Adiabatic calorimetry from about 5 to 350 K was overlapped The phase, association, and aggregation behavior of the binary with differential scanning calorimetry (DSC) from about 250 system [TlO 2 C(CH 2 ) 5 CH 3 / HO 2 C(CH 2 ) 5 CH 3 ] have been investito 550 K for heat capacity measurements and thermodynamic gated through temperature and enthalpy vs composition diagrams characterization of the polymorphism and thermotropic meby determination of the thermal behavior of the pure components somorphism that the different members exhibit, the first and and 30 mixtures over an interval of nearly 300 K, and by differenlatest papers corresponding to Refs. (1, 2). Sample preparatial scanning calorimetry and polarized microscopy. Total miscibiltion was performed by heterogeneous reaction between the ity was observed in liquid and liquid crystalline phases but essenalkanoic acid in methanolic solution and solid thallium(I) tially total immiscibility among crystalline phases. The phase becarbonate, initially by following the recipe of Duruz et al. havior exhibits: (1) Formation of two intermediate compounds which dissociate in the solid state at low temperature;
(2) A eutectic (3), who emphasized the convenience of using a small excess reaction at salt molar fraction x Å 0.08 about 3 K below the acid of acid in the aqueous reaction medium for sodium alkanoate melting point; (3) Unimolecular salt:acid association by very strong synthesis. According to them, the decomposition rates for hydrogen bonding, which undergoes a crystal/crystal transition the members of the Na homologous series are quite high prior to incongruent melting at 295.5 K, occurring by a highly unless this provision is observed to repress the action of energetic peritectic reaction in which the compound evolves more hydroxyl ions (the main salt instability source). This practice than the sum of the total transition enthalpies of both components; had been followed even in the case of other compounds (4) Lyotropic mesophase formation at 390.0 K over the x § 0.66 prepared not in aqueous but in dry alcoholic solutions, as composition range by aggregation of a solid and a liquid phase to Cs alkanoates (4) or in the present T1 case. However, this yield, on heating, a continuous series of liquid crystalline solutions ''small excess of acid'' proved to be hazardous and eventuwith mixed-lamellar structure and interlamellar spacings around ally deleterious for the alkanoate preparation, which led us 20 A ˚as determined by powder X-ray diffractometry, their texture being focal conic, consistent with the neat thermotropic mesophase to use just ''a small excess of carbonate'' instead. In fact, of the pure salt. A tridimensional phase diagram is presented. All it was noted subsequent to submission of the results on the these main reactions have been fully characterized by their nature, heptanoate for publication (5) that of the five transitions stoichiometry, and relevant thermodynamic data. ᭧ 1997 Academic observed in the 260 to 310 K region, one of the larger ones Press at about 296 K was extraneous (6). Further analysis revealed Key Words: thallium(I) hydrogen diheptanoate; polymorphism; the formation of a unimolecular salt:acid association (7); mesomorphism; phase transition thermodynamics; eutectic; perihence the contamination of the alkanoate by the correspondtectic; focal conic textures; lamellar structures; 3D phase diagram.
ing acid may constitute a serious and complex problem for the thermodynamics of the alkanoates. To evaluate its repercussions, it will be of most interest to characterize this chem-1 Present address: Instituto de Cata ´lisis y Petroleoquı ´mica (CSIC), Camwas soon noted that this is a common phenomenon for