Structural study of crystalline and columnar copper (II) soaps

Abstract

A homologous series of binuclear copper (II) linear chain alkanoates together with two branched chain and one aromatic substituted copper (II) alkanoates have been synthesized and studied by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction. All of these are crystalline at room temperature, they are mesomorphic in nature above c. 100°C, with the exception of copper propionate which remains crystalline up to its thermal decomposition above 200°C. A systematic study has shown that the linear chain alkanoates, starting from the pentanoic derivative, produce columnar mesophases with hexagonal symmetry. Columns of polar copper carboxylate groups are surrounded by disordered aliphatic chains, and form a two dimensional hexagonal lattice. The repeat unit in a column is a binuclear dicopper tetracarboxylate complex. Two transition regimes have been detected leading from the crystal to the columnar mesophase: one dominated by the interactions between the polar heads, the other by the interactions between aliphatic chains. In the special case of the butyric derivative, the columnar mesophase obtained is rectangular in symmetry. Instead of being oriented perpendicular to the columnar axis and superposed in a four fold helicoidal fashion, the repeat units in the columns are tilted and all shifted in the same direction with respect to one another.