Insoluble surface films

A second way of classifying the material is on the basis of the experimental methods involved. For mobile interfaces, surface tension is easily measured. For these it is easiest to examine the surface tension-adsorption relationship starting with surface tension data. When insoluble surface films are involved, we shall see how the difference in y between a clean surface and one with an adsorbed film may be measured directly. For solid surfaces, surface tension is not readily available from experiments. In this case adsorption may be measurable directly, and the relationship between adsorption and surface tension may be examined from the reverse perspective. 

Insoluble surface films can be studied by electron microscopy. The films are transferred from the substrate on to a collodion support and shadow-cast by a beam of metal atoms directed at an angle a (about 15°) to the surface (Figure 4.21). If the width x of the uncoated region is measured, the thickness of the film, x tan a, can be calculated for example, a /i-C36H73COOH film has been shown to be about 5 nm thick - i.e. consistent with a vertically orientated monomolecular layer. The technique has also been used for following the state of the surface as a film is compressed. 

Basically, the final choice of the cation has to relate strictly to the application. The presence of cations such as Li+ or Na+ in solutions may lead to precipitation of insoluble surface films or noble metal electrodes and thus interfere with the basic electrochemical behavior of many redox couples on nonactive metal electrodes in polar aprotic solvents [9], The use of tetraalkyl ammonium salts eliminates this problem because the thermodynamics of insoluble salt precipitation on electrodes differs in the presence of these bulky cations from that developed in the presence of cations of alkaline or alkaline earth metals [6-9],... 

Indications of further complexities in the coherent type of surface films were found by Langmuir, and have been further investigated by Labrouste,1 Adam, and others. It is now well established that there frequently exists a coherent, expanded state in insoluble surface films of fatty substances, intermediate in area between the very closely packed condensed films, and the gaseous films in which the molecules move about independently on the surface. This most interesting type of film will be discussed in 18 and 19. 

The action at these interfaces includes interfacial tensions, adsorption, the spreading of liquids on surfaces, insoluble surface films, and the catalytic activity of various solid surfaces for many types of chemical reactions. 

Dysthe K, Rabin Y (1986) Damping of short waves by insoluble surface films. In ONRL Workshop Proceedings - Role of surfactant films on the interfacial properties of the sea surface, ed. HerrFL and Williams J, U.S.Office of Naval Research, Arlington, Va, pp 187-213... 

Iron and steel are readily attacked by both dilute and concentrated orthophosphoric acid. The dilute acid will form insoluble surface films of acid iron phosphates which act as protective layers and prevent further oxidation (Section 12.7). 

In the presence of water, superficial pitting corrosion can occur. As shown by Bailey in 1920 [5] and confirmed by Takatani [6], corrosion will cease soon due to the formation of an insoluble surface film of calcium aluminate Ca(A102)2 by the reaction of aluminate ions AIO2 that are present at pH values above 12, with Ca ions originating from the dissociation of Ca(OH)2-... 

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