Phase Change Enthalpies and Entropies of Liquid Crystals (original) (raw)
Related papers
The Journal of Physical Chemistry B, 2001
The heat capacities of the binary system consisting of a nonionic surfactant, hexaoxyethylene n-dodecyl ether (C 12 E 6), and water were precisely measured as a function of temperature by adiabatic calorimetry over the temperature and the concentration ranges where lyotropic liquid crystals are formed. The enthalpy and entropy of transitions were determined for all transitions observed. The enthalpy and entropy of transition between liquid crystalline phases suggest that the liquid crystalline phases in this system are mainly constructed by C 12 E 6 molecules with a fixed amount of water. The excess heat capacities, as estimated by measuring the heat capacity of neat C 12 E 6 , are positive over the entire temperature and concentration ranges. The excess heat capacities also support the suggestion given above concerning the role of C 12 E 6 molecules on the structure building.
Estimating Solid–Liquid Phase Change Enthalpies and Entropies
Journal of Physical and Chemical Reference Data, 1999
A group additivity method based on molecular structure is described that can be used to estimate solid-liquid total phase change entropy (⌬ 0 T fus S tpce) and enthalpy (⌬ 0 T fus H tpce) of organic molecules. The estimation of these phase changes is described and numerous examples are provided to guide the user in evaluating these properties for a broad range of organic structures. A total of 1858 compounds were used in deriving the group values and these values are tested on a database of 260 additional compounds. The absolute average and relative errors between experimental and calculated values for these 1858 compounds are 9.9 J•mol Ϫ1 •K Ϫ1 and 3.52 kJ•mol Ϫ1 , and 0.154 and 0.17 for ⌬ 0 T fus S tpce and ⌬ 0 T fus H tpce , respectively. For the 260 test compounds, standard deviations of Ϯ13.0 J•mol Ϫ1 •K Ϫ1 (⌬ 0 T fus S tpce) and Ϯ4.88 kJ mol Ϫ1 (⌬ 0 T fus H tpce) between experimental and calculated values were obtained. Estimations are provided for both databases. Fusion enthalpies for some additional compounds not included in the statistics are also included in the tabulation.
Thermochimica Acta, 2000
Solidi®cation (supercooled nematic-to-solid or supercooled smectic-A-to-solid), melting (solid-to-nematic or solid-tosmectic-A), smectic-A-to-nematic, and clearing (nematic-to-isotropic or smectic-A-to-isotropic) transition temperatures W s , W m , W AN , and W c , respectively, and enthalpies DH s , DH m , DH AN , and DH c , respectively, have been determined by differential scanning calorimetry (DSC) for members of the p-(n-alkyl)-p H -cyanobiphenyl (RCB) and the p-(n-alkoxy)-p H -cyanobiphenyl (ROCB) homologous series of liquid crystals, where`R' is the number of carbon atoms in the n-alkyl or n-alkyloxy tails. The study shows that the transition of the less ordered nematic phase to the more ordered smectic-A phase, in liquid crystals that exhibit both phases (those with R8 or 9), involves a transient phase of intermediate order. Broad or overlapping solidi®cation exotherms indicate that two mesogenic aggregations supercool simultaneously. Finally, there is evidence for a transition between two solid forms that occurs at temperatures preceding W m or concurrent with it depending on`R' and the heating rate. This refutes an earlier view that during melting the solid form with the higher melting point solidi®es. #
Synthesis and properties of two new liquid crystals: an analytical and thermodynamic study
Journal of Chromatography A, 1999
Synthesis, analytical performances, thermodynamic and surface properties of two new liquid crystals substituted with poly(ethylene oxide) chains are described. The first of them is N, N9-diphenyl-[4-h2,3,4-tri[2-(2-metoxyethoxy)ethoxy]benzylidenejimine]piperidine (LC ) and the second is 2-hydroxy-3-methyl-4-h4-[2-(2-butoxyethoxy)ethoxy]j 49-h4-[2-(2-1 butoxyethoxy)ethoxy]styryljazobenzene (LC ). The nematic ranges of the two liquid crystals, determined by differential 2 scanning calorimetry do not interfere. The analytical and thermodynamic studies of LC and LC in the solid, nematic and 1 2
The phase transition behavior of the liquid crystal dimer α,ω-bis(4,4-cyanobiphenyl)nonane (CBC9CB), which has been reported to exhibit a nematic-nematic phase transition, has been investigated by means of high-resolution adiabatic scanning calorimetry. This nematic-nematic phase transition is weakly first-order with a latent heat of 0.24 ± 0.01 kJ kg −1. Mixtures up to 40 wt % with 4-pentyl-4-cyanobiphenyl (5CB) liquid crystals have also been investigated, which also show this nematic to nematic phase transition. The transition stays weakly first-order with a decreasing latent heat with increasing concentration of 5CB. For mixtures with more than 40 wt % uniaxial nematic-unknown nematic phase transition was not observed.
Chemical Characterization and Thermal Analysis of Recovered Liquid Crystals
Crystals
Chemical, structural, and thermal properties of recovered nematic Liquid Crystal (LC) mixtures were investigated by applying several analytical techniques. A large quantity (65,700) of End-Of-Life (EOL) Liquid Crystal Display (LCD) screens were used to extract these LC blends. The studied EOL-LCD screens were heterogeneous in nature, particularly due to their different brands, production years, and dimensions. The collected TV and computer screens, as well as tablets, presented an average diagonal size of 24 inches. Chemical characterization revealed that the recovered compounds present typical chemical structures of LC molecules by the simultaneous presence of aliphatic chains and aromatic and polar groups. POM observations of these samples exhibited Schlieren and marble-like textures at room temperature, which are typical of nematic LCs. Moreover, thermal characterization and thermo-optical analysis showed that these LC mixtures displayed a broad nematic phase between −90 °C and +...
Thermal analysis of binary liquid crystalline mixtures
Journal of Thermal Analysis and Calorimetry, 2007
Two series of binary mixtures composed of bent shaped and rod like molecules are reported. The first star shaped bent core molecules were synthesized and used as a component of binary mixtures. The chiral rod like compounds having commensurable length with the arms of the bent core compounds have been chosen for these mixtures. The resulted compositions show various thermotropic liquid crystalline phases that are characteristic to both types of liquid crystalline materials. In case of mixing the rod like molecules to the bent core compound the B2, B7 and induced B1 phases have been observed. While using the star-shaped bent core and chiral rod like compounds in mixture, the paraelectric smectic A, ferroelectric smectic C * and orthogonal hexatic smectic B phases were preferred. The appearing mesophases were investigated by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction methods.