Ionic Interactions of Fatty Acid Monolayers at the Air/Water Interface

Abstract

The static properties of stearic and arachidic acid monolayers at the air-water interface in the presence of various divalent metal ions (Mg2+, Ca2+, Ba2+, Co2+, Cd2+, and Pb2+) were examined by use of the ionizing electrode method for the measurement of surface potential coupled with the Wilhelmy plate method of surface tension measurement. At pH 6.0, the presence of all the divalent cations results in the condensation of the monolayer, with Pb2+ and Cd2+ having the most significant effects. Moreover, only Pb2+ has such an effect at and below pH 5.0. The surface potential of stearic acid at pH 6.0 and 20 A2/molecule changes with the concentration of the metal ion in the subphase, and the magnitude and direction of this change are dependent on the type of the metal ion and its respective counterion. The surface potential profiles relative to the metal ion concentration are shown to be concave upward with a ranked order of Pb2+ > Cd2+ > Co2+, whereas those of alkaline earth metals are respectively convex upward, flat, and slightly concave upward for Ba2+, Ca2+, and Mg2+. It is concluded that monolayer ordering induced by covalent binding of carboxylate-metal ion proceeds in the order Pb2+ > Cd2+ > Co2+, whereas alkaline earth ions exhibit no such ordering effect since their interactions with carboxylates are mainly ionic in nature. © 1990, American Chemical Society. All rights reserved.