Monolayers and Multilayers of 2-Heptadecylimidazole and Its Silver Complexes

Abstract

Stable insoluble monolayers of 2-heptadecylimidazole (2-HI) at the air/water interface'are formed for the acidic substrate water below pH 5, but the monolayers become unstable above pH 7. This is ascribed to the lack of hydrophilic ability of imidazolyl head group, in alkali region. At lower pH of the substrate water, the 3-nitrogen atoms of imidazolyl rings are protonated and this contributes to the proper hydrophobic-hydrophilic balance of the monolayer molecules. But if there are silver ions of concentration of the order of 10^-6 mol/l (0.1 ppm) in the substrate water, monolayers become very stable even in the alkali region and the collapsing pressures rise up to 50 mM/m. The complex formation of 2-HI with silver ion was discussed by equilibrium constants of the complex formation, and the result was confirmed by the change of the silver ion concentration before and after the spreading of a monolayer. The extremely high local concentration of 2-HI in the surface phase contributes to the formation of the complex in such a low concentration of silver ion. The 2-HI monolayers can be deposited on a solid substrate only when a suitable amount of silver ion in the substrate water is present. The 2-HI-silver complex monolayer deposited as a Y-type LB film and the long spacing of the film, determined by the X-ray diffraction, was 5.52 nm which was almost twice the length of the 2-HI molecule. The IR absorption spectrum of the LB film on a CaF₂ substrate also confirmed the formation of the complex in the LB film.