Heat capacity and other thermodynamic properties of CoTe2 from 5 to 1 030 K and of CoTe2.315 from 300 to 1 040 K

The heat capacity of orthorhombic (marcasite-type structure) cobalt ditelluride has been measured from 5 to 1 030 K by adiabatic-shield calorimetry with alternate energy inputs and equilibrations. Above 900 K a marked increase in heat capacity occurs which probably signals a change in the composition of the CoTe₂-phase towards higher tellurium content. Values at 298.15 and 1 000 K in J K^–1 mol^–1 of the heat capacity (C _p,m), entropy [S _m ^° (T)–S _m ^° (0)], andGibbs energy function – [G _m ^° (T)–H _m ^° (0)]T ^–1 are 75.23, 114.5, 49.93, and 132.4, 216.2, 139.17, respectively. Consistent with the metallic behavior of CoTe₂, deviation of the heat capacity from theDebye T ³-law was found at low temperatures. Comparison with the heat capacity of FeTe₂ shows aSchottky-like deviation with a maximum of 7.3 J K^–1 mol^–1 at 80 K and evidences the influence of the additional 3 d-electron in cobalt compared to iron. Heat capacity measurements were made on CoTe_2.33 to ascertain the existence range of the CoTe_2+x -phase and the entropy of the associated structural disorder.The portion of this research done at Ann Arbor was supported in part by the Structural Chemistry and Chemical Thermodynamics Program of the Division of Chemistry of the National Science Foundation under Grant No. CHE-7710049.     

Comparative Thermophysical Study of Some

Similarities and differences in the physical chemistry behaviour among several metal alkanoate series (thallium(I), lead(II) and copper(II)) previously studied are shown here. The presence of a 'condis' phase in the first two series is studied. The thallium and lead series present a smectic A mesogenic behaviour, while the copper(II) one forms molecular aggregates which develop columnar discotic liquid crystal phases. Finally, the monovalent cation salts form acid soaps with the acid; instead, when the cation is bivalent, the acid behaves as a polar solvent similarly to the water-surfactants systems. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermophysical study of lead(II) n-alkanoates by DSC, optical microscopy, FTIR and Raman spectroscopy

Thirteen members of the lead(II) n-alkanoate series (from the n-hexanoate to the n-octadecanoate) have been synthesized, purified, and studied by differential scanning calorimetry, polarizing light microscopy, and FTIR and Raman spectroscopy. A solid-solid transition was found for all members of the series. A mesomorphic state, implying a fusion and a clearing point, was identified as smectic C, and was present only in the lower members of the series below the n-tridecanoate. The higher members form the isotropic liquid through a regular fusion. Temperatures and enthalpies for the phase transitions have been measured and the corresponding entropy changes calculated. Heat capacity measurements between 280 K and 400 K were also carried out. A study of these magnitudes versus number of carbons was made. The solid phase previous to the liquid crystal in the lower members and to the isotropic liquid in the higher members could be identified as a 'condis' crystal (conformationally disordered phase) by an FTIR versus temperature study.     

The role of calorimetry in the structural study of mesophases and their glass states

Four mesomorphic states of matter are known: liquid crystal, plastic crystal, condis phase, and rotator phase, all of them are solid phases, except liquid crystal, which is fluid. Plastic crystal (also called ODIC, orientational disordered crystal), rotator phase, and even condis phase have been considered the same phase by many authors. Differences between them will be established to define their own characteristics. Two organic salts series have been used for discussion in this presentation: (1) thallium(I) alkanoate series, that presents a condis mesophase, and (2) lead(II) alkanoate series, that present a rotator one, both forming a smectic A-like liquid crystal phase. Based in the literature data of the alkyl ammonium bromide series it can be established that the short chain length members would present a rotator phase, and, the large chain ones, a condis phase. Five different glass states are known (four with partial crystalline order), corresponding with the above mentioned mesophases, and the ordinary (amorphous) glass state.     

Rubidium and lithium butanoates binary phase diagram

The temperature and enthalpy vs. composition diagrams of the binary system [xC₃H₇CO₂Li+(1–x)C₃H₇CO₂Rb], where x=mole fraction, were determined by differential scanning calorimetry (DSC). This binary systems displays the formation of two mixed salts with a composition 1:1 and 1:2, which melt incongruently at T _fus=590.5 K, with Δ_fus H _m=11.6 kJ mol^–1, and congruently at T _fus=614.5 K, with Δ_fus H _m=20.2 kJ mol^–1, respectively. The phase diagram also presents an ionic liquid-crystalline phase in a wide temperature range: 95 K.     

Short chain copper(II) n-alkanoate liquid crystals

Four short chain members of the copper(II) n-alkanoate series Cu(Cn)₂, from propanoate to hexanoate, have been synthesized, purified and characterized by means of optical microscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), FTIR and Raman spectroscopy. The TGA study shows that decomposition starts on heating above 420-470 K (depending on the sample) in a nitrogen atmosphere. In addition, thermal decomposition was investigated by DSC using special high pressure pans and endo- and exc-thermic processes were found which have not been reported previously. All but one of the compounds melt to a liquid crystal phase, which decomposes before the clearing point. The exception is the propanoate homologue, which decomposes directly from the solid state. Despite this problem of sample decomposition, the identification by optical microscopy of the tetragonal (butanoate) and hexagonal (pentanoate and hexanoate) discotic columnar phases was, for first time, possible by the addition of small amounts of the corresponding acid to the samples. These results are in agreement with the X-ray diffraction study performed using swelled mixtures of these salts with hydrocarbon solvents. Two solid-solid transitions, not previously reported in the literature, were found for the butanoate homologue at 395.9 and 422.9 K with ΔH=8.27 and 1.37 kJ mol^-1, respectively. The solid and liquid crystalline phases were investigated using variable temperature FTIR spectroscopy.