Self layers

A logical division is made for the adsorption of nonelectrolytes according to whether they are in dilute or concentrated solution. In dilute solutions, the treatment is very similar to that for gas adsorption, whereas in concentrated binary mixtures the role of the solvent becomes more explicit. An important class of adsorbed materials, self-assembling monolayers, are briefly reviewed along with an overview of the essential features of polymer adsorption. The adsorption of electrolytes is treated briefly, mainly in terms of the exchange of components in an electrical double layer. 

The self-assembly process can be continued to form multilayer films of up to 25 layers [33,48,49]. The reliability of this process is illustrated in Fig. XI-3, where the thickness grows linearly with the number of reacted layers. These thick layers have many interesting applications. 

Surface active electrolytes produce charged micelles whose effective charge can be measured by electrophoretic mobility [117,156]. The net charge is lower than the degree of aggregation, however, since some of the counterions remain associated with the micelle, presumably as part of a Stem layer (see Section V-3) [157]. Combination of self-diffusion with electrophoretic mobility measurements indicates that a typical micelle of a univalent surfactant contains about 1(X) monomer units and carries a net charge of 50-70. Additional colloidal characterization techniques are applicable to micelles such as ultrafiltration [158]. 

The atoms on the outennost surface of a solid are not necessarily static, particularly as the surface temperature is raised. There has been much theoretical [12, 13] and experimental work (described below) undertaken to investigate surface self-diffiision. These studies have shown that surfaces actually have dynamic, changing stmetures. For example, atoms can diflfiise along a terrace to or from step edges. When atoms diflfiise across a surface, they may move by hopping from one surface site to the next, or by exchanging places with second layer atoms. 

The power of optical spectroscopies is that they are often much better developed than their electron-, ion- and atom-based counterparts, and therefore provide results that are easier to interpret. Furtlienuore, photon-based teclmiques are uniquely poised to help in the characterization of liquid-liquid, liquid-solid and even solid-solid interfaces generally inaccessible by other means. There has certainly been a renewed interest in the use of optical spectroscopies for the study of more realistic systems such as catalysts, adsorbates, emulsions, surfactants, self-assembled layers, etc. 

Self-assembled monolayers (SAMs) are molecular layers tliat fonn spontaneously upon adsorjDtion by immersing a substrate into a dilute solution of tire surface-active material in an organic solvent [115]. This is probably tire most comprehensive definition and includes compounds tliat adsorb spontaneously but are neither specifically bonded to tire substrate nor have intennolecular interactions which force tire molecules to organize tliemselves in tire sense tliat a defined orientation is adopted. Some polymers, for example, belong to tliis class. They might be attached to tire substrate via weak van der Waals interactions only. 

Steiner U B, Cased W R, Suter U W, Rehahn M and Rau I U 1994 Self-assembled layers of substituted poly(p-phenylene)s on gold and copper Langmuir 10 1164-70... 

A second class of monolayers based on van der Waal s interactions within the monolayer and chemisorption (in contrast with physisorption in the case of LB films) on a soHd substrate are self-assembled monolayers (SAMs). SAMs are well-ordered layers, one molecule thick, that form spontaneously by the reaction of molecules, typically substituted-alkyl chains, with the surface of soHd materials (193—195). A wide variety of SAM-based supramolecular stmctures have been generated and used as functional components of materials systems in a wide range of technological appHcations ranging from nanoHthography (196,197) to chemical sensing (198—201). 

Constrained-Layer Treatments. Constrained-layer damping treatments consist of a thin layer (/ m) of viscoelastic material sandwiched between a base material and an outer constraining layer of sheet metal or other stmctural material. Some of these treatments are available with self-adhesives on both sides of the viscoelastic material and act as a bonding agent between the base and constraining layers others have the constraining layer already bonded to the inner layer so they need only be appHed to the base material. 

Various other soft materials without the layer—lattice stmcture are used as soHd lubricants (58), eg, basic white lead or lead carbonate [598-63-0] used in thread compounds, lime [1305-78-8] as a carrier in wire drawing, talc [14807-96-6] and bentonite [1302-78-9] as fillers for grease for cable pulling, and zinc oxide [1314-13-2] in high load capacity greases. Graphite fluoride is effective as a thin-film lubricant up to 400°C and is especially useful with a suitable binder such as polyimide varnish (59). Boric acid has been shown to have promise as a self-replenishing soHd composite (60). 

As fibers in the feed mat pass between the feed toU and feed plate, they ate separated by metallic wine teeth on the lickerin toU and carried to an air venturi where they ate stripped and tumbled until they strike a moving, perforated collection surface. At the collection surface, the airborne fibers foUow paths of least resistance and accumulate in a self-leveling manner while the air passes through perforations. Fiber orientation in the web is isotropic in layers corresponding to the number of fibers transferred from the wine teeth to the air-transportation 2one, the intensity of the air, and the speed of the collection surface. 

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