Study of the Polymorphism in Copper(II) Decanoate through Its Phase Diagram with Decanoic Acid, and Texture of the Columnar Thermotropic Liquid Crystal Developable Domains in This and Similar Systems
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A new third polymorph of copper(II) decanoate has been found by two methods: from the crystallization of the saltacid solution and from a specific thermal treatment of the polymorph obtained in n-heptane. The new polymorph, whose crystal structure has been solved for the first time, is the most stable from the thermodynamic point of view. It presents a bilayer structure, but with two different types of paddle-wheels catenae with opposite orientations. This polymorphism as well as the temperature vs composition phase diagram of the system, [(1 - x) C9H21CO2H + (x) (C9H21CO2)(2)Cu], were solved by differential scanning calorimetry, Fourier transform infrared spectroscopy, X-ray diffraction, pair distribution function, and optical microscopy. The singularities of the phase diagram are (a) a Krafft-like process of molecular association of the copper(II) decanoate into the acid solution, at about T = 361.5 K and x = 0.022 (forming salt-acid adducts of 1:1 stoichiometry); (b) a fusion transition as an invariant (T = 379.1 K) of the solid salt in excess from the copper(II) decanoate crystal phase to the discotic liquid crystal, forming homeotropic developable domains (seen for the first time in these systems) of the pure salt into the saturated acid solution. These domains are seen not only in the copper(II) decanoate but also in other members of the series, at high salt concentration in the corresponding acid solution, and allow identification of the hexagonal columnar discotic structure of the mesophase (with the exception of the copper(II) butanoate, where the tetragonal domains with tilted disks, paddle-wheels, were seen in the same salt-acid region).