Pockels-Langmuir films

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

The variant of 11 with an undecyl tail followed by a thioacetyl termination ( Cll thioacetyl ) gave disappointing results the Pockels-Langmuir film collapsed at relatively low surface pressures, compared to 11, and yielded disordered LB monolayers, with competition between strong physisorption by the dicyanomethanide end of the molecule and Au-to-thiolate chemisorption. The monolayer rectified in either direction, depending on where in the LB monolayer, that is, on which molecule ( right side up or upside down ) the STM tip was probing. ... 

Figure 5.3 Film balances a. Langmuir trough having a movable barrier and a Wilhelmy tensiometer to measure the spreading pressure, n, for water insoluble monolayers, b. PLAWM (Pockels, Langmuir, Adam, Wilson and McBain) trough used for partially water-soluble monolayers, where a flexible membrane, which is fixed to the barrier, separates the surfactant solution and pure water departments to prevent the passage of dissolved surfactant molecules into the pure water department beneath the barrier.

Film Balance A shallow trough that is filled with a liquid on top of which is placed material that may form a monolayer. The surface area available can be adjusted by movable barriers, and any surface pressure thus created can be measured by means of a float. Also called Langmuir Film Balance, Langmuir Trough, and Pockels—Langmuir—Adam—Wilson—McBain Trough or PLAWM Trough. 

For many years, the main experimental tool for these studies was the film balance (see Figure 4.19). Various workers, including Pockels, Langmuir, and Adam, made major contributions to its development (see Gaines, 1966). A small, known quantity of the surfactant to be studied is dissolved in a volatile solvent and deposited carefully by pipette on the surface of a pool of water. The solvent is chosen so that it spreads rapidly over the water and then evaporates, leaving the surfactant uniformly distributed as a monomolecular layer (or monolayer) in the region between the two barriers. One of the barriers is movable, so that the area occupied by the surfactant film can be varied. A torsion balance is provided to measure the surface pressure (i.e., the difference between the surface tension of pure water and that of the film-covered surface). More commonly in modem instruments, a Wilhelmy plate is used to measure surface tension in the film region. 

PLAWM Trough. Pockels-Langmuir-Adam-Wilson-McBain trough. See Film Balance. 

In 1916 Langmuir published his well-known paper on The fimdamental properties of liquids, in which he accepted the conclusion that films of oils on water were unimolecular and made further experiments on the subject by a direct method. Pockels and Rayleigh had measured the surface tension of their films by determining the force necessary to break them, a method open to some obvious objections. The French investigators did not as a rule measure the tensions of the films at all but determined their... 

While the experiments of Pockels and Rayleigh generated some activity on the properties of thin films, the area was essentially dormant until the work of Irving Langmuir (1881-1957 Nobel Prize, 1932, for surface chemistry) at the General Electric Corporate Research Laboratories in Schenectady, New York. Langmuir developed a number of new techniques... 

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 ... 

The first measurement of (what is now called) a t(A] curve comes from Pockels ). She did many experiments on her kitchen table and realized the relevance of sweeping the surface before measuring. Langmuir gave generous credit to her work, although he largely independently developed the film balance that nowa-... 

As part of his work on surfaces, Langmuir studied monolayer films on water surfaces, with techniques pioneered by a chemist who started her studies in greasy dishwater Agnes Pockels. 

Following observations by Lord Rayleigh and Miss A. Pockels, which showed that oil films on water are unimolecular (one molecule thick), Langmuir devised a film-pressure balance which confirmed this result. In the case of films of fatty acids the cross-section of the molecule was independent of the length of the hydrocarbon chain, which indicates that the carboxyl group at the end of the molecule is sunk in the water and the insoluble hydrocarbon chain is outside the water surface. Much work on the compressibility of surface films gave pressure-surface curves with a superficial resemblance to the pressure-volume curves of van der Waals s equation. 

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