Synthesis of coesite nanocrystals from ethane bridged periodic mesoporous organosilica at low temperature and extreme pressure

Coesite nanocrystals have been synthesized from periodic mesoporous organosilica (PMO) with (CH(2))(2) bridges heated at 300 °C for 150 min and 12 GPa. The crystals are not sintered, single crystalline, and have diameters of ca. 100-300 nm. Below 300 °C, an amorphous non-porous organosilica glass was obtained. Heating above 300 °C at 12 GPa results in the rapid crystal growth and micron size coesite crystals were formed.     

Recombination of the hydrated electron at high temperature and pressure in hydrogenated alkaline water

Pulse radiolysis experiments were performed on hydrogenated, alkaline water at high temperatures and pressures to obtain rate constants for the reaction of hydrated electrons with hydrogen atoms (H* + e-(aq) --> H(2) + OH-, reaction 1) and the bimolecular reaction of two hydrated electrons (e-(aq) + e-(aq) --> H(2) + 2 OH-, reaction 2). Values for the reaction 1 rate constant, k(1), were obtained from 100 - 325 degrees C, and those for the reaction 2 rate constant, k(2), were obtained from 100 - 250 degrees C, both in increments of 25 degrees C. Both k(1) and k(2) show non-Arrhenius behavior over the entire temperature range studied. k(1) shows a rapid increase with increasing temperature, where k(1) = 9.3 x 10(10) M(-1) s(-1) at 100 degrees C and 1.2 x 10(12) M(-1) s(-1) at 325 degrees C. This behavior is interpreted in terms of a long-range electron-transfer model, and we conclude that e-aq diffusion has a very high activation energy above 150 degrees C. The behavior of k(2) is similar to that previously reported, reaching a maximum value of 5.9 x 10(10) M(-1) s(-1) at 150 degrees C in the presence of 1.5 x 10(-3) m hydroxide. At higher temperatures, the value of k(2) decreases rapidly and above 250 degrees C is too small to measure reliably. We suggest that reaction 2 is a two-step reaction, where the first step is a proton transfer stimulated by the proximity of two hydrated electrons, followed immediately by reaction 1.     

X-ray scattering by water molecules studied by using synchrotron radiation

The energy spectra of free water molecules were measured at scattering angles 2θ ranging from 10.5° to 75.7°, using an angle-dispersive-type diffractometer and synchrotron radiation as an X-ray source. A silicon (111) monochrometer was used to obtain incident X-rays with the wavelengths of (1.543/n) Å (n = 1,3,4,5). Observed inelastic scattering peaks are clearly separated from eleastic ones at s values [s = (4π/λ) sin Å] larger than 8 Å^?1. The increase of the separation with an increasing s value was consistent with the classical theory of the Compton shift. The total (elastic plus inelastic) intensities were obtained over a range of s = 0.74–5.0 Å^?1. Experimental difference intensities Δσ_ee and Δσ_ne were obtained separately by combining the X-ray and high-energy electron scattering data. The experimental results are in reasonable agreement with the theoretical intensities calculated from SCF and CI molecular wave functions with a basis set of double-zeta plus polarization functions. © 1994 John Wiley & Sons, Inc.     

Molecular dynamics at constant pressure and temperature

Methods are discussed for generating by molecular dynamics isobaric-isoenthalpic, NPH, isochoric-isothermal, NVT, and isobaric-isothermal, NPT, ensembles. Andersen's constant-pressure method is reformulated so that the ensemble rather than the scaled system is directly calculated. Four constant-temperature schemes were considered. Two involve the addition of a stochastic collision term to the molecular trajectories. The Andersen method and a stochastic dynamics approach were examined. The latter employed a velocity damping term in addition to the random force. Two other methods employed uniform velocity scaling applied to all molecules. The NPT algorithm induces a transition to the dilute phase for a Lennard-Jones fluid in the spinodal region (p^* = 0.5, T^* = 1.28) of the phase diagram. The thermodynamic equivalence of the ensembles is demonstrated by long calculations of the chemical potential of Lennard-Jones states by the particle insertion method. The internal energy, pressure, constant volume and pressure specific heats, adiabatic compressibilities, pair radial distribution functions and self-diffusion coefficients are also evaluated. Only for second-order thermodynamic quantities is there evidence of an ensemble dependence.     

Raman scattering spectroscopic study of n-tetradecane under high pressure and ambient temperature

The Raman spectroscopy of n-tetradecane was investigated in a Moissanite anvil cell at pressure from 0.1 MPa to 1.4 GPa and ambient temperature. The result shows that the liquid-solid phase transition of n-tetradecane takes place at around 302.8 MPa and the corresponding DeltaV(m) obtained is about -9.6 cm(-3)/mol. Above 302.8 MPa, the frequencies of CH(2) and CH(3) symmetric stretching and asymmetric stretching vibration shift to higher wave numbers in a linear manner with increasing pressure, which can be expressed as: nu(s)(CH(3))=0.013P+2882.0; nu(as)(CH(3))=0.014P+2961.6; nu(s)(CH(2))=0.013P+2850.8; nu(as)(CH(2))=0.009P+2923.2. This relationship indicates that n-tetradecane can be a reliable pressure gauge for the experimental study within the pressure range of 0.3-1.4 GPa.     

X-ray diffraction and inelastic neutron scattering study of 1:1 tetramethylpyrazine chloranilic acid complex: temperature, isotope, and pressure effects

The x-ray diffraction studies of the title complex were carried out at room temperature and 14 K for H/D (in hydrogen bridge) isotopomers. At 82 K a phase transition takes place leading to a doubling of unit cells and alternation of the hydrogen bond lengths linking tetramethylpyrazine (TMP) and chloranilic acid molecules. A marked H/D isotope effect on these lengths was found at room temperature. The elongation is much smaller at 14 K. The infrared isotopic ratio for O-H(D)...N bands equals to 1.33. The four tunnel splittings of methyl librational ground states of the protonated complex required by the structure are determined at a temperature T=4.2 K up to pressures P=4.7 kbars by high resolution neutron spectroscopy. The tunnel mode at 20.6 microeV at ambient pressure shifts smoothly to 12.2 microeV at P=3.4 kbars. This is attributed to an increase of the strength of the rotational potential proportional to r(-5.6). The three other tunnel peaks show no or weak shifts only. The increasing interaction with diminishing intermolecular distances is assumed to be compensated by a charge transfer between the constituents of deltae/e approximately 0.02 kbar(-1). The phase transition observed between 3.4 and 4.7 kbars leads to increased symmetry with only two more intense tunneling bands. In the isotopomer with deuterated hydrogen bonds and P=1 bar all tunnel intensities become equal in consistency with the low temperature crystal structure. The effect of charge transfer is confirmed by a weakening of rotational potentials for those methyl groups whose tunnel splittings were independent of pressure. Density functional theory calculations for the model TMP.(HF)2 complex and fully ionized molecule TMP+ point out that the intramolecular rotational potential of methyl groups is weaker in the charged species. They do not allow for the unequivocal conclusions about the role of the intermolecular charge transfer effect on the torsional frequencies.     

Hard domain compression of triblock-copolymer gels by high pressure X-ray small-angle scattering

Small-angle X-ray scattering using synchrotron radiation at pressures up to 700 MPa has been performed on triblock-copolymer gels, polystyrene-rubber-polystyrene with poly(ethylene/butylene) as the rubber mid-blocks. A 2-stage pressure behaviour could be verified by comparison of the experimental data with the Tait-equation and a model of pressure screening.     

Mineral–water interfacial structures revealed by synchrotron X-ray scattering

Chemical reactions occurring at the mineral–water interface are controlled by an interfacial layer, nanometers thick, whose properties may deviate from those of the respective bulk mineral and water phases. The molecular-scale structure of this interfacial layer, however, is poorly constrained, and correlations between macroscopic phenomena and molecular-scale processes remain speculative. The application of high-resolution X-ray scattering techniques has begun to provide substantial new insights into the molecular-scale structure of the mineral–water interface. In this review, we describe the characteristics of synchrotron-based X-ray scattering techniques that make them uniquely powerful probes of mineral–water interfacial structures and discuss the new insights that have been derived from their application. In particular, we focus on efforts to understand the structure and distribution of interfacial water as well as their dependence on substrate properties for major mineral classes including oxides, carbonates, sulfates, phosphates, silicates, halides and chromates. We compare these X-ray scattering results with those from other structural and spectroscopic techniques and integrate these to provide a conceptual framework upon which to base an understanding of the systematic variation of mineral–water interfacial structures.     

HgWO4 synthesized at high pressure and temperature

Crystals of mercury(II) tungstate(VI), HgWO₄, were grown in sealed gold tubes under an Ar atmosphere at 300 MPa and 973 K. The monoclinic crystal structure (C2/c) of HgWO₄ consists of zigzag chains of edge‐sharing WO₆ octahedra running along the c axis and layers of very distorted corner‐sharing HgO₆ octahedra in the bc plane. The Hg atom lies on an inversion centre and the W atom is on a twofold axis. No structural effects which can be ascribed to the high pressure used in the synthesis were found.     

Simultaneous small-angle X-ray scattering and wide-angle X-ray diffraction

The simultaneous SAXS/WAXD technique is shown to provide an unambiguous method for following structural changes taking place during the programmed heating of a range of multiphase polymeric materials. Results are given for polyethylene, block copolyurethanes and block copolyesters containing liquid crystalline hard segments. UK Thermal methods Group Award Lecture     

Concentration dependence of the zero-angle X-ray scattering from liquid mixtures of water and methanol

Measurements on the concentration dependence of the zero-angle X-ray scattering from liquid mixtures of water and methanol at 25°C and atmospheric pressure are compared with theoretical data calculated from thermodynamic quantities. Good agreement between theory and experiment is observed.     

Degradation effects in the extraction of antioxidants from birch bark using water at elevated temperature and pressure

Experiments with birch bark samples have been carried to enable a distinction between extraction and degradation effects during pressurised hot water extraction. Two samples, E80 and E180, contained birch bark extracts obtained after extraction at 80 and 180 °C for up to 45 min, respectively. Two other samples, P80 and P180, were only extracted for 5 min at the two temperatures and were thereafter filtered and hydrothermally treated at 80 and 180 °C, respectively. During the latter treatment, samples were collected at different times to assess the stability of the extracted compounds. An offline DPPH (2,2-diphenyl-1-picrylhydrazyl) assay, as well as a high performance liquid chromatographic separation coupled to an electrochemical detector, were used to determine the antioxidant capacity of the processed samples. The results obtained with the different techniques were compared to assess the yield of the extraction and degradation processes. In addition, an online hyphenated system comprising high performance liquid chromatography coupled to diode-array; electrochemical; and tandem mass spectrometric detection (HPLC-DAD-ECD-MS/MS) was used to study the compositions of the extracts in more detail. The results for the samples processed at 80 °C showed that the extraction reached a steady-state already after 5 min, and that the extracted compounds were stable throughout the entire extraction process. Processing at 180 °C, on the other hand, gave rise to partly degraded extracts with a multitude of peaks in both the diode array and electrochemical detectors, and a higher antioxidant capacity compared to for the extracts obtained at 80 °C. It is concluded that HPLC-DAD-ECD is a more appropriate technique for the determination of antioxidants than the DPPH assay. The mass spectrometric results indicate that one of the extracted antioxidants, catechin, was isomerised to its diastereoisomers; (+)-catechin, (−)-catechin, (+)-epicatechin, and (−)-epicatechin.     

Radiation-induced polymerization of hexafluoropropylene at high temperature and pressure

The radiation-induced polymerization of hexafluoropropylene was studied in the pressure and temperature ranges of 4,500–15,000 atm. and 100–230°C., respectively. Retardation was a serious problem; data thought to apply to the unretarded polymerization are summarized below. At 1,500 rad/hr. the polymerization rate was 15%/hr. at 230°C. and 15,000 atm. The activation enthalpy and volume are 9.5 kcal./mole and ?10 cc./mole, respectively. The rate varies as the square root of the radiation intensity. The largest intrinsic viscosity of the polymer is 2.0 dl./g.; values increase with temperature and pressure. At 130°C. and 10,000 atm. the intrinsic viscosity was the same at two radiation intensities.     

X-ray Raman spectroscopic study of benzene at high pressure

We have used X-ray Raman spectroscopy (XRS) to study benzene up to approximately 20 GPa in a diamond anvil cell at ambient temperature. The experiments were performed at the High-Pressure Collaborative Access Team's 16 ID-D undulator beamline at the Advanced Photon Source. Scanned monochromatic X-rays near 10 keV were used to probe the carbon X-ray edge near 284 eV via inelastic scattering. The diamond cell axis was oriented perpendicular to the X-ray beam axis to prevent carbon signal contamination from the diamonds. Beryllium gaskets confined the sample because of their high transmission throughput in this geometry. Spectral alterations with pressure indicate bonding changes that occur with pressure because of phase changes (liquid: phase I, II, III, and III') and possibly due to changes in the hybridization of the bonds. Changes in the XRS spectra were especially evident in the data taken when the sample was in phase III', which may be related to a rate process observed in earlier shock wave studies.     

Hydrogen confinement in carbon nanopores: extreme densification at ambient temperature

In-situ small-angle neutron scattering studies of H(2) confined in small pores of polyfurfuryl alcohol-derived activated carbon at room temperature have provided for the first time its phase behavior in equilibrium with external H(2) at pressures up to 200 bar. The data were used to evaluate the density of the adsorbed fluid, which appears to be a function of both pore size and pressure and is comparable to the density of liquid H(2) in narrow nanopores at ～200 bar. The surface-molecule interactions responsible for densification of H(2) within the pores create internal pressures that exceed the external gas pressure by a factor of up to ～50, confirming the benefits of adsorptive storage over compressive storage. These results can be used to guide the development of new carbon adsorbents tailored for maximum H(2) storage capacities at near-ambient temperatures.     

X-ray scattering studies of fatty acid films on water and on Cdcl2 solutions

X-ray diffraction methods for Langmuir films on the surface of water are briefly presented, together with recent results for docosanoic acid monolayers on pure water and for eicosanoic acid monolayers on an ionic subphase.     

Neutron scattering experiments on fully deuterated liquid N-methylformamide DCONDCD3 at various temperatures and under pressure. Comparison to X-ray results

Neutron scattering experiments are performed on fully deuterated liquid N-methylformamide (C2D5NO) at various temperatures and under pressure. The recorded data at atmospheric pressure and room temperature are analyzed to yield the molecular form factor and the distinct pair correlation function. Our measurements clearly show that the hydrogen-bond network, of which the parameters are deduced, persists locally in the liquid. The experimental structure factor could be explained in terms of short-range crystal structure. The r(N...O) distance decreases with increasing temperature from 293 to 398 K, whereas no significant variation of the intermolecular structure is detected when varying pressure from 1 bar to 4 kbar. Along the study, some comparison is made with complementary X-ray results.     

Neutron scattering in polymer gels at the theta temperature

Small angle elastic neutron scattering was performed on three polystyrene networks swollen to equilibrium in cyclohexane at the theta temperature. Comparison of the SANS spectra with similar measurements in uncross-linked solutions reveals that in the intermediate Q region the gels behave like solutions of lower concentration than the equivalent solutions. We assume that the gel spectra are composed of a static component plus a dynamic, or solution-like part. By a suitable fitting procedure, the former can be estimated, yielding the mean square static concentration fluctuation <Δφ²> generated by cross-linking contraints in the gel. Independent measurement of the swelling pressure of these gels permits an estimate to be made of the scattering intensity I(Q=0) of the dynamic part of the spectrum. Plausible agreement is found between the macroscopic and microscopic estimations of the osmotic compressibility if allowance is made for the concentration polydispersity in the gels.