Interaction of monomolecular films of biological significance with heavy metal ions and complexes at the air-water interface: I. Stearic acid and zinc, cadmium, and mercury

Abstract

The interfacial interactions between stearic acid monolayers and the heavy metal ions, zinc, cadmium, and mercury have been investigated as part of a broader study investigating the effects of heavy metals on model monolayers simulating the human alveolus. The work has shown a strong interaction between the stearic acid film and ions in solutions of Zn, Cd, and Hg chloride. Changes in surface properties were caused by an interaction of the divalent cation of the salts with the negatively charged carboxyl head group of stearic acid. The affinity of the metal ions for the film molecules is in the order Cd > Zn > Hg. It is postulated that the interaction occurs in a 2:1 ratio of fatty acid to metal ion. While the cadmium as Cd 2+ ion is present at a lower percentage at the same salt concentration compared to Zn, it had a greater effect on surface potential and a nearly equivalent effect on the surface pressure of the stearic acid films. The activity of the Hg 2+ ion for causing changes in the surface properties of the films was much less than that observed for Zn and Cd. This may be related to the very low concentrations of Hg 2+ present in the solutions studied, and may not be a true indication of the intrinsic Hg 2+ ion affinity of the charged film site. Despite this low concentration, Hg 2+ still had an observable effect on the stearic acid compression isotherm. The interactions studied may have implications related to the toxic and physiologic effects of heavy metals on living tissues.