Surface spread technique

An alternative method for the visualization of small structures is shadowing with heavy metals (platinum, tungsten, etc.). Rotary shadowing in combination with the surface spreading technique of Kleinsmidt [7] appeared to be very powerful. In this method macromolecules are decorated with platinum grains of about 40 A which indirectly visualize the shape of these molecules. Double-stranded and even single-... 

By using a laser with less power and the beam spread over a larger area, it is possible to sample a surface. In this approach, after each laser shot, the laser is directed onto a new area of surface, a technique known as surface profiling (Figure 2.4c). At the low power used, only the top few nanometers of surface are removed, and the method is suited to investigate surface contamination. The normal surface yields characteristic ions but, where there are impurities on the surface, additional ions appear. 

In recent years there has been an increased interest in cellular leathercloth using modified spreading techniques. In some leathercloth formulations the surface has an undesirable tackiness. In such cases this can be overcome by applying a PVC-acrylic lacquer. 

Next let us consider some of the physical properties of the spread monolayer we have described. Equation (1) states that the surface tension of the covered surface will be less than that of pure water. It is quite clear, however, that the magnitude of 7 must depend on both the amount of material adsorbed and the area over which it is distributed. The spreading technique already described enables us to control the quantity of solute added, but so far we have been vague about the area over which it spreads. Fortunately, once the material is deposited on the surface, it stays there —it has been specified as insoluble and nonvolatile for precisely this reason. This means that some sort of barrier resting on the surface of the water may be used to corral the adsorbed molecules. Furthermore, moving such a barrier permits the area accessible to the surface film to be varied systematically. In the laboratory this adjustment of area is quite easy to do in principle. As we see below, the actual experiments must be performed with great care to prevent contamination. 

Next let us consider those difficulties associated with the determination of the amount of material deposited on the surface. We have already noted that the method of depositing insoluble monolayers by spreading permits the accurate determination of n. Since the spreading technique requires solvent volatility, care must be exercised to prevent the stock solutions from changing concentration due to evaporation prior to their application to the surface. Also, precise microvolumetric methods must be used to dispense the solution on the aqueous surface since the quantity used is small. The solvent (as well as the solute) must be free from contaminants. There is also the possibility that the solvent will extract spreadable contaminants from the waxed surfaces of the float, barriers, and tray. Some workers advocate addition and evaporation of one drop at a time to minimize this. Oily contaminants may also reach the water surface from the fingers and from the atmosphere. These last sources are particularly hard to control Tests for reproducibility and blank compressions (i.e., moving the barrier toward the float on a clean surface) are the best evidence of their absence. 

Low energy ion scattering (LEIS) is the most surface-sensitive technique available today and is therefore exceptionally well suited for the study of spreading and wetting in... 

TiTany authors (I, 2, 3) have compared the surface behavior of macro- molecules, especially proteins, with the behavior of low molecular weight monolayers. This paper notes a series of effects that occurred when bovine serum albumin (BSA) was spread on various clean liquid surfaces and was compressed or decompressed. The transfer of the protein monolayer and of some small chain monolayers was also studied using a surface distillation technique. 

In this approach the polymer is dissolved in a suitable solvent which is then applied as a droplet (of volume several microliters) to the surface. Spreading and evaporation produce a thin film. With this technique the loading of the polymer can be readily calculated from the solution concentration and droplet volume. However, although simple to use, the technique is only really suitable for modification of small areas (up to about 1 cm ) and there can be problems in achieving uniform film coverage. This approach has also been used with organosilanes to prepare copolymer films held together by -Si-O-Si- bonds formed by hydrolysis [105-111]. 

Chemical modification of surface silanol groups on the glass surface affects both the spreading of liquid films [97] and the residual adsorptive properties of glass capillary columns. Thus, the surface wettability problems can largely be solved through different individual surface treatment techniques, or their combination. Most of the recent developments in this area have been reviewed [100]. 

The electron beam spreads out as the primary electrons strike the surface. This means that the spatial resolution is limited by how much the beam spreads within the material ( 2 pm). The probability of X-ray emission reduces with atomic number, so elements with low atomic number are difficult to detect and helium and hydrogen have never been detected using EPMA. The depth of penetration of the electron beam into the material is 2 pm. This means that EPMA is not a truly surface sensitive technique but is more sensitive to the bulk. 

The main reason for this was the lack of surface sensitive techniques that could probe interfaces at the proper molecular level. Therefore most attention was directed to understand the bulk properties whereas interface properties were only addressed by macroscopic, empirical studies like wetting and spreading. 

Fig. 10.3 Approaches available for the preparation of honeycranb-structured porous films via (i) solvent casting (ii) spin-coating (iii) dynamic airflow technique (iv) dip-coating, and (v) on-water surface spreading. Reprinted with permission fi-om ref [27]...

In many laminating operations, plastic films are used as part of the composite. Since many of these will have poor surface-wetting characteristics, especially to water, it is necessary to use surface-modification techniques in line with the laminating operation to enhance adhesion (see Wetting and spreading). One system commonly used is Corona discharge treatment. 

Two major techniques have been used to study the dimension of chromosome fibers by electron microscopy. The first is the surface spreading method, in which the cells are spread on a water-air interphase, and all the intracellular components are dispersed. But the chromatin and the spindle remain close together and can be picked up on a grid, which can then be prepared for electron microscopic examination. The second technique is the thin-sectioning method. After fixation the tissue is embedded in plastic and cut less than 1000 A thick. Refer to specialized texts for descriptions of these techniques [119]. The important finding is that with the first method, the dimension of the fibers diameter is estimated to be 200-300 A, whereas it is only 80-180 A with the second. 

Many Immunological and enzymatic reactions have been studied by surface film technique. In addition, various aspects of protein structure can be deduced by investigation of spread protein films, for example, the study of compressibility of a protein film permits the determination of the molecular weight of the protein molecules forming the film. Tumit in 1954 has described a method for obftiining protein diffusion coefficient (D) by monolayer technique which is less cumbersome and is speedy too. Lastly, surface films have also been used to study the effect of irradiation of proteins and the results supplement the infonnatlon obtained by more conventional methods. 

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