Surface Films on Liquid Substrates

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 

A sketch of Langmuir s film balance is shown in Ref. 7 and a modem version of a film balance, in Fig. IV-5. 

Langmuir also gave needed emphasis to the importance of employing pure substances rather than the various natural oils previously used. He thus found that the limiting area (at the Pockels point) was the same for palmitic, stearic, and cerotic acids, namely, 21 per molecule. (For convenience to the reader, the common names associated with the various hydrocarbon derivatives most frequently mentioned in this chapter are given in Table IV-1.) 

This observation that the length of the hydrocarbon chain could be varied from 16 to 26 carbon atoms without affecting the limiting area could only mean that at this point the molecules were oriented vertically. From the molecular weight and density of palmitic acid, one computes a molecular volume of 495 A a molecule occupying only 21 A on the surface could then be about 4.5 A on the side but must be about 23 A long. In this way one begins to obtain information about the shape and orientation as well as the size of molecules. 

The preceding evidence for orientation at the interface plus the considerations given in Section III-3 make it clear that the polar end is directed toward the water and the hydrocarbon tails toward the air. On the other hand, the evidence from the study of the Gibbs monolayers (Section III-7) was that the smaller molecules tended to lie flat on the surface. It will be seen that the orientation 

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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. 

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