A study of the properties and composition of stearic acid monolayers on an aqueous subphase containing cadmium ions

Compression isotherms and chemical compositions of stearic acid (HSt) monolayers applied onto an aqueous subphase containing cadmium ions have been studied in a wide pH range. The data obtained have been used to calculate the dependences of surface elasticity E2.5, change ΔA2.5 in the surface area per molecule at a constant two-dimensional pressure in the monolayer of 2.5 mN/m, and change Δ|ζ| in the absolute value of the ζ potential of particles formed from the material of a monolayer collapsed on the surfaces of aqueous solutions with the same compositions on the pH of the subphase. The obtained data are in good agreement with each other and unambiguously attest to the dependences of the surface (E2.5, ΔA2.5) and electrosurface (Δ|ζ|) characteristics of the monolayers on their chemical composition. This leads us to make the reasonable suggestion that two alternative reactions may occur between cadmium ions and the HSt monolayer with the formation of two salts, CdSt2 and Cd(OH)St.