Contact Potentials and the Effects of Unimolecular Films on Surface Potentials. I. Films of Acids and Alcohols (1933)

Simultaneous measurements of film pressure and surface potential have been carried out by the use of an apparatus designed in such a way that the potential may be determined for any location on the surface of the film. At film pressures above that of the gaseous films, organic substances with homo-heteropolar molecules give a single smooth curve for the relation between surface potential and molecular area. At areas sufficiently great to reduce the pressure to that of the gaseous film the surface potential becomes variable and remains variable until the area becomes so great that the continents and islands of condensed film evaporate in the two-dimensional system to give a gaseous film alone. For example with films of myristic acid at 17° the surface potential is represented by a single curve below a molecular area of about 50 sq. Å, and by any value below 170 mv at higher areas, at which islands in the film persist. The areas above which the surface potentials become variable, due to the effects of islands or continents of film, are found to be about as follows: stearic acid, 28; palmitic acid, 28; pentadecylic acid, 41; lauric acid (on NaCl-HCl solution), 36; oleic acid, 52; cetyl alcohol, 23.7; myristyl alcohol, 27. The surface potential rises rapidly with the number of carbon atoms in the hydrocarbon chain, and is about 400 mv for condensed films of stearic acid. It is higher for alcohols than for acids with the same number of carbon atoms. The increment is about that given by the addition of two carbon atoms to the acid. The change of one of the single bonds of stearic acid to the double bond of oleic acid as the film forming substance lowers the surface potential by from 37 to 65 percent, and makes this potential much more greatly affected by the molecular area. The following numbers give the number of carbon atoms in several normal acids and the corresponding potentials at 20 sq. Å at 17°: (12), 276; (14), 336; (15), 375; (16) 383; and (18), 410. The surface potential increases with decrease of temperature, with an increase of concentration in the film, and for the substances investigated, by a replacement of a basic by an acid solution. A collapse of the film lowers the surface potential since it changes the molecular orientation in the film. The films exhibit in the condensed state considerable hysteresis, both with respect to film pressure and surface potential. This feature has not been sufficiently noted in other work. The relation between surface potential and the molecular moments of the molecules which constitute the film is discussed. The dipole moment as calculated from surface potentials by the equation of Helmholtz are only about one-sixth as large as those found by ordinary methods for independent molecules in nonpolar solvents. The discrepancy is attributed partly to a lowering of potential by an orientation of the dipoles with respect to each other, and partly to the too great simplicity of the equation. The apparatus used for this work has been found to give good determinations of the contact potentials of metals. In its design use was made of ideas suggested principally by Lord Kelvin, and by Guyot and by Frumkin.