The structure of surface films XXI—Surface potentials of dibasic esters, alcohols, aldoximes, and ketones

Abstract

The object of this work was to obtain further information on several points affecting the dependence of surface potentials of unimolecular films on water surfaces, on the constitution and orientation of the molecules in the films. Measurements on the dibasic esters were expected to throw light on the relation between surface potential and density of packing of molecules in films of “gaseous” type; the measurements on alcohols, aldoximes, and methyl ketones were undertaken primarily in order to obtain values for the surface potential produced by the polar groups in these compounds, which could be compared with the potentials found with derivatives of the sterols, in which the same polar groups are attached to a complex ring system, instead of to an aliphatic straight carbon chain. The surface potential ΔV is the change in contact potential between the aqueous solution on which the film is spread, and air, caused by the presence of the surface film. The simplest basis of comparison is the quantity μ given by the equation ΔV = 4π n μ, n being the number of molecules per square centimetre of the films. If the dipoles of the film molecules were arranged as a parallel plate condenser in the surface, Kμ cos θ would be the dipole moment of the molecules, θ being the angle at which these dipoles are inclined to the vertical, and K the dielectric constant of the surface film. θ is in general unknown; but by comparsion of the highest values of μ obtained, with the values for the dipole moments of various polar groups in organic molecules obtained by other methods, it may be concluded that the value of K in the surface films is usually of the order 5 to 10.