Interactions in the Thallium(I) Heptanoate and Heptanoic Acid System: Association, Aggregation, and Phase Behavior

The phase, association, and aggregation behavior of the binary system [TlO2C(CH2)5CH3 + HO2C(CH2)5CH3] have been investigated through temperature and enthalpy vs composition diagrams by determination of the thermal behavior of the pure components and 30 mixtures over an interval of nearly 300 K, and by differential scanning calorimetry and polarized microscopy. Total miscibility was observed in liquid and liquid crystalline phases but essentially total immiscibility among crystalline phases. The phase behavior exhibits: (1) Formation of two intermediate compounds which dissociate in the solid state at low temperature; (2) A eutectic reaction at salt molar fraction x = 0.08 about 3 K below the acid melting point; (3) Unimolecular salt:acid association by very strong hydrogen bonding, which undergoes a crystal/crystal transition prior to incongruent melting at 295.5 K, occurring by a highly energetic peritectic reaction in which the compound evolves more than the sum of the total transition enthalpies of both components; (4) Lyotropic mesophase formation at 390.0 K over the x >/= 0.66 composition range by aggregation of a solid and a liquid phase to yield, on heating, a continuous series of liquid crystalline solutions with mixed-lamellar structure and interlamellar spacings around 20 A as determined by powder X-ray diffractometry, their texture being focal conic, consistent with the neat thermotropic mesophase of the pure salt. A tridimensional phase diagram is presented. All these main reactions have been fully characterized by their nature, stoichiometry, and relevant thermodynamic data.