Contact Potentials of Reversible Soluble Films of Lauric Acid

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WHEN a nearly saturated solution of lauric acid in distilled water is placed in an Adam-Jessop film-pressure trough, the soluble adsorbed film may be compressed or swept in the same manner as an in soluble film of a higher fatty acid. However, the surface is rapidly replenished by diffusion from below, after sweeping, or the compressed film diffuses down-ward. In either case the reading of the film-balance returns to zero, in a period of about thirty minutes at 25° C. Similar effects have been noticed with benzopurpurin solutions1 and with soap solutions2. Since the contact potential of solutions is proportional to the concentration of the surface-active solute, simultaneous contact potential measurements with a polonium-covered silver electrode on the above surfaces might be expected to yield potential-time curves nearly parallel to the force-time curves. Such proved to be the case, with the following notable differences.

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  1. 1

    Doss, Current Sci., 4, 405 (1935).

  2. 2

    McBain and Wilson, private communication, and J. Amer. Chem. Soc., 58, 379 (1936).

  3. 3

    J. Chem. Phys., 1, 852 (1933).

  4. 4

    Ann. Phys., x, 15, 5 (1931).

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