Phase Transitions of Urea Adducts with n-Paraffins under High Pressure

The temperatures of the phase transition between orthorhombic and hexagonal structures of the urea adducts with n-paraffins, n-C_nH_2n+2 (n = 14–44), have been measured by differential thermal analysis for the pressure range up to 250MPa. The transition temperatures increase both with pressure and with the number of carbon atoms of the guest paraffins. The increasing rate of the transition temperatures with pressure is about half of that of pure paraffin crystals. The pressure dependences of the lattice constants have also been measured by X-ray diffraction. The specific volume changes discontinuously at the transition pressure; the result suggests that the phase transition of the urea adducts with n-paraffins is a first-order one. Furthermore, the specific volume change at the transition pressure decreases with the number of carbon atoms of the guest paraffins; 3.4 × 10-⁶ m³/Kg for n = 16 and 2.1 × 10-⁶ m³/Kg for n = 44.     

Crystal Structure and Phase Transition of Diglycine Perchlorate

Abstract  

Diglycine perchlorate (DGPCl), a new 2:1 adduct formed between glycine and perchloric acid has been obtained and studied using differential scanning calorimetry and single crystal X-ray diffraction. DGPCl undergoes a reversible first-order phase transition at 261.5 K. The crystal structures at 150 and 293 K have been determined to be triclinic, space group P‐1, Z = 2, suggesting the first-order phase transition to be an isostructural phase transition. The DGPCl crystal consists of five glycinium-monoprotonated glycinium dimers and five perchlorate anions in an asymmetric unit. The glycine moieties in the glycinium-monoprotonated glycinium dimers are non-planar. Two types of hydrogen bonds are present in the crystal, strong O–H···O hydrogen bonds and a weak N–H···O hydrogen bonds. The short, strong O–H···O hydrogen bond connects the glycinium ion and mono protonated glycinium ion. In four of the dimers, the O–H and H···O bond lengths are different, indicating the hydrogen atom to be located more close to the monoprotonated glycinium ion. However, in one of the glycinium-monoprotonated glycinium dimer the O··H and H··O bond lengths are nearly equal, suggesting the hydrogen atom (O···H···O) to be attached to the oxygen atoms of both glycine moieties. On thermal transition some of these hydrogen bonds are weakened and in all dimers the hydrogen atom seems to be located more close to the mono protonated glycinium ion.     

Phase Diagrams of New Compounds with Nematic Reentrant Mesophases

Two new series of compounds exhibiting nematic and smectic reentrant mesophases are presented here; the first one is 4-n-alkoxy benzoyloxybenzylidene 4'-cyanoaniline series the second one the 1-(4-n-alkylbenzoyloxyphenyl) 2-(4'-cyanophenyl) ethane series. Isobaric binary phase diagrams are plotted for each compound by means of the contact method and the mesophases so identified.     

Phase Transition of 1,16-Hexadecanediol

It was found that 1,16-hexadecanediol exhibited a solid-solid phase transition. X-ray diffraction experiments and dielectric constant measurements were carried out in order to investigate the structure of the high-temperature phase. A model of molecular arrangement in the high-temperature phase was presented. The high-dielectric constant values were observed in the high-temperature phase. It is considered that the high-temperature phase of 1,16-hexadecanediol is a rotator phase in analogy with n-higher alcohols.     

Phase Transition Studies in Mixed Polymesomorphic Liquid Crystals with Cholesteric and Smectic A Phase

Cholesteric mesomorphism was induced in a polymesomorphic liquid crystal with nematic and smectic A phases by mixing it with a low pitch cholesteric. A variety of mixtures for a wide range of concentrations were studied. It was observed that the addition of cholesteric liquid crystal disrupts the smectic phase of the parent polymesomorphic liquid crystal. The measured valuses of ultrasound velocity and specific volume and their related parameters were found to exhibit anomalies near each of the phase transitions. The mean interaction parameter $ \bar \alpha $ was estimated for different mixtures and was found to decrease with increasing concentration of cholesteric liquid crystal in the mixture. It was observed that smectic A-cholesteric transition becomes second-order at a particular concentration in agreement with the theoretical predictions.     

Internal Friction in Copolymer Vinylidenefluoride with Trifluoroethylene at Ferroelectric Phase Transition

Crystallography Reports - Temperature dependences of internal friction Q–1, obtained at different heating rates of a copolymer VDF/TrFE (75/25) sample, are studied in the temperature range of...     

ENDOR Study of the Phase Transition in Benzil

ENDOR spectroscopy was used to elucidate the domain structure of benzil accompanying its phase transition at 84°K. Spectral twinning was observed on a single-crystalline sample as a result of multidomain formation below the phase transition temperature. This twinning was explained by applying the Landau theory of phase transitions. With knowledge of the anisotropy of the proton hyperfine interaction, we extracted the angle between the domains and the morphologic two-fold axis to be 1.55° at 2°K.     

Synthesis,Characterisation and Phase Transition Studies of 50.9 and 90.5

The synthesis, characterisation of phases of the compounds N(p-n-pentyloxy benzylidene) p-n-nonylaniline, 50.9 and N(p-n-nonyloxy benzylidene) p-n-pentylaniline, 90.5 and phase transition studies using different experimental techniques are presented. The compound 50.9 exhibits a phase sequence NAB whereas 90.5 shows a phase variant ABG. A large density jump across IN transition is observed in 50.9 similar to that observed in 50.10 compound.     

Observation of Intermediate Smectic A Structure at the Phase Transition Nematic-Smectic C in 4,4′-di-n-Heptyloxyazoxybenzene

Using different boundary conditions in thin film of 4,4′-di-n-heptyloxyazoxy benzene and studying the microscopic textures as well as the depolarized Rayleigh light scattering, an intermediate smectic A-like state during the nematic-smectic C phase transition was observed.     

Synthesis and Molecular Structure of Sulfur Containing Cobalt Compounds

Abstract  

The reaction of cobalt (II) chloride hexahydrate and N-tert-butyl-2-thioimidazole (tm^t-Bu) yielded products [k²-(tm^t-Bu)₂]CoCl₂ (1) and [(tm^t-Bu)₂Co₂(μ-tm^t-Bu)₂Cl₂]·CH₃CN (2) at different reaction time. The single crystal X-ray diffraction analysis was carried out for 1 and 2 {Bruker Kappa Apex-II CCD diffractometer, MoK_α radiation}. Crystal data for 1: orthorhombic P212121 unit cell a = 8.195(6) ?, b = 13.778(10) ?, c = 17.354(11) ? and V = 1960(2) ?³ whereas the compound 2 crystallizes in monoclinic system P21/c with unit cell a = 10.069(7) ?, b = 11.181(7) ?, c = 18.156 ?, β = 95.322(4)° and V = 2035.4(2) ?³. The complex 1 contains one dimensional chain running along the a-axis and the complex 2 contains one dimensional chain running along the b-axis due to existence of different types of intermolecular interaction. The change in structure of the product is directly related to the reaction time i.e. reaction for lesser time produces mononuclear product and for longer time produces binuclear compound.     

Structure and Electrochemical Properties of Carbon Nanotube Intercalation Compounds

Abstract

This article summarizes the current status of carbon nanotube intercalation compounds. It focuses on the structure and electrochemical properties of intercalated single-walled carbon nanotubes (SWNTs). Materials synthesis, purification and characterization methods are also discussed. This article draws mostly from works performed at UNC.     

Phase Transition of Magnesium Sulphite Hydrates in Aqueous Suspension

Solution mediated phase transitions in aqueous suspension (pH 9.5) of MgSO₃ hydrates were studied at 50, 60, 65, and 70 °C for hexa- to trihydrate and 25, 30, and 35 °C for tri- to hexahydrate conversion. The former conversion is controlled by the diffusion growth of the new phase crystals whereas the latter process is controlled by both the initial phase dissolution and the new phase growth.     

Phase Transition Studies of the Liquid Crystal Pentyloxybenzylidene Phenylazoaniline

Phase transition studies of the liquid crystal pentyloxybenzylidene phenylazoaniline are investigated with density and ultrasonic velocity measurements. The density variation with temperature confirms that the isotropic liquid-nematic, nematic-smectic A and smectic A-smectic B transitions are of first order. The temperature variation of ultrasonic velocity confirms all the phase transitions. The adiabatic compressibility (β_ad), Rao number (R_n) and Molar compressibility (B) are estimated from density and ultrasonic velocity.     

The Order-Disorder Phase Transition in Liquid Crystals as a Function of Molecular Structure. I. The Alkyl Cyanobiphenyls

The pretransitional behaviour of a homologous series of mesomorphic compounds, the alkyl cyanobiphenyls (CNρρC _n H _{2n + 1}) for n = 5–12, has been studied as a function of temperature by light scattering in the isotropic phase. The higher homologues, n = 8–12, exhibit a smectic A phase that becomes increasingly important at the expense of the nematic phase for increasing n and for n = 10 and 12 no nematic phase is observed. We have shown from light scattering and differential scanning calorimetry measurements that the presence of the smectic A phase changes markedly the pretransitional behaviour, and gives rise to two distinct pretransitional regions. The first region is for temperatures greater than ～3°C above the clearing temperature T_c where the systems exhibit a typical pretransitional behaviour adequately described by the phenomenological Landau-de Gennes model of a second order phase transition with an intervering first order transition at T_c . The constants of this model for this region are given and show a distinct odd-even effect. The second region is close to T_c where a strong divergence from the already critical behaviour is observed. This secondary divergence is observed to be a function of the alkyl chain length and its conformation, and is attributed to coupling between smectic A like layers and the orientational order parameter in the isotropic phase.     

Highly Electrical Conductivity of Hybrid Langmuir-Blodgett Films of Transition Metal Dichalcogenide and Amphiphilic Compounds

Abstract

Hybrid alternate layered films of transition metal dichalcogenides and amphiphilic compounds were prepared by Langmuir-Blodgett (LB) technique. The conductivity at room temperature depended on the transition metal dichalcogenide species, showing the highest value for the hybrid LB films of MoS₂ system. This methodology was successfully applied to the fabrication of the hybrid LB films using various organic amphiphiles such as cyanine dyes, phthalocyanines, azobenzene, and ferrocene derivatives. The conductivity depended on the interlayer spacing: the conductivity decreased with increasing interlayer spacing of the film. The highest electrical conductivity of the hybrid LB films was over 100 Scm^?.     

Kinetics of Desolvation and Structure of Bulky Xanthenol Inclusion Compounds

Abstract  The inclusion compounds of 14-hydroxy-14-toluenyldibenzo[a:j]xanthene (H) with the guests acetone (H·ACE), pyridine (H·PYR) and N,N-dimethylformamide (H·DMF) were all successfully solved in the triclinic spacegroup P-1 with unit cell dimensions for H·ACE: a = 9.415(2) ?, b = 9.730(2) ?, c = 12.994(3) ?, α = 82.65(3), β = 76.69(3), γ = 88.67(3), Z = 2; for H·PYR: a = 9.482(2) ?, b = 10.169(2) ?, c = 13.089(3) ?, α = 106.59(3), β = 95.91(3), γ = 90.35(3), Z = 2 and for H·DMF: a = 9.537(19) ?, b = 10.055(2) ?, c = 13.129(3) ?, α = 79.18(3), β = 74.42(3), γ = 86.46(3), Z = 2. The thermal stabilities for all compounds were investigated and in particular the kinetics of desolvation for the DMF compound was determined using both isothermal and non-isothermal methods. Index Abstract  The host compound 14-hydroxy-14-toluenyldibenzo[a:j]xanthene forms inclusion compounds with the guests acetone, pyridine and N,N-dimethylformamide. The crystal structures were determined and their thermal stabilities investigated. The kinetics of desolvation for the DMF compound was determined using both isothermal and non-isothermal methods.      

Thermal Expansion of Thiourea in the Range of Phase Transition

By means of a quartz dilatometer three principal expansion coefficients have been determined in the temperature range 100–273°K, and their anisotropy within (100) plane. The appearance of singular points, corresponding to phase transformations, depends on direction of measurements but most drastic changes are observed along the crystallographic c-axis, in agreement with the direction of the weakest intermolecular interactions. The calculated pressure necessary to shift the phase equilibrium I/II to room temperature is in good agreement with the value observed in a direct experiment.¹¹