Spectroscopic Investigation of Nitrate−Metal and Metal−Surfactant Interactions in the Solid AgNO₃/C₁₂EO₁₀ and Liquid-Crystalline [M(H₂O)<i>_n</i>](NO₃)₂/C₁₂EO₁₀ Systems

Abstract

Interactions of the nitrate ions in various metal nitrate salts with C n H 2 n +1 (CH 2 CH 2 O) m OH (C n EO m )-type nonionic surfactants have been investigated both in the solid and in the liquid-crystalline (LC) systems. In the ternary system, the mixture of salt/water/C n EO m has a mesophase up to a certain concentration of salt, and the nitrate ions in this phase are usually in a free-ion form. However, upon the evaporation of the water phase, the nitrate ion interacts with the metal center and coordinates as either a bidentate or unidentate ligand. It is this interaction that makes the AgNO 3 ternary system undergo a phase separation by releasing solid Ag(C n EO m ) x NO 3 complex crystals. In contrast, the salt/surfactant systems maintain their stable LC phases for months. Note also that the salt/surfactant systems consist of transition-metal aqua complexes in which the coordinated water molecules play a significant role in the self-assembly and organization of the nonionic surfactant molecules into an LC mesophase. Throughout this work, Fourier transform infrared spectroscopy has been extensively used to investigate the interactions of the nitrate ions with a metal center and the metal ions with the surfactant molecules. Polarized optical microscopy and X-ray diffraction techniques have been applied to investigate the nature of the crystalline and LC phases.