Films surfaces

FIGURE 22.11 Surface pressure IT versus area. For each material, thex-axis corresponds to the average area per molecule in the film. Source Ya. Gerasimov, Physical Chemistry, Mir Publishers, Moscow, 1974. 

Unless othetwise noted, all art on this page is Cengage Learning 2014. 

Certain insoluble substances will spread on the surface of a liquid such as water until they form a monomolecular layer. Long-chain fatty acids, stearic acid and oleic acid, are classical examples. The —COOH group at one end of the molecule is strongly attracted to the water, while the long hydrocarbon chain is hydrophobic. 

A shallow tray, the Langmuir tray, is filled to the brim with water (Fig. 18.13). The film is spread in the area between the float and the barrier by adding a drop of a dilute solution of stearic acid in benzene. The benzene evaporates leaving the stearic acid on the surface. The float is attached rigidly to a superstructure that allows any lateral force, indicated by the arrow, to be measured by means of a torsion wire. 

By moving the barrier, we can vary the area confining the film. If the area is reduced, the force on the barrier is practically zero until a critical area is reached, whereupon the force rises rapidly (Fig. 18.14a). The extrapolated value of the critical area is 0.205 nm per molecule. This is the area at which the film becomes close packed. In this state the molecules in the film have the polar heads attached to the surface and the hydrocarbon tails extended upward. The cross-sectional area of the molecule is therefore 0.205 nm.  

The force F is a consequence of the lower surface tension on the film-covered surface as compared with that of the clean surface. If the length of the barrier is /, and it moves a 

Note that T is a force per unit length of the barrier, which is equal to that on the float. From curve 1 in Fig. 18.14(a) and Eq. (18.40), we see that the surface tension of the film-covered surface is not very different from that of the clean surface until the film becomes close packed. 

Most metals and alloys are susceptible to erosion-corrosion damage. Many depend upon the development of a surface film of some sort (passivity), for resistance to corrosion. Examples are aluminum, lead, and stainless steels. Erosion-corrosion results when these protective surfaces are damaged or 

Material Typical corrosion rates (Mdd weight loss In milligrams per square decimetre per day)  

The entry of hydrogen into metals is promoted by various compounds, especially those involving elements from groups VA (P, As, and Sb) and VIA (S, Se, and Te) of the periodic table. In an effort to explain differences in the effectiveness of promoters, several studies have been performed to determine the manner in which they influence the behavior of adsorbed hydrogen. 

The experimental evidence indicates that the hydride of the promoter is the species which primarily affects hydrogen entry and that the amount of hydrogen adsorbed is related to the bond strength of the hydride. Further details on the role of promoters can be found elsewhere. 

If the film is reduced during cathodic polarization, the surface condition will clearly be time-dependent, so the absorption charac- 

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Radiographic image of 800 mm thick reinforced concrete wall with vertical and horizontal bars clearly visible, in this case some 150 mm from the concrete surface (film side). 

W. D. Harkins, The Physical Chemistry of Surface Films, Reinhold, New York, 1952. 

The surface elasticity E is found to vary linearly with t and with a slope of 2. Obtain the corresponding equation of state for the surface film, that is, the function relating t and a. 

About this time Miss Pockelsf [3] showed how films could be confined by means of barriers thus she found little change in the surface tension of fatty-acid films until they were confined to an area corresponding to about 20 per molecule (the Pockels point). In 1899, Rayleigh [5] commented that a reasonable interpretation of the Pockels point was that at this area the molecules of the surface material were just touching each other. The picture of a surface film... 

This chapter and the two that follow are introduced at this time to illustrate some of the many extensive areas in which there are important applications of surface chemistry. Friction and lubrication as topics properly deserve mention in a textbook on surface chemistiy, partly because these subjects do involve surfaces directly and partly because many aspects of lubrication depend on the properties of surface films. The subject of adhesion is treated briefly in this chapter mainly because it, too, depends greatly on the behavior of surface films at a solid interface and also because friction and adhesion have some interrelations. Studies of the interaction between two solid surfaces, with or without an intervening liquid phase, have been stimulated in recent years by the development of equipment capable of the direct measurement of the forces between macroscopic bodies. 

Macromolecular Surface Films, Charged Films, and Langmuir-Blodgett Layers... 

This chapter concludes our discussion of applications of surface chemistry with the possible exception of some of the materials on heterogeneous catalysis in Chapter XVIII. The subjects touched on here are a continuation of Chapter IV on surface films on liquid substrates. There has been an explosion of research in this subject area, and, again, we are limited to providing just an overview of the more fundamental topics. 

Photoelectrochemistry may be used as an in situ teclmique for the characterization of surface films fonned on metal electrodes during corrosion. Analysis of the spectra allows the identification of semiconductor surface phases and the characterization of their thickness and electronic properties. 

Concentrated nitric acid renders the metal passive , i.e. chemically unreactive, due to formation of a thin oxide surface film (which can be removed by scratching or heating in hydrogen). 

Sodium wire, produced with a sodium press (Fig. II, 47, 1), is 6rst collected in sodium-dried ether, the necessary quantity removed, rapidly dried between 61ter paper,and transferred to the flask. Tlirn shavings of sodium, although less satisfactoiy may also be employed, but it is important to avoid undue exposure of the sodium to the atmosphere which produces a surface film of sodium hydroxide. 

The diffusion coefficient depends upon the characteristics of the absorption process. Reducing the thickness of the surface films increases the coefficient and correspondingly speeds up the absorption rate. Therefore, agitation of the Hquid increases diffusion through the Hquid film and a higher gas velocity past the Hquid surface could cause more rapid diffusion through the gas film. 

Some nonhygroscopic materials such as metals, glass, and plastics, have the abiUty to capture water molecules within microscopic surface crevices, thus forming an invisible, noncontinuous surface film. The density of the film increases as the relative humidity increases. Thus, relative humidity must be held below the critical point at which metals may etch or at which the electrical resistance of insulating materials is significantly decreased. 

In steel-on-steel lubrication with a zinc dialkyl dithiophosphate additive, a complex surface paste appears to form first of zinc particles and iron dithiophosphate. The iron dithiophosphate then thermally degrades to a brown surface film of ZnS, ZnO, FeO, plus some iron and zinc... 

In neutral and alkaline environments, the magnesium hydroxide product can form a surface film which offers considerable protection to the pure metal or its common alloys. Electron diffraction studies of the film formed ia humid air iadicate that it is amorphous, with the oxidation rate reported to be less than 0.01 /rni/yr. If the humidity level is sufficiently high, so that condensation occurs on the surface of the sample, the amorphous film is found to contain at least some crystalline magnesium hydroxide (bmcite). The crystalline magnesium hydroxide is also protective ia deionized water at room temperature. The aeration of the water has Httie or no measurable effect on the corrosion resistance. However, as the water temperature is iacreased to 100°C, the protective capacity of the film begias to erode, particularly ia the presence of certain cathodic contaminants ia either the metal or the water (121,122). 

The seminal discovery that transformed membrane separation from a laboratory to an industrial process was the development, in the early 1960s, of the Loeb-Sourirajan process for making defect-free, high flux, asymmetric reverse osmosis membranes (5). These membranes consist of an ultrathin, selective surface film on a microporous support, which provides the mechanical strength. The flux of the first Loeb-Sourirajan reverse osmosis membrane was 10 times higher than that of any membrane then avaUable and made reverse osmosis practical. The work of Loeb and Sourirajan, and the timely infusion of large sums of research doUars from the U.S. Department of Interior, Office of Saline Water (OSW), resulted in the commercialization of reverse osmosis (qv) and was a primary factor in the development of ultrafiltration (qv) and microfiltration. The development of electro dialysis was also aided by OSW funding. 

Metals having tenacious surface films that make roU bonding difficult, eg, stainless steel/Cr—Mo steels, can be explosion clad. 

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