Pockels point

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... 

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.)... 

Pockels Point. When surfactant molecules are added into a system and form an insoluble film at an interface, surface tension does not decrease very strongly until enough is added to form a complete monolayer. The transition point is termed the Pockels point and corresponds to a surface area occupied per molecule of about 20 A for soaps. 

The thermal motions of the molocules arc neglected in this argument. It happened that all the oils used in the earlier researches formed films in which the thermal motions could be neglected, as the molecules had such a large attraction for one another laterally in the films that they were united into compact masses much too large to show independent thermal motion. For such coherent films Pockels s law that the surface tension does not drop appreciably till the critical point is reached, and then drops suddenly, is true. 

The clearest results were obtained with the normal, saturated fatty acids and alcohols. These formed stable films, which would stand considerable compression laterally, and (at room temperature on distilled water) gave a very clearly marked critical area at which the surface pressure first appeared, this point being of course Pockels s critical point of the first diminution of surface tension. As the area was reduced from large initial areas no surface pressure could be detected until the area had reached about 22 sq. A. per molecule and at 20 5 sq. A. the pressure increased very rapidly indeed with further increase of pressure. The curve I of Fig. 15 shows the relation between surface pressure and area per molecule, which is obtained with accurate apparatus for the fatty acids on water curve III is that obtained with the alcohols.3... 

Exact experimental methods. Surface-pressure measurements. Nearly all the methods in use at present are modifications of Langmuir s method, of directly measuring the outward force exerted on a light floating barrier which divides the film-covered surface from a clean surface and for the manipulation of the films, and adjustment of the area available for them, modifications of Pockels s barriers are almost always used. Ths essential points requiring attention for accuracy are as follows ... 

Figure 7 Schematic diagram of the laser-induced electrical birefringence apparatus. Path of Nd YAG laser pulse EP, extracavity polarizers PC, Pockels cell L, lens (focal length 63.5 cm, focal point about 20 cm past the sample cell) D, dichroic mirror SC, sample cell C, calorimeter BS, beam splitter FI, UV-Vis cutoff filter F2, narrow-bandpass filter (1.060 /im) PD, photodiode LT, light trap. Path of He-Ne monitoring beam P, polarizer 2/4, quarter-wave plate A, analyzer F3, narrow-bandpass filter (632.8 nm) G, glass plate diffuser PM, photomultiplier. OSC, oscilloscope X timing and trigger. (Reprinted with permission from Ref 27. Copyright 1995 American Chemical Society.)...

Linear (Pockels effect). This effect is characterized by a square hysteresis loop having a large E. It occurs for tetragonal type compositions at the PbTiOa-rich end of the solid-solution range such as 8/36/60. An is linear with respect to E from a practical point of view, grain size has a very significant effect on the linearity. 

Table 2. Pressure-Area isotherm data for Pockels-Langmuir films. IT and A. are the pressure and molecular area, respectively, at the collapse point.

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