Lead(ii) soaps: crystal structures, polymorphism, and solid and liquid mesophases.

The long-chain members of the lead(ii) alkanoate series or soaps, from octanoate to octadecanoate, have been thoroughly characterized by means of XRD, PDF analysis, DSC, FTIR, ssNMR and other techniques, in all their phases and mesophases. The crystal structures at room temperature of all of the members of the series are now solved, showing the existence of two polymorphic forms in the room temperature crystal phase, different to short and long-chain members. Only nonanoate and decanoate present both forms, and this polymorphism is proven to be monotropic. At higher temperature, these compounds present a solid mesophase, defined as rotator, a liquid crystal phase and a liquid phase, all of which have a similar local arrangement. Since some lead(ii) soaps appear as degradation compounds in oil paintings, the solved crystal structures of lead(ii) soaps can now be used as fingerprints for their detection using X-ray diffraction. Pair distribution function analysis on these compounds is very similar in the same phases and mesophases for the different members, showing the same short range order. This observation suggests that this technique could also be used in the detection of these compounds in disordered phases or in the initial stages of formation in paintings.

Thermal and structural study of the crystal phases and mesophases in the lithium and thallium(i) propanoates and pentanoates binary systems: formation of mixed salts and stabilization of the ionic liquid crystal phase.

The temperature and enthalpy vs composition phase diagrams of the binary systems [xC(2)H(5)CO(2)Li + (1 - x)C(2)H(5)CO(2)Tl], and [x(n-C(4)H(9)CO(2)Li) + (1 - x)n-C(4)H(9)CO(2)Tl], where x is the mole fraction, were determined by DSC. Both binary systems display the formation of one 2:1 mixed salt each (at x = 0.667) that appear as a peritectic (incongruent melting) at T(fus) = 512.0 K, and T(fus) = 461.1 K, with Delta(fus)H(m) = 13.76 and 8.08 kJ.mol(-1) for Li-Tl (I) propanoates, and n-pentanoate mixed salts, respectively. The thermotropic liquid crystal of the thallium(I) n-pentanoate transforms into a more stable liquid-crystal phase, which appears in the phase diagram between 380 and 488 K and for x = 0 up to x = 0.56. The crystal structure of thallium(I) propanoate and of the two mixed salts were obtained via X-ray synchrotron radiation diffraction measurements. These compounds present a bilayered structure similar to the two pure lithium salts previously found by our group.

Structural and thermodynamic study on short metal alkanoates: lithium propanoate and pentanoate.

Lithium propanoate and pentanoate were characterized by DSC, single crystal and powder XRD and FTIR and impedance spectroscopies. Lithium propanoate presents a solid-to-solid transition (SII-SI) at T(ss) = (549.1 +/- 0.7) K on first heating that varies on the second and next ones, followed by a fusion at T(f) = (606.1 +/- 0.5) K. For lithium pentanoate, two solid-to-solid transitions (SIII-SII and SII-SI), at T(ss) = (205.5 +/- 0.5) K and T(ss) = (325.2 +/- 0.7) K, respectively, and a melting point at T(f) = (576.5 +/- 0.3) K were found. The crystal structures for both compounds were characterized at 100 and 298 K (and for the lithium propanoate also at 160 K). Single-crystal XRD showed that the SII phase of both compounds has a monoclinic structure with the same symmetry group (P2(1)/c). This is the first time that a single-crystal structure has been reported for any member of the lithium alkanoates series, so far. FTIR and impedance spectroscopies were also carried out to better characterize the solid phases in these compounds.

A novel rotator glass in lead(II) pentanoate: calorimetric and spectroscopic study.

Lead(II) pentanoate was studied by DSC, XRD, and FTIR and solid state CP/MAS-NMR spectroscopies. A transition from the crystal to the intermediate phase, at T(ss) = 328.2 +/- 0.6 K, with delta(ss)H = 8.8 +/- 0.1 kJ x mol(-1), and a melting at T(f) = 355.6 +/- 0.3 K, with delta(f)H = 12.6 +/- 0.1 kJ x mol(-1), were observed on first heating. The thermal and structural behavior of the lead(II) pentanoate shows as a link between those of the shorter and longer members of the previously studied lead(II) alkanoate series. The optical microscopy and FTIR vs temperature studies show structural changes from the crystal to the intermediate phase and its solid state nature. Moreover, X-ray diffraction and C-13 and Pb-207 CP/MAS-NMR studies confirm the rotator nature of the intermediate phase in this compound. Two different glass states, one from the isotropic liquid and another from the rotator phase, were obtained by quenching at high and low rates, respectively. The glass transition temperatures (measured at 5 K x min(-1)) were 322.9 and 275.7K, respectively.

Molecular association of normal alkanoic acids with their thallium(I) salts: a new homologous series of fatty acid metal soaps.

A new homologous series of thallium(I) hydrogen dialkanoates, fatty acid thallium soaps, from the dipropane up to the ditetradecane is reported for the first time. This association with 1:1 stoichiometry is the only one exhibited by the thallium derivatives. They have been prepared by solidification of molten mixtures with equimolar proportions of acid and corresponding neutral salt, through crystallization from an anhydrous ethanolic solution of the mixture has also been successful in getting pure compounds with largest chain lengths. Vibrational spectroscopies clearly characterize these crystalline compounds as very strong hydrogen bonding systems. Assignations of active modes in proton and carbon nuclear magnetic resonance spectrometry (NMR) (in ethanol) and infrared (IR) and Raman spectra (in solid state) are reported. According to X-ray diffraction (XRD) they have monomolecular lamellar structures with the acyl chains arranged up and down to the cation/H-bond network in a methyl-to-methyl fashion, and vertically oriented to the basal plane. The acyl chains present all-trans conformation and alternating configuration (perpendicular orthorhombic subcell), like the beta'-phases of other kinds of lipids. Lamellar thickness is reported for the six room-temperature crystalline members. The molecular compounds present polymorphism, one crystal/crystal transition at temperatures close to the peritectical melting. Phase transition thermodynamics are also given and discussed with respect to their acid and salt parents. Their incongruent melting involves nearly 90% of the total enthalpic increments of both constituents' melting processes, making these compounds potential thermal energy storage materials.