DIVALENT-CATION STEARIC-ACID MONOLAYER INTERACTIONS AT THE AIR-WATER-INTERFACE

Abstract

The structure and dynamics of stearic acid monolayers at the air/water interface have been examined by means of in situ ellipsometry, surface potential, and surface light scattering. The results of the ellipsometry and surface potential experiments are completely in accord with our model proposed earlier that Pb2+ and Cd2+, unlike those of alkaline-earth ions, are specific in modifying the aqueous head group structure of fatty acid monolayers without affecting the thickness of the hydrocarbon tail layer. Pursuing the head group structure modifications by these ions, we determined the viscoelastic parameters of the monolayer with various cations, Na+, Mg2+, Ca2+, Ba2+, Co2+, Cd2+, and Pb2+, with the technique of surface light scattering. We conclude that Pb2+ and Cd2+ are anomalous compared to all others, and both ions give rise to larger dilational elastic and loss moduli. In addition, compared to all other ions, lead ion has a greater loss tangent, the ratio of loss modulus to elastic modulus, while cadmium ion has a smaller loss tangent. Hence, we attribute the unique property of the cadmium salt of stearic acid in forming Langmuir-Blodgett films to the least loss tangent in conjunction with an enhanced elastic modulus.