Abstract
This paper presents certain results on the electrical properties of multilayers of stearic acid and various stearates plated on insulating materials. The experimental data lead to the conclusion that the electrical charges of these multilayers arise from the adsorption of ions from the substrate on the carboxyl groups of the floating stearate monolayer. With a calcium-bearing substrate, for example, the first stage in the process is a conversion of stearic acid to calcium stearate and this reaction is a function of the substrate pH and consequently of the degree of ionization of stearic acid. The next stage is represented by an adsorption of calcium ions on the carboxyl groups of the calcium stearate which are carried along with the monolayer in the plating process. The electric charges of X-multilayers thus arise from volume distributions of positive adsorbed ions. The positive ions adsorbed on the floating monolayer induce a cloud of negative ions in their neighborhood which, under certain conditions such as dependence on size and valency of the negative ions, may lead to electrically neutral and wetted films. With certain types of mixed films, i.e., where the percentage of stearic acid present is appreciable, the rate at which the plating process is carried out becomes an important factor in determining whether X- or Y-type films can be built up, and for these cases fast dipping and withdrawal speeds can neutralize an existing multilayer charge. In plating X-multilayers it has been observed that after about 500 layers have been deposited, and the electrostatic repulsive field of the multilayer has thus reached a certain value, the upper portion of the submerged probe has a silvery appearance which gradually moves down the probe with increasing number of dips, and film does not adhere to the multilayer over this portion. This electrostatic repulsive field thus sets a limit to the thickness of X-multilayers on insulators. Under certain conditions the outer layer of multilayers dipped in film-free solutions overturns and adsorbs ions from the solution, resulting, in some cases, in wetted films and, in others, in an electrical charging of the multilayers.
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