The electrical conductivity caused by insoluble monomolecular films of fatty acid on water

The well-known researches on thin films of insoluble fatty substances on water have brilliantly demonstrated that such films are monomolecular and that the molecules are oriented with their polar groups in contact with the water. Thus stearic acid spreads on water with its carboxyl groups on the water surface. No one, however, has as yet investigated the possible dissociation of such fatty acids with formation of free hydrogen ions in the water, leaving the fatty ions behind in a monomolecular film on the surface. Such dissociation need not and probably should not involve more than a small fraction of the molecules on the surface. One of us has pointed out the enormous voltages which would be caused between a complete monomolecular layer of fresh ions and the charges of opposite sign, vastly exceeding the electromotive forces of vigorous chemical reactions. This and many other lines of evidence have led to the theory of sparse surface dissociation," according to which a surface such as that of a colloidal particle may be largely covered or consist of undissociated molecules or accorded ion pairs, forming a double layer in the strict Helmholtz sense, but with a further portion of the surface covered only with unbalanced or singly adsorbed ions whose partner remain free and mobile somewhere in the solution.