Dilatometric investigation of thermotropic linear chain copper (II) alkanoates

The molar volumes of three binuclear copper (II) alkanoates (cupric octadecanoate, docosanoate, and tetracosanoate) were measured over the temperature range 30°–200 °C, which encompasses their phase transition to a columnar mesophase. A sharp volume increase in a narrow temperature interval about the transition was observed, consistent with a first-order phase transition. Values obtained for the partial molar volume of a methylene group in the columnar mesophase (16.9±0.3 cm³/mol) are between those for crystalline paraffins and for other smectic or discotic mesogens, respectively, indicating a disordered state, but not a complete fusion, of the aliphatic chains of the copper soaps in their columnar mesophase. The stacking period of the binuclear copper complexes in a column was derived from a combination of dilatometric and X-ray data. Its value, 4.64±0.08 Å, is independent of the chain length of the complex and of the temperature.