In situ observation of the pressure-induced mesophase for 4′-n-hexadecyloxy-3′-nitrobiphenyl-4-carboxylic acid

In situ observation of the optical texture, and X-ray patterns of the pressure-induced mesophase seen for 4′-n-hexadecyloxy-3′-nitrobiphenyl-4-carboxylic acid (ANBC-16) was performed under hydrostatic pressures up to 100MPa using a polarizing optical microscope equipped with a high pressure hot stage and a wide angle X-ray diffractometer equipped with a high pressure vessel respectively. It was found that the pressure-induced mesophase (hereafter refered to as ‘X’) appeared at pressures above 60 MPa, and exhibits a birefringent broken-fan or a sand-like texture that remain unaltered in the SmC phase. The POM-transmitted light intensity curve measured on heating clearly showed the Cr₄ → Cr₁ → SmC → ‘X’ → SmA → I transition sequence at 80 MPa. The optical texture and the POM-transmitted light intensity measured during a pressure cycle at 185°C showed a reversible change between the cubic and ‘X’ phases. The WAXD pattern of the ‘X’ phase showed a spot-like pattern, suggesting no layered structure for this phase, and also revealed a substantial decrease in the d-spacing of the low angle reflection at 80 and 100 MPa, compared with the d-spacings of the (0 0 1) reflection of the SmC phase and also the (2 1 1) reflection of the cubic phase. It is concluded from these data that the ‘X’ phase is a birefringent hexagonal columnar phase.     

High pressure differential thermal analysis of nematic compounds with different molecular shapes

Abstract

By means of high pressure differential thermal analysis we measured the pressure/temperature phase diagrams of several nematic compounds with different molecular structures. Using pressures up to about 500 M Pa we found strongly nonlinear clearing curves. This behaviour is interpreted as changes from ‘weak’ intermolecular potentials to ‘hard’ potentials, or using thermodynarmic arguments, strong pressure dependence of the volume discontinuity at the clearing transitions.     

Structural and vibrational properties of solid nitromethane under high pressure by density functional theory

The structural, vibrational, and electronic properties of solid nitromethane under hydrostatic pressure of up to 20 GPa have been studied using density functional theory. The changes of cell volume, the lattice constants, and the molecular geometry of solid nitromethane under hydrostatic loading are examined, and the bulk modulus B0 and its pressure derivative B0' are fitted from the volume-pressure relation. Our theoretical results are compared with available experiments. The change of electron band gap of nitromethane under high pressure is also discussed. Based on the optimized crystal structures, the vibrational frequencies for the internal and lattice modes of the nitromethane crystal at ambient and high pressures are computed, and the pressure-induced frequency shifts of these modes are discussed.     

Effect of high hydrostatic pressure on prebiotic peptide synthesis

Here we reported the high hydrostatic pressure, as a key factor of deep-sea environment conditions, promoted the peptide formation and should be considered as one of the significant factors in studying the origin of life.     

Equilibrium surface properties of lipid mixtures of retinal,phosphatidylcholine and fatty acid derivatives at the air/water interface

The miscibilities of phosphatidylcholine, retinal and saturated fatty acid derivatives in surface phases at the air/water interface are investigated on the basis of the thermodynamic two-dimensional phase rule. The latter is applied to the ‘collapse’ pressure and the equilibrium surface pressure characteristics of binary lipid monolayers or spread amphiphilic mixtures, respectively. The equilibrium surface pressures (ESPs), at which insoluble lipid monolayers are in equilibrium with three-dimensional lipid phases, are determined by spreading of single-component or binary solutions of lipids in organic solvent up to supersaturation at the air/water interface. The kinetics of establishment of steady surface pressure values at supersaturation is followed depending on the nature of the lipid samples. ESPs and ‘collapse’ pressures of mixtures of dilaur-oylphosphatidylcholine (DLPC), all-trans retinal (t-R) and lauric acid (LA) are studied at various lipid molar ratios. The compositional phase diagrams of the ESPs and ‘collapse’ pressures, obtained at a constant temperature, indicate that the interfacial miscibilities of both DLPC and t-R and DLPC and LA are non-ideal. Owing to its ‘bulky’ molecular structure and the tendency towards self-aggregation, dominated by intermolecular π-π interactions, the t-R component could be accommodated in the hydrophobic portion of the phospholipid membrane at mole fractions less than 0.5. The accommodation of the other neutral, rod-like fatty acid component in the DLPC matrix is probably favoured by the formation of intermolecular hydrogen bonding. Phase separation between DLPC and LA is evident from the thermodynamic results at high LA mole fractions (> 0.75) in the surface mixtures.     

Liquid-liquid phase equilibria in polymer solutions and polymer mixtures

The pressure dependence of liquid-liquid equilibria in weakly interacting binary macromolecular systems (homopolymer solutions and blends) will be discussed. The common origin of the separate high-temperature/low-temperature and high-pressure/low-pressure branches of demixing curves will be demonstrated by extending the study into the region of metastable liquid states including the undercooled, overheated and stretched states (i.e. states at negative pressures). The seemingly different response of the UCST-branch of solutions and blends when pressurized (pressure induced mixing for most polymer solutions, pressure induced demixing for most blends) will be explained in terms of the location of a hypercritical point found either at positive (most solutions) or negative pressure (most blends). Further, it is shown that the pressure dependence of demixing of homopolymer solutions and blends may be described using a ‘master-curve’ which, however, is sometimes partly masked by degradation or by vapour-liquid and/or solid-liquid phase transitions. Experimental results demonstrating the extension of liquid-liquid phase boundary curves into the metastable regions will be presented, and the existence of solubility islands in the vicinity of the hypercritical points discussed.     

High-pressure Raman study of liquid and molecular crystal at room temperature. Methylene chloride

Raman spectra of liquid and crystalline CH₂Cl₂ were measured at hydrostatic pressures up to 85 kbar and at room temperature. The pressure dependences of the internal modes (ν₄, ν₃, ν₉, ν₂ and ν₁) and the two external (lattice) modes are reported; the ν₄ symmetric CCl₂ bending mode is split into two major peaks at the liquid—solid phase transition point (at 11.3 kbar), and the discontinuities of the slopes for their peak frequencies against pressure suggest a second-order phase transition at ≈ 45 kbar. The pressure data are used to test the applicability of the vibrational scaling law proposed by Zallen for a molecular crystal.     

The effect of aqueous solutions of trimethylamine-N-oxide on pressure induced modifications of hydrophobic interactions

To understand the mechanism of protein protection by the osmolyte trimethylamine-N-oxide (TMAO) at high pressure, using molecular dynamics (MD) simulations, solvation of hydrophobic group is probed in aqueous solutions of TMAO over a wide range of pressures relevant to protein denaturation. The hydrophobic solute considered in this study is neopentane which is a considerably large molecule. The concentrations of TMAO range from 0 to 4 M and for each TMAO concentration, simulations are performed at five different pressures ranging from 1 atm to 8000 atm. Potentials of mean force are calculated and the relative stability of solvent-separated state over the associated state of hydrophobic solute are estimated. Results suggest that high pressure reduces association of hydrophobic solutes. From computations of site-site radial distribution function followed by analysis of coordination number, it is found that water molecules are tightly packed around the nonpolar particle at high pressure and the hydration number increases with increasing pressure. On the other hand, neopentane interacts preferentially with TMAO over water and although hydration of neopentane reduces in presence of this osmolyte, TMAO does not show any tendency to prevent the pressure-induced dispersion of neopentane moieties. It is also observed that TMAO molecules prefer a side-on orientation near the neopentane surface, allowing its oxygen atom to form favorable hydrogen bonds with water while maintaining some hydrophobic contacts with neopentane. Analysis of hydrogen-bond properties and solvation characteristics of TMAO reveals that TMAO can form hydrogen bonds with water and it reduces the identical nearest neighbor water molecules caused by high hydrostatic pressures. Moreover, TMAO enhances life-time of water-water hydrogen bonds and makes these hydrogen bonds more attractive. Implication of these results for counteracting effect of TMAO against protein denaturation at high pressures are discussed.     

Structural Properties of Nitromethane Molecular Crystal under High Pressure： An ab initio Investigation

The pressure-dependent structural properties under hydrostatic pressure up to 120 GPa and the decomposition under uniaxial compression along the b lattice vector up to 40 GPa of nitromethane molecular crystal using ab initio method are presented. The internal molecular bond lengths and bond angles were calculated for different pressures. All bond lengths decrease as the pressures are increased under hydrostatic compression. The obvious rotation of methyl group is 33.89° under hydrostatic pressure at 120 GPa. In addition, we observe the change of C-H bonds, which have been stretched under uniaxial compression along b lattice vector in the range of 0-40 GPa of nitromethane.     

Study of the hydrostatic pressure dependence of the Raman spectrum of single-walled carbon nanotubes and nanospheres

We have investigated the behavior of single-walled carbon nanotubes and nanospheres (C(60)) under high hydrostatic pressure using Raman spectroscopy over the pressure range 0.2-10 GPa using a diamond anvil cell. Different liquid mixtures were used as pressure transmission fluids (PTF). Comparing the pressure dependence of the Raman peak positions for the nanotubes and the nanospheres in different PTF leads to the observation of a number of new phenomena. The observed shift in Raman peak position of both radial and tangential modes as a function of applied pressure and their dependence on the PTF chemical composition can be rationalized in terms of adsorption of molecular species from the of PTF on to the surface of the carbon nanotubes and/or nanospheres. The peak shifts are fully reversible and take place at a comparatively modest pressure (2-3 GPa) that is far below pressures that might be required to collapse the nanoparticles. Surface adsorption of molecular species on the nanotube or nanospheres provides a far more plausible rational for the observed phenomena than ideas based on the notion of tube collapse that have been put forward in the recent literature.     

Structural phase transitions in Zn(CN)2 under high pressures

High pressure behavior of zinc cyanide (Zn(CN)₂) has been investigated with the help of synchrotron-based X-ray diffraction measurements. Our studies reveal that under pressure this compound undergoes phase transformations and the structures of the new phases depend on whether the pressure is hydrostatic or not. Under hydrostatic conditions, Zn(CN)₂ transforms from cubic to orthorhombic to cubic-II to amorphous phases. In contrast, the non-hydrostatic pressure conditions drive the ambient cubic phase to a partially disordered crystalline phase, which eventually evolves to a substantially disordered phase. The final disordered phase in the latter case is distinct from the amorphous phase observed under the hydrostatic pressures.     

Isochronal temperature–pressure superpositioning of the α-relaxation in type-A glass formers

Dielectric spectra were obtained at ambient and elevated pressures for three ‘type-A’ glass formers, which exhibit excess intensity on the high frequency side of the structural relaxation peak. The response to pressure of the peak maximum and the excess wing suggests categorization of such glass formers into two groups: associated liquids, in which the α-relaxation and the excess wing have a different pressure dependence, and van der Waals liquids, which at fixed value of the α-relaxation time, conform to temperature–pressure superpositioning. This distinction is believed to arise from the change in the number of intermolecular bonds (non-dispersive interactions) with volume.     

Ionic Liquid: A Good Pressure Transmitting Medium

A potential new use of room temperature ionic liquid for a pressure transmitting medium is introduced in detail. A systematic study of the pressure-induced solidification of 1-butyl-3-methylimidazolium tetrafluoroborate ([C₄MIM][BF₄]) is presented in a diamond anvil cell at pressures up to 30 GPa by combining ruby fluorescence and synchrotron X-ray diffraction measurements. Its hydrostatic properties have been determined with hydrostatic limit up to about 6 GPa, and a slight pressure gradient was found up to 21 GPa. These results indicate that this kind of ionic liquid is a good hydrostatic pressure transmitting medium.     

First-principles study of structural and vibrational properties of crystalline silver azide under high pressure

A detailed first-principles study of the structural and vibrational properties of crystalline silver azide under hydrostatic pressure of 0–500 GPa has been performed with density functional theory in the generalized gradient approximation. The crystal structure is relaxed to allow ionic configurations, cell shape, and volume to change without any symmetry constraints. It is found that the silver azide crystal remains orthorhombic structure with Ibam space group for pressures up to 7 GPa, where there is a transition to an I4/mcm tetragonal symmetry. The lattice parameter and electronic structure are investigated as functions of pressure. The calculated vibrational frequencies at ambient pressure are in agreement with available experimental data. We also discuss the pressure-induced frequency shifts for the internal and lattice modes of silver azide crystal upon compression.     

Phase behaviour of the thermotropic cubic mesogens 1,2-bis-(4-n-undecyl- and 4-n-dodecyl-oxybenzoyl)hydrazine under pressure

The phase transition behaviour of two optically isotropic, thermotropic cubic mesogens 1,2-bis-(4-n-undecyloxy- and 4-n-dodecyloxy-benzoyl)hydrazine, BABH(11) and BABH(12), was investigated under hydrostatic pressures up to 300 MPa using a high pressure differential thermal analyser, a wide angle X-ray diffractometer and a polarizing optical microscope equipped with a high pressure optical cell. It is found that for BABH(11) and BABH(12), a smectic C (SmC) phase is induced between the isotropic liquid (I) and the cubic (Cub) phases by applying pressures above 10-12 and 16-17 MPa, respectively. A sea-island texture consisting of bright sand-like sea regions (SmC phase) and areas of dark islands (Cub phase) appears in the mesophase under pressures up to 140 MPa, while the sand-like texture of the SmC phase is formed predominantly on cooling under pressure. These observations indicate the destabilization of the cubic phase with increasing pressure. The phase transition sequence of BABH(11) and BABH(12), Cr-Cub-I at atmospheric pressure, changes to Cr-Cub-SmC-I under intermediate pressures and would change to Cr-SmC-I under elevated pressure.     

Pressure effect on the three-dimensional charge-transfer ferromagnet [{Ru₂(m-FPhCO₂)₄}₂(BTDA-TCNQ)

The magnetic properties of a three-dimensional infinite network compound, [{Ru?(m-FPhCO?)?}?(BTDA-TCNQ)] (m-FPhCO?? = m-fluorobenzoate; BTDA-TCNQ = bis(1,2,5-thiadiazolo)tetracyanoquinodimethane), were investigated under hydrostatic pressures up to 12.9 kbar. The applied pressure caused the intralattice exchange couplings to weaken, inducing a decrease in the magnetic phase transition temperature from 107 K at ambient pressure to 57 K at 12.9 kbar.     

High-pressure Raman scattering on Fe2(MoO4)3 microcrystals obtained by a hydrothermal method

This study reports on the effect of high hydrostatic pressure on the vibrational spectra of Fe₂(MoO₄)₃ microcrystals using Raman scattering in combination with a membrane diamond-anvil cell. The ferric molybdate was obtained by the conventional hydrothermal method, and the structural and morphological properties of the sample were characterized using additionally X-ray diffraction, scanning electron microscopy, as well as energy dispersive spectroscopy. The X-ray diffraction measurements have shown that the crystals have a monoclinic structure. At pressures higher than 4.8 GPa, the disappearance of external modes was observed, suggesting that pressure has induced a breakdown in the translational symmetry of the crystal system. The high-pressure amorphous phase is reversible and attributed to an incomplete crystal-amorphous transformation.     

Pressure-induced reduction of shielding for improving sonochemical activity

The effect of hydrostatic pressure on chemical reactions induced by 20 kHz ultrasound has been studied using three different methods: the oxidation of potassium iodide, bubble cloud visualization studies, and sound attenuation measurements. The latter two have demonstrated that shielding of the ultrasonic wave is less pronounced at elevated pressures. Accordingly, the yield of iodine liberation increases with increasing pressure. At high static pressures, however, the less efficient cavitation dynamics dominate and the chemical reactivity decreases rapidly.     

Lattice dynamics of cuprous iodide

The lattice dynamics of cuprous iodide have been investigated on the basis of an ‘11-parameter’ rigid-ion model (RIM). The reported neutron scattering data of CuI are reasonably well decribed by the model, especially when an optimized procedure for the selection of parameters is used. Results are presented for the phonon dispersion curves of CuI. The critical point phonon (CPP) frequencies inferred from the model are shown to provide satisfactory assignments of the observed two-phonon features in the available Raman spectrum.     

A specialized apparatus for the study of liquid crystals at high pressures

Abstract

A pressure system specially designed for the study of liquid crystal materials at high pressures up to 4kbar is presented. The pressure system is based on a hydrostatic screw injector and uses either oil or gas as the pressure transmitting medium. The type of measurements which can be performed with the instrument include polarized microscopy, optical spectroscopy, electrooptic and electrical measurements. The different measurements performed place specific constraints on the design of the apparatus and the pressure cells, and details are given. A preliminary study of the smectic A modification of the twist grain boundary phase (TGB_A) at high pressures is presented. The pressure versus temperature phase diagram shows (i) a negative gradient of the TGB_A/isotropic phase boundary line and (ii) that the TGB_A phase does not exist at pressures above about 250 bar. Following Lubensky's analogy between the TGB_A phase and type II superconductors, the disappearance of the TGB_A phase at high pressure may imply that the Landau-Ginsburg parameter K decreases with pressure.