On the surface potentials of unimolecular films of long chain fatty acids.–Part I. Experimental method

Molecular orientation in a film formed at an air liquid interface was ascribed by Hardy to an inwardly directive attractive force along the normal to the surface, the condition of minimum potential involving two terms, one relating to the variation in density, the other to the orientation of the field with the "polar" groups, which are accordingly attracted inwards towards the bulk phase. That "polar" groups do in fact posses large fields is confirmed from an examination of the electric moments of compounds containing these groups. In previous communications it was shown that the characteristics and phase of films could be affected not only by alterations in the variables of temperature and two dimensional pressure as shown experimentally by Langmuir and by Adam, but also by altering the extent of the adhesion of the asymmetrically situated polar group to the substrate. Alterations in the extent of this adhesion will naturally affect the field and it is the object of this communication to attempt an examination of the behaviour of films from this point of view. The modification of an interfacial potential difference produced by the presence of a unimolecular film was shown by Helmholtz to be given by the expression 4 πn μ = ∆V where the change in potential ∆V is caused by n molecules per square centimetre of the film forming substance, each molecule possessing an average electric moment of effective vertical component μ.