formed by this process contains all of the molecules injected on the surface there is no equilibrium with the bulk solution because of the low water solubility of the surfactant. Conse-... 

The structure of these amphiphiles is radically different from that of conventional Langmuir film-forming amphiphiles, where segregation of the hydrophobic and hydrophilic parts of molecules at the air/water interface is a prerequisite. This is the first example of a Langmuir-Blodgett film that is fabricated with a micellelike substance. It is intriguing that amphiphilicity is still required to form a stable monolayer film of hyperbranched polymers, in spite of their fundamental structural differences from conventional amphiphiles. 

Monomolecular layers and LB-multilavers. Compound 21 exhibits amphiphilic properties. Spreading at the air-water interface leads to the formation of oriented monomolecular layers. The monolayer forms a solid condensed phase at 20 C with a collapse pressure near SOmN/m. It can easily be transferred onto various hydrophobic substrates such as CaF, ZnS, AgCl, Si, Ga or metal surfaces, and Langmuir-Blodgett-type multilayers (21> of variable thickness can be built up. These multilayers also exhibit a rapid reaction if exposed to UV- or y-irradiation. From an infrared spectroscopic study described recently (ll-JA) a 1,4-addition reaction is evident, as it also occurs in microcrystalline powders of 21- The solid state polymerization of 21 is schematically represented by Figure 14. 

Planar polymer films (either free-standing or at interfaces) are of particular interest, not only because they are preferred in many applications, but also because they allow for surface studies which could not be performed on vesicles. Langmuir monolayers from a vesicle-forming amphiphilic triblock copolymer were studied towards understanding the polymer interactions with a cation transporting peptide, alamethicin [275]. Planar solid supported block copolymer membranes are... 

In the LB technique, a monolayer of amphiphilic molecules, prepared at the air-water interface, is transferred to a substrate, thus giving a monomolecular film. The molecules must be solvable in a volatile, water-insoluble (organic) solvent, but not, or to a very limited extent, in water. Thus, when the solution of the molecules in the organic solvent is spread over the surface of water, the solvent evaporates, leaving a monolayer of molecules at the air-water interface. This monolayer can be compressed and transferred to a substrate. When the molecules are replaced by colloidal, nanosized particles, monolayers of these particles on a substrate are obtained. Smectites are especially well suited for the LB technique. The elementary clay sheets are about 1 nm thick and a few tens to hundreds nm wide and long. In the alkali- or alkaline earth form, they are hydrophilic, but by ion exchange with suitable amphiphilic cations, they become hydrophobic. There are then two ways to prepare mono-layers of smectite clay particles by the LB technique. 

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