Lanthanide(III) Dodecanoates: Structure, Thermal Behaviour, and Ion-Size Effects on the Mesomorphism

Rare-earth salts of dodecanoic acid (lauric acid) have been prepared and characterized. The compounds have the stoichiometry Ln(C11H23COO)3, where Ln = Y, La, Ce−Lu (except Pm). The thermal behaviour of these rare-earth dodecanoates has been investigated by hot-stage polarized optical microscopy, differential scanning calorimetry (DSC), thermogravimetry (TG), and high-temperature X-ray diffraction analysis. The compounds have a lamellar bilayer structure in the solid state, with the alkyl chains in an all-trans conformation normal to the ionic sublayer. The size of the trivalent lanthanide ion has proved to be a critical factor with regard to the existence of mesophases of this type of metallomesogens bearing carboxylate ligands. Thus, it was found that a mesophase can only be formed with the light lanthanide ions having large ionic radii. The mesophase stability range decreases rapidly on traversing the lanthanide series. Whereas a smectic A phase has been observed for the LaIII, CeIII, PrIII, and NdIII dodecanoates, no mesomorphism is apparent for the other lanthanide complexes.