Monomolecular Films of Molecules Which Lie Flat on the Surface of Water I. Surface Pressures and Potentials of Films of Long Molecules: Polymers of ω-Hydroxy Decanoic Acid

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Abstract
Film pressures and surface potentials have been measured for a series of linear polymers of ω‐hydroxy decanoic acid, ranging in molecular weight from 780 to 25,200. The molecules are oriented parallel to the surface. An improved form of the apparatus of Harkins and Fischer was used. The chief modification in the procedure was the use of two radioactive polonium electrodes over the film. The pressure‐area relations show that the area per molecule in the condensed films is nearly proportional to the molecular weight and that the compressibility of the films is large. The film density is found to be lowest for the polymers of smallest molecular weight. The films are not very sensitive to changes in hydrogen ion concentration, but on changing from an acid to an alkaline substrate the films in the expanded state give higher molecular areas at given pressures, and the pressures for film collapse are considerably higher. Dimensions of the molecules are calculated and a complete table of mean values is included. The vertical cross section of the molecules varies between 19.4 and 19.9 sq. A or is nearly the same as the horizontal cross section (20.5) of vertically oriented molecules such as stearic acid. Some differences between films with perpendicular orientation to the surface and those with parallel orientation are tabulated. The surface potentials of the polymer films rise to between 400 and 450 mv, although the surface density of the polar groups is only 1/3 to 2/5 that of stearic acid type films, which also give potentials of about 400 mv. The contribution of each polar group is, therefore, considerably greater than that of the dipole of the vertically oriented stearic acid molecule. The surface potential of a condensed film was found to increase about 13 percent less rapidly than the surface density of the dipoles.

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