Chain orientation and headgroup structure in Langmuir monolayers of stearic acid and metal stearate (Ag, Co, Zn, and Pb) studied by infrared reflection-absorption spectroscopy

The monolayers of stearic acid at the air-water interface on pure water and ion-containing subphases have been studied using infrared reflection-absorption spectroscopy. In the presence of Co2+, Zn2+, and Pb2+, ordered hydrocarbon chains and hexagonal subcell structure remain almost unchanged in comparison with those for the monolayer on pure water at the surface pressure of 20 mN/m. In the cases of Co2+ and Zn2+, the H-bonded monodentate and unsymmetric bidentate chelating structure within the headgroups were formed, and in the case of Pb2+, three types of structures, bidentate chelating, unsymmetric chelating, and bidentate bridging coordinations, were formed. The hydrocarbon chains in the monolayers are uniaxially oriented at a tilt angle of similar to 0 degrees with respect to the surface normal in contrast to a tilt angle of similar to 20 degrees on pure water surface at 20 mN/m by the computer simulation of theoretical calculation to experimental data. In the presence of Ag+, multilayers were developed with the highly ordered hydrocarbon chains in a triclinic subcell structure and a bidentate bridging structure within the headgroups. The multilayers were composed of three monolayers and the hydrocarbon chains in each monolayer were oriented at an angle of similar to 30 degrees away from the surface normal with their C-C-C planes almost perpendicular to the water surface. (c) 2006 American Institute of Physics.