Effect of anion type on monolayers and Langmuir–Blodgett films of gadolinium stearate

This work is focused on the preparation and characterization of gadolinium-containing stearic acid (SA) Langmuir–Blodgett (LB) films. SA monolayer behavior on the gadolinium-containing aqueous subphase has been analyzed by thermodynamic measurements. The compression isotherm shape changed substantially under varying anion type at the constant rare-earth cation content and pH in aqueous subphase. Those changes were ascribed to different monolayer surface charging under gadolinium binding. Small angle X-ray diffraction was used to study the structure of formed multilayer rare-earth-containing LB films. Considerable structural defects in multilayer LB films were found when gadolinium chloride solution was exploited as aqueous subphase for LB film formation, caused presumably by electrostatic interactions under charged monolayer deposition. The use of monodentate complexions like acetic acid residue was proposed to form bulk phase low-charged complexes with trivalent cations. The electroneutral ligand exchange reactions of such complexes with fatty acid Langmuir monolayer results in the formation of quasi-neutral trivalent cation complexes bound to the monolayer surface. This approach allowed effective neutralization of the monolayer surface under gadolinium binding and formation of condensed monolayer below the triple-point temperature. The compression and subsequent deposition of such monolayers resulted in the formation of highly ordered LB films with high content of adsorbed gadolinium cations arranged in two-dimensional (2-D) monatomic layers.