This paper extends the measurements on the change in contact potential caused by a monomolecular insoluble film, at an air-water interface, to the series of nitriles, C
n
H
2
n
+1
CN. The length of the carbon chain was varied from 14 to 22 total carbon atoms, including the carbon in the end CN group. The nitriles were chosen because their end group, ON, is one of the few organic groups in which the resultant dipole moment of the end group cannot be altered by internal rotation of the various parts of the group. In nearly all previous studies, those on the fatty acids, alcohols, esters, a comparison of the vertical component of the dipole moment of the molecules in the films, with the moments of similar molecules in free space, is hindered by the very large variations which might occur in the resultant dipole moment of a polar group, when the atoms of the group assume different relative positions owing to rotation round the linkages between atoms. For instance, using Eucken and Meyer's data for the component parts of the dipole of the carboxyl group, the vertical component of the total dipole moment can assume any value between —4·2 and + 13·2 × 10
-19
e.s.u.§ The experimental technique was the same as that described in our previous paper (
loc. cit
.), except that sometimes a copper wire bearing a little polonium at the lower end was used in place of the disc. This renders it somewhat easier to detect inhomogeneities in the film. Simultaneous measurements were taken of surface pressure and surface potential. All experiments were done at room temperature, which was never above 23·5° 0. or below 16° C. Petroleum ether (60—70° 0.) was used as the solvent for dropping on the surface in all experiments; the underlying solution was N/100 hydrochloric acid.
Surface potential study of monolayers of surfactant donor and acceptor molecules
