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

A new third polymorph of copper­(II)
decanoate has been found by
two methods: from the crystallization of the salt–acid 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)2Cu], 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).