Structural control of Langmuir–Blodgett films containing metal cations by ligands exchange

Abstract

The structure of multilayer gadolinium stearate Langmuir–Blodgett (LB) films has been studied in dependence on the gadolinium salt nature and aqueous subphase composition used for the LB film preparation. Stearic acid Langmuir monolayers were formed on the surface of aqueous gadolinium acetate or gadolinium chloride solutions (10 −4 M) with different monovalent electrolyte concentrations at the same pH value (5.65) at which all monolayer stearic acid molecules were ionized and bound with Gd 3+ cations. The structure of the deposited films was characterised by X-ray diffraction, Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM). It was found that Gd stearate LB films formed with the use of gadolinium acetate solution as a subphase had quasi-crystalline domain morphology with ordered layered structure, in contrast to the rather disordered structure of LB films formed using GdCl 3 solution as a subphase. The structure characteristic for the Gd stearate LB film formed with the use of Gd acetate solution as a subphase can be obtained using GdCl 3 solution with an excess of acetic acid (9×10 −4 M). The substantially different morphology of Gd stearate LB film characterized by the perfect layered and planar terrace structure was obtained in the presence of NaCl (10 −2 M) in Gd acetate or Gd chloride solutions used as subphases. The data obtained show that the formation of polyvalent metal cation complexes with Langmuir monolayer and bulk phase ligands along with ligands exchange and substitution interactions are substantial factors determining structural organization of the corresponding deposited metal-containing multilayer LB films.