Clathrate hydrate formation after CO2-H2O vapour deposition

We study vapour condensation of carbon dioxide and water at 77 K in a high-vacuum apparatus, transfer the sample to a piston-cylinder apparatus kept at 77 K and subsequently heat it at 20 MPa to 200 K. Samples are monitored by in situ volumetric experiments and after quench-recovery to 77 K and 1 bar by powder X-ray diffraction. At 77 K a heterogeneous mixture of amorphous solid water (ASW) and crystalline carbon dioxide is produced, both by co-deposition and sequential deposition of CO(2) and H(2)O. This heterogeneous mixture transforms to a mixture of cubic structure I carbon dioxide clathrate and crystalline carbon dioxide in the temperature range 160-200 K at 20 MPa. However, no crystalline ice is detected. This is, to the best of our knowledge, the first report of CO(2) clathrate hydrate formation from co-deposits of ASW and CO(2). The presence of external CO(2) vapour pressure in the annealing stage is not necessary for clathrate formation. The solid-solid transformation is accompanied by a density increase. Desorption of crystalline CO(2) atop the ASW sample is inhibited by applying 20 MPa in a piston-cylinder apparatus, and ultimately the clathrate is stabilized inside layers of crystalline CO(2) rather than in cubic or hexagonal ice. The vapour pressure of carbon dioxide needed for clathrate hydrate formation is lower by a few orders of magnitude compared to other known routes of CO(2) clathrate formation. The route described here is, thus, of relevance for understanding formation of CO(2) clathrate hydrates in astrophysical environments.     

Methane adsorption on microporous carbon adsorbent with wide pore size distribution

Methane adsorption on a microporous carbon adsorbent with a bimodal pore size distribution is studied at temperatures of 303–333 K at pressures up to 30 MPa. The total micropore volume of the adsorbent, as determined by the Dubinin method, is as large as 1.02 cm³/g. Maximum values of methane adsorption of ≈18 mmol/g are attained at a temperature of 303 K and a pressure of 30 MPa. Methane adsorption isosteres are plotted based on experimental data, and adsorption equilibria at low temperatures are calculated using the linearity of the plots. Experimental isotherms of methane adsorption are compared with the isotherms calculated by the Dubinin–Nikolaev equation with variations in parameters E and n. Temperature dependences of these parameters are determined. Specific characteristics of methane adsorption accumulation are calculated.     

Hydrogen adsorption on nanoporous carbon adsorbents prepared from furaldehyde by thermochemical synthesis

Hydrogen adsorption on two samples of active carbon (FAS) produced from furaldehyde by the thermochemical synthesis method is investigated. Maximum hydrogen adsorption on these active carbons at hydrogen boiling temperature of 20.38 K and a pressure of 0.101 MPa is calculated in terms of the theory of the volume filling of micropores. Hydrogen adsorption on FAS-1-05 active carbon at temperatures of 77, 196, and 300 K and pressures of 7 and 20 MPa is calculated using the condition of linear isosteres. The calculated data are compared with the experimental results obtained for the same adsorbent at temperatures of 77 and 293 K and pressures below 5.1 and 16.1 MPa, respectively. The maximum adsorption value of hydrogen on FAS-1-05 amounts to 6.2 wt % at 5.1 MPa and 77 K.     

Volume Change for Hydrophobic Hydration of Biphenyl

The solubility of biphenyl in water at 298.2 K was measured at pressures up to 200 MPa. The logarithm of the solubility linearly decreased with increasing pressure. From the value of the slope, the volume change for the dissolution of biphenyl in water was thermodynamically estimated to be 11.8±0.5 cm³⋅mol⁻¹. The solution density of biphenyl in carbon tetrachloride at 298.2 K and 0.10 MPa was also measured to estimate the partial molar volume of the solute. Using these values and the molar volume of solid biphenyl, the volume change for the hydrophobic hydration of the biphenyl was estimated to be −6.5±1.6 cm³⋅mol⁻¹. The value was compared with those of methylene group and other aromatic hydrocarbons as a function of the rotational molecular diameter of these hydrophobic solutes.     

Association of ionic liquids in solution: a combined dielectric and conductivity study of [bmim][Cl] in water and in acetonitrile

Ion association of the ionic liquid [bmim][Cl] in acetonitrile and in water was studied by dielectric spectroscopy for salt concentrations c ≤ 1.3 M at 298.15 K and by measurement of molar electrical conductivities, Λ, of dilute solutions (c ≤ 0.006 M) in the temperature range 273.15 ? T/K ≤ 313.15. Whilst acetonitrile solutions of [bmim][Cl] exhibit moderate ion pairing, with an association constant of K°(A) ≈ 60 M(-1) and increasing with temperature, [bmim][Cl] is only weakly associated in water (K°(A) ≈ 6 M(-1)) and ion pairing decreases with rising temperature. Only contact ion pairs were detected in both solvents. Standard-state enthalpy, entropy and heat capacity changes of ion association were derived, as well as the activation enthalpy of charge transport and the limiting conductivity of the cation, λ(∞)?([bmim](+)). These data, in conjunction with effective solvation numbers obtained from the dielectric spectra, suggest that the solvation of [bmim](+) is much weaker in water than in acetonitrile.     

Gas hydrate formation in the system C2H6–H2–H2O at pressures up to 250 MPa

Decomposition curves of gas hydrates formed in the ethane–hydrogen–water system were studied in the pressure interval 2–250 MPa. Gas hydrates synthesized at low (up to 5 MPa) pressures were also studied with use of X-ray powder diffraction and Raman spectroscopy. It was shown that ethane–hydrogen mixtures with hydrogen contents 0–30 mol.% form cubic structure I gas hydrates. Higher hydrogen concentration most probably results in appearance of another hydrate phase. We speculate that the gas mixtures with the hydrogen content above 60 mol.% form cubic structure II double hydrate of hydrogen and ethane at temperatures below ≈280 K and pressures above 25 MPa.     

Anomalous dielectric behavior and thermal motion of water molecules confined in channels of porous coordination polymer crystals

Guest water molecules confined in channels of porous coordination polymer crystals [Ln(2)Cu(3)(IDA)(6)]·nH(2)O (Ln = La, Nd, Sm, Gd, Ho, Er; IDA = [NH(CH(2)COO)(2)](2-); n ≈ 9) exhibited large dielectric constants (ε) and antiferroelectric behaviors at high temperatures (e.g., ε(Sm) ≈ 1300 at 400 K). In addition, plots of the temperature dependence of ε showed broad peaks at ～170 K, below which ε became very small. These puzzling temperature dependences of ε are consistent with the results of molecular dynamics simulations, suggesting the "freezing of thermal motion" of water molecules at ～170 K.     

Aggregation-induced amplified quenching in conjugated polyelectrolytes with interrupted conjugation

A pair of anionic conjugated polyelectrolytes that contain three-ring (phenylene ethynylene) units linked by a single -CH(2)- or -O- tether (P1 and P2, respectively) are studied. The linkers serve to interrupt the π conjugation along the polymer backbone. Fluorescence spectroscopy reveals that P2 forms a fluorescent aggregate in methanol and water; however, the fluorescence of P1 is much weaker in water, and P1 exhibits only weak aggregate fluorescence. Fluorescence quenching of the polymers was examined using methyl viologen (MV(2+)) as a cationic quencher. P1 shows only a weak amplified quenching effect, with a Stern-Volmer quenching constant of K(SV) ≈ 6 × 10(5) M(-1) in methanol. Interestingly, for P2 in methanol, the aggregate emission is strongly quenched with K(SV) ≈ 5 × 10(6) M(-1), which is comparable to the highest quenching efficiency observed for fully π-conjugated polyelectrolytes. By contrast, the monomer emission is quenched much less efficiently, with K(SV) ≈ 2 × 10(5) M(-1). The results are explained by a model in which -O- linked polymer P2 is able to fold into a helical conformation in solution, which facilitates the formation of extended π-stacked aggregates allowing long-distance exciton transport.     

Solubility of α-amino acids in water under high pressure: glycine, -alanine, -valine, -leucine, and -isoleucine

The solubility of glycine, -alanine, -valine, -leucine, and -isoleucine in water was measured at 298.15 K and pressures up to 400 MPa. The standard deviation of the logarithm of the solubility is 0.001–0.003, equal to or better than the accuracy of atmospheric pressure measurement in the literature (0.001–0.05). A variety of solubility phenomena were observed. The solubility of glycine decreased with increasing pressure, whereas that of -alanine increased. The -valine and -isoleucine have a solubility maximum at around 100 MPa, and -leucine seems to exhibit a solid-phase phase transition at around 200 MPa. Pressure coefficient of the solubilities at 0.10 MPa is compared with that thermodynamically estimated in reference to aqueous density measurements of glycine and -alanine at 298.15 K and 0.10 MPa, supporting a reliability of our high-pressure measurements.     

Neutron diffraction study of unusual phase separation in the antiperovskite nitride Mn3ZnN

The antiperovskite Mn(3)ZnN is studied by neutron diffraction at temperatures between 50 and 295 K. Mn(3)ZnN crystallizes to form a cubic structure at room temperature (C1 phase). Upon cooling, another cubic structure (C2 phase) appears at around 177 K. Interestingly, the C2 phase disappears below 140 K. The maximum mass concentration of the C2 phase is approximately 85% (at 160 K). The coexistence of C1 and C2 phase in the temperature interval of 140-177 K implies that phase separation occurs. Although the C1 and C2 phases share their composition and lattice symmetry, the C2 phase has a slightly larger lattice parameter (Δa ≈ 0.53%) and a different magnetic structure. The C2 phase is further investigated by neutron diffraction under high-pressure conditions (up to 270 MPa). The results show that the unusual appearance and disappearance of the C2 phase is accompanied by magnetic ordering. Mn(3)ZnN is thus a valuable subject for study of the magneto-lattice effect and phase separation behavior because this is rarely observed in nonoxide materials.     

Inelastic collisions in molecular oxygen at low temperature (4 ≤ T ≤ 34 K). Close-coupling calculations versus experiment

Close-coupling calculations and experiment are combined in this work, which is aimed at establishing a set of state-to-state rate coefficients for elementary processes ij → lm in O(2):O(2) collisions at low temperature involving the rotational states i, j, l, m of the vibrational ground state of (16)O(2)((3)Σ(g)(-)). First, a set of cross sections for inelastic collisions is calculated as a function of the collision energy at the converged close-coupled level via the MOLSCAT code, using a recent ab-initio potential energy surface for O(2)-O(2) [M. Bartolomei et al., J. Chem. Phys. 133, 124311 (2010)]. Then, the corresponding rates for the temperature range 4 ≤ T ≤ 34 K are derived from the cross sections. The link between theory and experiment is a Master Equation which accounts for the time evolution of rotational populations in a reference volume of gas in terms of the collision rates. This Master Equation provides a linear function of the rates for each rotational state and temperature. In the experiment, the evolution of rotational populations is measured by Raman spectroscopy in a tiny reference volume (≈2 × 10(-4) mm(3)) of O(2) travelling along the axis of a supersonic jet at a velocity of ≈700 m/s. The accuracy of the calculated rates is assessed experimentally for 10 ≤ T ≤ 34 K by means of the Master Equation. The rates, jointly with their confidence interval estimated by Monte Carlo simulation, account to within the experimental uncertainty for the evolution of the populations of the N = 1, 3, 5, 7 rotational triads along the supersonic jet. Confidence intervals range from ≈6% for the dominant rates at 34 K, up to ≈17% at 10 K. These results provide an experimental validation of state-to-state rates for O(2):O(2) inelastic collisions calculated in the close-coupling approach and, indirectly, of the anisotropy of the O(2)-O(2) intermolecular potential employed in the calculation for energies up to 300 cm(-1).     

Effects of Pressure and pH on the Physical Stability of an I-Motif DNA Structure

The thermodynamic stability of a cytosine(C)-rich i-motif tract of DNA, which features pH-sensitive [C..H..C]⁺ moieties, has been studied as function of both pressure (0.1–200 MPa) and pH (3.7–6.2). Careful attention was paid to correcting citrate buffer pH for known variations that stem from changes in pressure. Once pH-corrected, (i) at pH >4.6 the i-motif becomes less stable as pressure is increased (K_D decreases), giving a small negative volume change for dissociation (Δ_DV°) of the i-motif – a conclusion opposite to that which would be drawn if the buffer pH was not corrected for the effects of pressure; (ii) the i-motif's melting temperature increases by more than 30 K between pH 6.5 and 4.5, the consequence of an enthalpy for dissociation (Δ_DH°) of 77(3) and 90(3) kJ (mol H⁺)⁻¹ at 0.1 and 200 MPa, respectively; (iii) below pH 4.6 at 0.1 MPa (pH 4.3 at 200 MPa) the melting temperature decreases as a result of double protonation of cytosine pairs, and Δ_DH° and Δ_DV° change signs; and (iv) the combination of Δ_DH° and Δ_DV° lead to the melting temperature at pH 4.3 being 3 K higher at 200 MPa than at 0.1 MPa.     

PVT measurements on n -octyl-isothiocyanato-biphenyl (8BT) at elevated pressures

For the first time pressure-volume-temperature (PVT) measurements for the crystal-like smectic E phase have been performed. The phase diagram of 4′-n -octyl-4-isothiocyanatobiphenyl 8BT has been recently established using differential thermal analysis up to 250 MPa. 8BT exhibits a splitting of the clearing line above 170 MPa. PVT data have been measured in the same pressure range for temperatures between 313 and 393 K. Volume and enthalpy changes accompanying the clearing line of 8BT are also presented. The configurational part of the entropy change at the CrE-I transition of 8BT amounts to ≈ 60%. Using the PVT data and recently published dielectric relaxation results, the isochoric activation energy was calculated (giving ≈ 50% of the activation enthalpy); this is compared with analogous results for other liquid crystals.     

Preparation of flexible and elastic poly(trimethylene carbonate) structures by stereolithography

3D porous and non-porous structures are designed and prepared by stereolithography using resins based on PTMC macromers. Tough, flexible network films prepared in this manner show E moduli of ≈3.8 MPa and high elongations at break >900%; tensile strengths are ≈4.2 MPa. These values increase with increasing PTMC macromer molecular weight. To reach suitable viscosities for processing, up to 45 wt% propylene carbonate is added as non-reactive diluent. The solid specimens have compression moduli of 3.1-4.2 MPa, similar to the values determined in tensile testing. The built porous structures show porosities of 53-66% and average pore sizes of 309-407 μm. The compression moduli of the porous structures are significantly lower than those of the solid structures.     

Proton conduction in hydrous glasses of the join CaAl2Si2O8-CaMgSi2O6: an impedance and infrared spectroscopic study

Hydrous silicate glasses with compositions along the join diopside-anorthite (An, CaAl(2)Si(2)O(8))-(Di, CaMgSi(2)O(6)) containing up to 3 wt. % H(2)O were synthesized at temperatures 1523-1723 K and pressures of 200 MPa in an internally heated gas pressure vessel. The water content of the glasses was analyzed by Karl-Fischer titration. Infrared microspectroscopy was used to test the homogeneity of the water distribution and to measure the concentrations of OH groups and H(2)O molecules before and after conductivity measurements. The electrical conductivity was measured by impedance spectroscopy at temperature up to 685 K. A positive correlation between water content and conductivity was observed for An(100) from 0 to 1.8 wt.% H(2)O, for An(50)Di(50) (in mol.%) from 1.5 to 2.8 wt.% H(2)O, and for Di(100) from 0 to 1.2 wt.% H(2)O. At same water content of ～1.2 wt.%, the conductivity was three orders of magnitude higher in Di(100) than in the other two glasses, emphasizing the importance of non-bridging oxygens on the transport of hydrous charge carriers. Consistent with findings in literature, we conclude that protons are the predominant mobile charge carriers in alkali-free hydrous silicate glasses. Conductivity data were evaluated in terms of proton diffusivity by the Nernst-Einstein equation. The obtained diffusion coefficients range from 10(-17) m(2)/s for An(50)Di(50) with 1.50 wt.% of H(2)O at 596 K to 10(-12) m(2)/s for An(50)Di(50) with 2.77 wt.% of H(2)O at 685 K.     

Hydrogen adsorption on a carbon adsorbent with slitlike micropores below and above the critical temperature

Hydrogen adsorption is calculated for model microporous adsorbents with slitlike micropore widths of 0.538, 0.878, and 1.218 nm obtained by the consecutive exclusion of one, two, and three layers of hexagonal carbon from graphite structure taken as a model cell. Calculations are performed using the basic concepts of the theory of volume filling of micropores, Dubinin-Radushkevich equation, and linear adsorption isosteres. For structures with one-and two-layer carbon walls, the calculation is carried out at temperatures of 20, 33, 77, 200, 300, and 400 K and pressures up to 20 MPa. For AC3:5 structure, the maximum hydrogen adsorption amounts to 7.9 wt % at 20 MPa and 300 K. The parameters of adsorbent porous structure are established. Hydrogen adsorption is shown to be governed by the capacity and the energy of adsorption.     

Effect of high-pressure ammonia treatment on the activity of Ge3N4 photocatalyst for overall water splitting

The photocatalytic activity of beta-Ge(3)N(4) powder for overall water splitting is successfully enhanced by ammonia treatment at 823 K for 5-24 h at ammonia pressures of 20 MPa or greater. The surface and bulk nitrogen content in the treated samples varies according to the treatment temperature and treatment time, related to the stability of beta-Ge(3)N(4) powder under pressurized ammonia. The change in nitrogen content resulted in a change in the photocatalytic activity for overall water splitting. A beta-Ge(3)N(4) powder treated at 823 K for 5 h under ammonia at 20 MPa exhibited a photocatalytic activity 4 times higher than that of the as-synthesized powder, attributable to a decrease in the density of anion defects in the bulk and surface.     

Water exchange from the oxo-centered rhodium(III) trimer [Rh3(mu3-O)(mu-O2CCH3)6(OH2)3]+: a high-pressure 17O NMR study

Water exchange from the oxo-centered rhodium(III) trimer, [Rh3(mu3-O)(mu-O2CCH3)6(OH2)3]+, was investigated using variable-temperature (272.8-281.6 K) and variable-pressure (0.1-200 MPa) 17O NMR spectroscopy. The exchange reaction was also monitored at three different acidities (pH = 1.8, 2.9, and 5.7) in which the molecule is in the fully protonated form (pKa = 8.3 (+/-0.2), I = 0.1 M, T = 298 K). The temperature dependence of the pseudo-first-order rate coefficient for water exchange yields the following kinetic parameters: k(ex)298 = 5 x 10(-3) s(-1), deltaH(double dagger) = 99 (+/-3) kJ mol(-1), and deltaS(double dagger) = 43 (+/-10) J K(-1) mol(-1). The enhanced reactivity of the terminal waters, some 6 orders of magnitude faster than water exchange from Rh(H2O)6(3+), is likely due to trans-labilization from the central oxide ion. Also, another contributing factor is the low average charge on the metal ions (+0.33/Rh). Variation of reaction rate with pressure results in a deltaV(double dagger) = +5.3 (+/-0.4) cm3 mol(-1), indicative of an interchange-dissociative (I(d)) pathway. These results are consistent with those published by Sasaki et al. who proposed that water substitution from rhodium(III) and ruthenium(III) oxo-centered trimers follows a dissociative mechanism based on highly positive activation parameters (Sasaki, Y.; Nagasawa, A.; Tokiwa-Yamanoto, A.; Ito, T. Inorg. Chim. Acta 1993, 212, 175-182).     

Excess Volume of Ternary Water + Diacetone Alcohol + 2-Propanol as a Function of Pressure,Temperature and Composition

Abstract

The excess volume V^E of the ternary water + diacetone alcohol (or DAA) + 2-propanol and of the three binaries water + DAA, water + 2-propanol and DAA + 2-propanol was evaluated from experimental density data (2772 values) as a function of the pressure P (between 0.1 MPa and 65 MPa), the temperature T (303.15K, 323.15K and 343.15K) and the composition. Various representative models are discussed. It is possible to account for the values of the density with an average absolute deviation of about 0.06% in the experimental P-T domain.     

Melting points and thermal expansivities of proton-disordered hexagonal ice with several model potentials

A method of free energy calculation is proposed, which enables to cover a wide range of pressure and temperature. The free energies of proton-disordered hexagonal ice (ice Ih) and liquid water are calculated for the TIP4P [J. Chem. Phys. 79, 926 (1983)] model and the TIP5P [J. Chem. Phys. 112, 8910 (2000)] model. From the calculated free energy curves, we determine the melting point of the proton-disordered hexagonal ice at 0.1 MPa (atmospheric pressure), 50 MPa, 100 MPa, and 200 MPa. The melting temperatures at atmospheric pressure for the TIP4P ice and the TIP5P ice are found to be about T(m)=229 K and T(m)=268 K, respectively. The melting temperatures decrease as the pressure is increased, a feature consistent with the pressure dependence of the melting point for realistic proton-disordered hexagonal ice. We also calculate the thermal expansivity of the model ices. Negative thermal expansivity is observed at the low temperature region for the TIP4P ice, but not for the TIP5P ice at the ambient pressure.