Surface Films on Liquids

When an insoluble liquid a is placed at the liquid air interface of another liquid P, the first liquid can spread out as a thin film which becomes monomolecular if the area of liquid P is sufficiently large. Everyone has experienced the colors which are seen when oil is spread on water in a muddy puddle on the road. The colors result from the interference of the light rays reflected from the oil air interface with those reflected at the water oil interface. The interference occurs because the oil film is very thin. The properties of thin films are especially interesting when their thickness corresponds to one molecule. 

The first question regarding thin films is how they spread on the surface of the substrate liquid P (see fig. 8.17). In order for this to occur, the Gibbs energy 

Examine the spreading of benzene on water, given that the interfacial tension of pure benzene is 28.18 mJ m , using the data in tables 8.1 and 8.10. 

From table 8.1, the interfacial tension of pure water is 71.81 mJ m , and from table 8.10, the interfacial tension between water and benzene is 34.4 mJm . Thus, the work of spreading is 

As a result, a small amount of benzene placed on the surface of water rapidly spreads. However, this simple calculation is naive in that it ignores the effect of benzene on the interfacial tension of pure water. When the benzene and water are in contact, a very small amount of benzene dissolves in the water to yield a saturated solution. Since the benzene is hydrophobic it accumulates at the water air interface and causes a significant lowering of the interfacial tension of the aqueous phase. After equilibrium is reached, yp is 62.2 mJ m. The work of spreading of benzene on water saturated with benzene is thus 

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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. 

A device for direct measurement of the surface pressure exerted by a surface film on liquids. The basic features of the trough include a horizontally positioned tray with a... 

Before proceeding to the main subject of this chapter—namely, the behavior and properties of spread films on liquid substrates—it is of interest to consider the somewhat wider topic of the spreading of a substance on a liquid surface. Certain general statements can be made as to whether spreading will occur, and the phenomenon itself is of some interest. 

Fuerstenau and Healy [100] and to Gaudin and Fuerstenau [101] that some type of near phase transition can occur in the adsorbed film of surfactant. They proposed, in fact, that surface micelle formation set in, reminiscent of Langmuir s explanation of intermediate type film on liquid substrates (Section IV-6). 

A third definition of surface mobility is essentially a rheological one it represents the extension to films of the criteria we use for bulk phases and, of course, it is the basis for distinguishing states of films on liquid substrates. Thus as discussed in Chapter IV, solid films should be ordered and should show elastic and yield point behavior liquid films should be coherent and show viscous flow gaseous films should be in rapid equilibrium with all parts of the surface. 

A Spread Lipid Films on Liquid Surfaces (Langmuir-Blodgett Films)... 

Langmuir s differential method for thin insoluble films on liquid surfaces. 

When an adsorbing surface is exposed to a gas or vapour adsorption will take place, being accompanied by the absorption or evolution of heat. Such thermal changes have already been noted in the extension and contraction of surface films of liquids. Although the direct determination of the surface energy of solid surfaces presents many experimental difficulties yet of its existence there is no doubt. On the adsorption of a gas or vapour a diminution in the free surface energy of the system likewise occurs. From the Gibbs-Helmholtz relationship dcr... 

Surface Viscosity Molecular films on liquid surfaces may be either readily mobile or slow to flow under the action of a two-dimensional stress. Some surface films show surface plasticity and behave as solids until a critical stress is applied. These films exhibit surface viscosity. 

The mechanical characteristics of thin films on liquids are described in a similar way to the three-dimensional case. The surface elasticity [618] is defined as... 

Amphiphilic molecules form monomolecular films on liquid surfaces. Some amphiphiles, such as phospholipids, with a large hydrophobic tail are practically insoluble in water and therefore form insoluble monolayers at the air-water interface. 

First, the absorption spectra of the Outer Segments of the photoreceptor cells rely on the resonant condition of one of the Rhodonines when it is present under specific conditions (Col. R of the Table). These conditions are that it is present as a liquid crystalline surface film on the disks of the segment (8R) and in contact with a de-excitation agent associated with the Inner Segment of the same cell (12R). 

Lenard2 suggested that the molecules in the surface of a liquid are oriented, and Langmuir3 and others gave experimental evidence of this by measurement of the area of insoluble films on liquid surfaces. Jung,4 by assuming electrically polarisable molecules of polarisability a, found ... 

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