Pockels surface balance

Surface studies of insoluble monolayers of all the common unconjugated bile acids, including the unsubstituted cholanoic acid, have been carried out by a number of workers and thoroughly reviewed [5]. Being insoluble non-swelling amphiphiles with limited aqueous solubility, their surface pressure-area (v-A) isotherms can be measured satisfactorily with a Langmuir-Pockels surface balance on an aqueous subphase containing 3-6 M NaCl to salt out polar functions and at sufficient acidic pH (1-3) to prevent ionization [5,6). 

Pockels described in the letter her design of a rectangular tin trough with a thin tin strip laid across it. The trough was filled to the brim with water, with a thin layer of oil covering the surface of the water on one side of the tin strip and clean water on the other side. The tin strip served to vary the area of the oil-contaminated surface, and a balance measured the force necessary to lift a small disk (a button) from the surface. Pockels used this setup to study the surface tension of the oil-contaminated layer. 

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-... 

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. 

By the age of 20 Pockels had invented a surface film balance and was conducting studies of surface films and monomolecular layers. The surface film balance consists of a fixed barrier and a movable barrier on a water surface. A film of some insoluble material like a soap or detergent is spread on the water between the two barriers, and the movable barrier is then used to compress the surface film. The amount... 

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. 

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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