Transfer film-substrate interface

Figure 3.9 Schematic illustration of the processes that can occur at a modified electrode, where P represents a reducible substance in a film on the electrode surface and A a species in solution. The processes shown are as follows (1) heterogeneous electron transfer to P to produce the reduced form Q (2) electron transfer from Q to another P in the film (electron diffusion or electron hopping in the film) (3) electron transfer from Q to A at the film/solution interface (4) penetration of A into the film (where it can also react with Q or at the substrate/film interface) (5) movement (mass transfer) of Q within the film (6) movement of A through a pinhole or channel in the film to the substrate, where it can be reduced. From A.J. Bard and L.R. Faulkner, Electrochemical Methods Fundamentals and Applications, 2nd Edition, Wiley, 2001. Reprinted by permission of John Wiley Sons, Inc...

An alternative explanation is suggested by some later work of Fleischauer and Bauer °°. They found that the best performance of transfer films of molybdenum disulphide was obtained when molybdic oxide was present in the lowest layers of the film adjacent to the steel surface. Oxidation to molybdic oxide is increased in the presence of moisture, so that reduction of moisture content due to frictional heating may reduce the amount of molybdic oxide present, and thus have a direct adverse effect on transfer film life as well as reducing the rate of film formation. Fleischauer and Bauer also found indications that transfer film life was improved if a slight excess of sulphur was present at the interface between the film and the substrate. The presence of molybdic oxide or excess sulphur are undesirable in the bulk of the lubricant and especially on the sliding surface and they suggested that for optimum... 

Langmuir-BIodgett-Kuhn (LBK) films are prepared by a dipping process in which monolayers floating at the air/water interface are deposited on a solid substrate. LBK films are highly ordered supramolecular assemblies. By selection of the amphiphilic molecules and adjustment of the transfer conditions, the LBK technique offers extensive structural control and opens a wav to optimize matrices for photoactive dyes. 

A method to obtain nano-particles at air/water interfaces has been described in [287]. Spreading of surfactant-coated metallic, semi-conducting, magnetic and ferroelectric nanoparticles on water surfaces results in the formation of monoparticulate thick films which then can be transferred, layer by layer, to solid substrates. These films can find potential applications in advanced electronic and electro-optical devices. Here and further, we give only typical examples of using surfactants in novel technologies. A more detailed description can be found in a new edition of Surfactants Science Series [288]. 

A discussion of the wear of PTFE would not be complete without some reference to PTFE composites. This has been a popular field of study simply because without fillers the wear of PTFE is normally unacceptable. A good filler will reduce transfer wear rates by up to three orders of magnitude. Various mechanisms have been proposed and the subject has been reviewed by the present author (8,9) and others (2,52). The simplest idea is that fillers wear less than the polymer when exposed at the interface. They may also suppress transfer and improve transfer film adhesion, A good deal of effort of high quality has been put into the search for chemically induced adhesion promotion at the transferred film-substrate interface but the evidence is equivocal (53,54). Chemical changes are detected but their precise contribution to the adhesion is uncertain in commercial applications. PTFE is a remarkably stable polymer to chemical attack even at sliding interfaces. 

The kinetics of the electron transfer at the electrode-polymer film interface, which initiates electron transport in the surface layer, is generally considered as a fast process, which is not rate limiting. It was also presumed that the direct electron transfer between the metal substrate and the polymer involves only those redox sites situated in the layer immediately adjacent to the metal surface. As follows from the theory (Eq. 8) the measured charge transport diffusion coefficient should increase linearly with c, whenever the contribution from the electron-exchange reaction is important therefore the concentration dependence of D may be the test of theories based on the electron-exchange reaction mechanism. Despite the fact that considerable efforts have been made to find the predicted linear concentration dependence of D, it has been observed only in a few cases and for a limited concentration range. 

In this way (multi)layers, referred to as Langmuir-Blodgett films or, for short, LB films, are formed with alternating head-head and tail-tail orientations of the surfactant molecules. This type of transfer in which a monolayer is deposited during each passage of the substrate through the interface is called F-transfer. It follows that with hydrophihc supports at the end of each completed cycle films containing an odd number of layers are deposited, whereas for hydrophobic surfaces an even number of layers are transferred. 

When interpreting the data it should be realized that the equilibrium structure of the transferred monolayer may not be identical to that of the parent monolayer in the Uqnid interface. Interaction of the amphiphiles with the solid substrate (or with the predeposited layers) is probably different from that with the liquid interface and, furthermore, at the solid surface the amphiphiles do not experience the same, constant, interfacial pressure that keeps the parent monolayer in the original state. Due to all of this, structural rearrangements may occur during or after deposition. During the ageing process of the transferred film phase separation, terrace and crack formation may take place. Where the films kept in air are stable over several weeks or months, the ageing in water may occur within minutes or hours. 

Aoki also considers the stochastic aspects of phase propagation mechanism and relates his analysis to the theory of percolation and the fractal dimension of the system. In this approach the Nemst equation for charge transfer at the substrate/film interface is used to compute the probability of the presence of a conductive seed or nucleus. When the potential is incremented, this seed can then grow in a one-dimensional manner governed by the propagation rate constant kp or the kinetic parameter to form a conductive pillar of a definite length. New nuclei can also form at the support electrode/film interface during the potential... 

LB films of 1,4,8,11,15,18-hexaoctyl-22,25-bis-(carboxypropyl)-phthalocyanine (2), an asymmetrically substituted phthalocyanine, were stable monolayers formed at the water—air interface that could be transferred onto hydrophilic siUca substrates (32—34). When a monolayer film of the phthalocyanine derivative was heated, there was a remarkable change in the optical spectmm. This, by comparison to the spectmm of the bulk material, indicated a phase transition from the low temperature herringbone packing, to a high temperature hexagonal packing. 

LB Films of Polymerizable Amphiphiles. Stxidies of LB films of polymerizable amphiphiles include simple olefinic amphiphiles, conjugated double bonds, dienes, and diacetylenes (4). In general, a monomeric ampbipbile can be spread and polymerization can be induced either at tbe air—water interface or after transfer to a soHd substrate. Tbe former polymerization results in a rigid layer tbat is difficult to transfer. 

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. 

The second way of preparing L-B monolayer structures, the horizontal lifting method, was introduced by Langmuir and Schaefer. In this method, a compressed monolayer first is formed at the water-air interface, and a flat substrate is then placed horizontally on the monolayer film. When the substrate is lifted and separated from the water surface, the monolayer is transferred onto the substrate, as depicted in Fig. 15(d). 

The other method of monolayer transfer from the air/water interface onto solid substrates is illustrated in Figure 2. This method is called the Langmuir-Schaefer technique, or horizontal lift. It was developed in 1938 by I. Langmuir and V. Schaefer for deposition of protein layers. Prepared substrate horizontally touches the monolayer, and the layer transfers itself onto the substrate surface. The method is often used for the deposition of rigid monolayers and for protein monolayers, hi both cases the apphcation of the Lang-muir-Blodgett method produces defective films. 

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