Phase behavior and local structure of a binary mixture in pores: mean-field lattice model calculations for analyzing neutron scattering data

We investigate the phase behavior of an asymmetric binary liquid A-W mixture confined between two planar homogenous substrates (slit pore). Molecules of species W interact preferentially with the solid walls via a long-range potential. Assuming nearest-neighbor attractions between the liquid molecules, we employ a lattice-gas model and a mean-field approximation for the grand potential. Minimization of this potential yields the density profiles of thermodynamically stable phases for fixed temperature, chemical potentials of both species, pore width and strengths of attraction. This model is used to analyze experimental small-angle neutron-scattering (SANS) data on the microscopic structure of the binary system isobutyric acid (iBA)+heavy water (D2O) inside a mesoscopic porous matrix (controlled-pore glass of about 10 nm mean pore width). Confinement-independent model parameters are adjusted so that the theoretical liquid-liquid coexistence curve in the bulk matches its experimental counterpart. By choosing appropriate values of the pore width and the attraction strength between substrates and water we analyze the effect of confinement on the phase diagram. In addition to a depression of the liquid-liquid critical point we observe surface induced phase transitions as well as water-film adsorption near the walls. The temperature dependence of the structure of water-rich and iBA-rich phases of constant composition are discussed in detail. The theoretical predictions are consistent with results of the SANS study and assist their interpretation.     

Structure and decompression melting of a novel, high-pressure nanoconfined 2-D ice

Molecular dynamics (MD) simulations of water confined in nanospaces between layers of talc (system composition Mg(3)Si(4)O(10)(OH)(2) + 2H(2)O) at 300 K and pressures of approximately 0.45 GPa show the presence of a novel 2-D ice structure, and the simulation results at lower pressures provide insight into the mechanisms of its decompression melting. Talc is hydrophobic at ambient pressure and temperature, but weak hydrogen bonding between the talc surface and the water molecules plays an important role in stabilizing the hydrated structure at high pressure. The simulation results suggest that experimentally accessible elevated pressures may cause formation of a wide range of previously unknown water structures in nanoconfinement. In the talc 2-D ice, each water molecule is coordinated by six O(b) atoms of one basal siloxane sheet and three water molecules. The water molecules are arranged in a buckled hexagonal array in the a-b crystallographic plane with two sublayers along [001]. Each H(2)O molecule has four H-bonds, accepting one from the talc OH group and one from another water molecule and donating one to an O(b) and one to another water molecule. In plan view, the molecules are arranged in six-member rings reflecting the substrate talc structure. Decompression melting occurs by migration of water molecules to interstitial sites in the centers of six-member rings and eventual formation of separate empty and water-filled regions.     

Density minimum of confined water at low temperatures: a combined study by small-angle scattering of X-rays and neutrons

A simple explanation is given for the low-temperature density minimum of water confined within cylindrical pores of ordered nanoporous materials of different pore size. The experimental evidence is based on combined data from in-situ small-angle scattering of X-rays (SAXS) and neutrons (SANS), corroborated by additional wide-angle X-ray scattering (WAXS). The combined scattering data cannot be described by a homogeneous density distribution of water within the pores, as was originally suggested from SANS data alone. A two-step density model reveals a wall layer covering approximately two layers of water molecules with higher density than the residual core water in the central part of the pores. The temperature-induced changes of the scattering signal from both X-rays and neutrons are consistent with a minimum of the average water density. We show that the temperature at which this minimum occurs depends monotonically on the pore size. Therefore we attribute this minimum to a liquid-solid transition of water influenced by confinement. For water confined in the smallest pores of only 2 nm in diameter, the density minimum is explained in terms of a structural transition of the surface water layer closest to the hydrophilic pore walls.     

Phase separation in perfluorosulfonate ionomer membranes

The extent of phase separation in Nafion® perfluorosulfonate ionomer membranes has been studied by small-angle neutron scattering (SANS). These polymers, which consist of a perfluorocarbon main chain and a sulfonate-containing side group, can absorb up to 30% by weight of water. Previous studies have shown that clustering of water occurs, forming particles in the size range observable by SANS. The current study is concerned with the fraction of water molecules which participate in the clustering and the chemical composition of the phases present. Experiments have been made on melt-quenched samples which have no fluorocarbon crystallinity. The analysis is based on isotopic replacement experiments in which SANS measurements are made on samples hydrated with mixtures of H₂O and D₂O. Values of the small-angle x-ray scattering (SAXS), mean-square electron density fluctuation, and mass density are used as additional criteria. It is shown that at high water content (more than 15% absorption by weight), a two-phase model can explain the data with a majority (>60%) of the water molecules in one phase and most (>90%) of the perfluorocarbon in the other phase; a sample hydrated to a lower extent (8% by weight) shows deviations from the two-phase model. These results are consistent with the scattering behavior at large angles observed by SAXS.     

Neutron and beta/gamma radiolysis of water up to supercritical conditions. 1. beta/gamma yields for H(2), H(.) atom, and hydrated electron

Yields for H2, H(.) atom, and hydrated electron production in beta/gamma radiolysis of water have been measured from room temperature up to 400 degrees C on a 250 bar isobar, and also as a function of pressure (density) at 380 and 400 degrees C. Radiolysis was carried out using a beam of 2-3 MeV electrons from a van de Graaff accelerator, and detection was by mass spectrometer analysis of gases sparged from the irradiated water. N2O was used as a specific scavenger for hydrated electrons giving N2 as product. Ethanol-d(6) was used to scavenge H(.) atoms, giving HD as a stable product. It is found that the hydrated electron yield decreases and the H(.) atom yield increases dramatically at lower densities in supercritical water, and the overall escape yield increases. The yield of molecular H2 increases with temperature and does not tend toward zero at low density, indicating that it is formed promptly rather than in spur recombination. A minimum in both the radical and H2 yields is observed around 0.4 kg/dm(3) density in supercritical water.     

Ce（H2O）n3+（n=1～6）水合性质的密度泛函理论研究

以TZ2P为基组,采用Becke- Perdew(BP)等10种密度泛函分别对Ce(H2O)n3+(n=1～6)体系进行几何优化.运用绝对平均误差分析得出在所计算的10种密度泛函中Becke-PBEc为最优泛函.振动光谱分析表明,Ce(H2O)n3+(n=1～6)体系随着n增大,即H2O配位数的增加,Ce和O之间的相互作用越来越弱,O和H之间的相互作用越来越强,振动吸收频率与键长的呈现良好的线性关系.电子结构分析表明,f轨道的孤对电子不参与Ce—O之间的成键.热力学分析表明,Ce3+不易形成低配位的水合离子,但是在适当温度压强以及动力学条件下铈的低配位水合离子有可能实现转变.     

Interactions between block copolymers and single-walled carbon nanotubes in aqueous solutions: a small-angle neutron scattering study

The amphiphilic copolymers of the Pluronic family are known to be excellent dispersants for single-walled carbon nanotubes (SWCNT) in water, especially F108 and F127, which have rather long end-blocks of poly(ethylene oxide) (PEO). In this study, the structure of the CNT/polymer hybrid formed in water is evaluated by measurements of small-angle neutron scattering (SANS) with contrast variation, as supported by cryo-transmission electron microscopy (cryo-TEM) imaging. The homogeneous, stable, inklike dispersions exhibited very small isolated bundles of carbon nanotubes in cryo-TEM images. SANS experiments were conducted at different D(2)O/H(2)O content of the dispersing solvent. The data for both systems showed surprisingly minimal intensity values at 70% D(2)O solvent composition, which is much higher than the expected value of 17% D(2)O that is based on the scattering length density (SLD) of PEO. At this near match point, the data exhibited a q(-1) power law relation of intensity to the scattering vector (q), indicating rodlike entities. Two models are evaluated, as extensions to Pederson's block copolymer micelles models. One is loosely adsorbed polymer chains on a rodlike CNT bundle. In the other, the hydrophobic block is considered to form a continuous hydrated shell on the CNT surface, whereas the hydrophilic blocks emanate into the solvent. Both models were found to fit the experimental data reasonably well. The model fit required special considerations of the tight association of water molecules around PEO chains and slight isotopic selectivity.     

Observation of entropic effect on conformation changes of complex systems under well-controlled temperature conditions

We report direct observation of an entropic effect in determining the folding of a linear dicarboxylate dianion with a flexible aliphatic chain [(-)O(2)C-(CH(2))(6)-CO(2)(-)] by photoelectron spectroscopy as a function of temperature (18-300 K) and degree of solvation from 1 to 18 water molecules. A folding transition is observed to occur at 16 solvent water molecules at room temperature and at 14 solvent molecules below 120 K due to the entropic effect. The (-)O(2)C-(CH(2))(6)-CO(2)(-)(H(2)O)(14) hydrated cluster exhibits interesting temperature-dependent behaviors, and its ratio of folded over linear conformations can be precisely controlled as a function of temperature, yielding the enthalpy and entropy differences between the two conformations. A folding barrier is observed at very low temperatures, resulting in kinetic trapping of the linear conformation. The current work provides a simple model system to study the dynamics and entropic effect in complex systems and may be important for understanding the hydration and conformation changes of biological molecules.     

The Structure of Adsorbed CO(2) in Slitlike Micropores at Low and High Temperature and the Resulting Micropore Size Distribution Based on GCMC Simulations

The Monte Carlo method is used in its grand ensemble variant in combination with CO(2) experimental isotherm data at low (195.5 K) and high temperatures (at 298 and 308 K, i.e., slightly below and above the CO(2) critical temperature, respectively) to characterize microporous carbons and obtain the corresponding pore size distribution (PSD). Specifically, the CO(2) density inside a single, slit-shaped, graphitic pore of given width is found on the basis of grand canonical Monte Carlo (GCMC) simulations for a predefined temperature and different relative pressures. The simulation results provide useful insights concerning the densification process in the micropores and the structure of the CO(2) molecules packing in the individual pores as the temperature or pressure changes from 195.5 K to ambient or from very low to 70 bar, respectively. Effects of temperature, pore size, quadrupole interactions, and molecule elongation on the local density profile within the pore are examined and discussed. In an additional step, we determine the optimal PSD for which the best match is obtained between computed and measured CO(2) isotherms. Comparisons are made between the PSDs found for the same carbon sample at low and high temperatures and conclusions are drawn concerning the applicability of the method and the reliability of the resulting micropore size distributions. Copyright 2000 Academic Press.     

Reaction CH(3) + OH Studied over the 294-714 K Temperature and 1-100 bar Pressure Ranges

Reaction of methyl radicals with hydroxyl radicals, CH(3) + OH → products (1) was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy over the 294-714 K temperature and 1-100 bar pressure ranges (bath gas He). Methyl radicals were produced by photolysis of acetone at 193.3 nm. Hydroxyl radicals were generated in reaction of electronically excited oxygen atoms O((1)D), produced in the photolysis of N(2)O at 193.3 nm, with H(2)O. Temporal profiles of CH(3) were recorded via absorption at 216.4 nm using xenon arc lamp and a spectrograph; OH radicals were monitored via transient absorption of light from a dc discharge H(2)O/Ar low pressure resonance lamp at ca. 308 nm. The absolute intensity of the photolysis light inside the reactor was determined by an accurate in situ actinometry based on the ozone formation in the presence of molecular oxygen. The results of this study indicate that the rate constant of reaction 1 is pressure independent within the studied pressure and temperature ranges and has slight negative temperature dependence, k(1) = (1.20 ± 0.20) × 10(-10)(T/300)(-0.49) cm(3) molecule(-1) s(-1).     

Reversible guest exchange and ferrimagnetism (T(C) = 60.5 K) in a porous cobalt(II)-hydroxide layer structure pillared with trans-1,4-cyclohexanedicarboxylate

The synthesis, characterization, and reversible guest-exchange chemistry of a new porous magnetic material that orders ferrimagnetically at 60.5 K are described. The material, Co(5)(OH)(8)(chdc).4H(2)O (chdc = trans-1,4-cyclohexanedicarboxylate), contains tetrahedral-octahedral-tetrahedral Co(II)-hydroxide layers of composition Co((oct))(3)Co((tet))(2)(OH)(8) that are linked together by bis(unidentate) chdc pillars. Noncoordinated water molecules occupy 1-D channels situated between the chdc pillars. The material remains monocrystalline during dehydration from Co(5)(OH)(8)(chdc).4H(2)O (CDCC.4H(2)O) to Co(5)(OH)(8)(chdc) (CDCC) via an intermediate Co(5)(OH)(8)(chdc).2H(2)O (CDCC.2H(2)O) upon heating or evacuation. In-situ single crystal and powder X-ray diffraction analyses indicate that the interlayer spacing decreases in two steps, each corresponding to the loss of two water molecules per formula unit as determined by thermogravimetry. The single crystal structure of the fully dehydrated material, CDCC, has no void volume due to a tilting of the pillars and 9% decrease of the interlayer spacing with water removal. Exposure of CDCC to air causes rapid rehydration of this material to CDCC.4H(2)O, as determined by single crystal X-ray diffraction, powder X-ray diffraction, thermogravimetry, and vibrational spectroscopy. Both the hydrated and dehydrated forms order magnetically below 60.5 K. The susceptibility data are consistent with ferrimagnetic behavior, and the value of the saturation magnetization at 2 K (ca. 2 micro(B)) is explained by a model of two sublattices, one comprising three octahedral cobalt atoms and another comprising two tetrahedral cobalt atoms. There is an enhanced 2-D correlation within the layer at temperatures just above the Curie temperature, as seen by nonlinearity in the ac susceptibility data and remanence in the isothermal magnetization. The crossover from 2-D to 3-D ordering occurs at T(C). The large anisotropy in the magnetization data on a single crystal suggests either a 2-D Ising or an XY magnet while the critical exponent of 0.25 is in favor of the latter. Both magnetization data in a small field in the ac and dc mode and isothermal magnetization data provide evidence of a further change in behavior at 23 K, which may originate from a reorientation of the moments within the layer. Variation of the pillar and of the guest-exchange chemistry, including the exchange of magnetic guests such as O(2), offers the possibility of tailoring the magnetic properties of this material.     

Quantum simulation of a hydrated noradrenaline analog with the torsional path integral method

An extended version of the torsional path integral Monte Carlo (TPIMC) method is presented and shown to be useful for studying the conformation of flexible molecules in solvated clusters. The new technique is applied to the hydrated clusters of the 2-amino-1-phenyl-ethanol (APE) molecule. APE + nH2O clusters with n = 0-4 are studied at 100 and 300 K using both classical and quantum simulations. Only at the lower temperature is the hydration number n found to impact the conformational distribution of the APE molecule. This is shown to be a result of the temperature-dependent balance between the internal energy and entropy contributions to the relative conformer free energies. Furthermore, at 100 K, large quantum effects are observed in the calculated conformer populations. A particularly large quantum shift of 30% of the total population is calculated for the APE + 2H2O cluster, which is explained in terms of the relative zero point energy of the lowest-energy hydrated structures for this cluster. Finally, qualitative agreement is found between the reported calculations and recent spectroscopy experiments on the hydrated clusters of APE, including an entropically driven preference for the formation of AG-type hydrated structures and the formation of a water "droplet" in the APE + 4H2O cluster.     

Hydration of lanthanoid(III) ions in aqueous solution and crystalline hydrates studied by EXAFS spectroscopy and crystallography: the myth of the "gadolinium break"

The structures of the hydrated lanthanoid(III) ions including lanthanum(III) have been characterized in aqueous solution and in the solid trifluoromethanesulfonate salts by extended X-ray absorption fine structure (EXAFS) spectroscopy. At ambient temperature the water oxygen atoms appear as a tricapped trigonal prism around the lanthanoid(III) ions in the solid nonaaqualanthanoid(III) trifluoromethanesulfonates. Water deficiency in the capping positions for the smallest ions starts at Ho and increases with increasing atomic number in the [Ln(H(2)O)(9-x)](CF(3)SO(3))(3) compounds with x=0.8 at Lu. The crystal structures of [Ho(H(2)O)(8.91)](CF(3)SO(3))(3) and [Lu(H(2)O)(8.2)](CF(3)SO(3))(3) were re-determined by X-ray crystallography at room temperature, and the latter also at 100 K after a phase-transition at about 190 K. The very similar Ln K- and L(3)-edge EXAFS spectra of each solid compound and its aqueous solution indicate indistinguishable structures of the hydrated lanthanoid(III) ions in aqueous solution and in the hydrated trifluoromethanesulfonate salt. The mean Ln--O bond lengths obtained from the EXAFS spectra for the largest ions, La-Nd, agree with estimates from the tabulated ionic radii for ninefold coordination but become shorter than expected starting at samarium. The deviation increases gradually with increasing atomic number, reaches the mean Ln-O bond length expected for eightfold coordination at Ho, and increases further for the smallest lanthanoid(III) ions, Er-Lu, which have an increasing water deficit. The low-temperature crystal structure of [Lu(H(2)O)(8.2)](CF(3)SO(3))(3) shows one strongly bound capping water molecule (Lu-O 2.395(4) A) and two more distant capping sites corresponding to Lu-O at 2.56(1) A, with occupancy factors of 0.58(1) and 0.59(1). There is no indication of a sudden change in hydration number, as proposed in the "gadolinium break" hypothesis.     

Relationship between the bubble temperature and main oxidant created inside an air bubble under ultrasound

Numerical simulations of nonequilibrium chemical reactions in a pulsating air bubble have been performed for various ultrasonic frequencies (20 kHz, 100 kHz, 300 kHz, and 1 MHz) and pressure amplitudes (up to 10 bars). The results of the numerical simulations have indicated that the main oxidant is OH radical inside a nearly vaporous or vaporous bubble which is defined as a bubble with higher molar fraction of water vapor than 0.5 at the end of the bubble collapse. Inside a gaseous bubble which is defined as a bubble with much lower vapor fraction than 0.5, the main oxidant is H2O2 when the bubble temperature at the end of the bubble collapse is in the range of 4000-6500 K and O atom when it is above 6500 K. From the interior of a gaseous bubble, an appreciable amount of OH radical also dissolves into the liquid. When the bubble temperature at the end of the bubble collapse is higher than 7000 K, oxidants are strongly consumed inside a bubble by oxidizing nitrogen and the main chemical products inside a bubble are HNO2, NO, and HNO3.     

Experimental evidence of fragile-to-strong dynamic crossover in DNA hydration water

We used high-resolution quasielastic neutron scattering spectroscopy to study the single-particle dynamics of water molecules on the surface of hydrated DNA samples. Both H(2)O and D(2)O hydrated samples were measured. The contribution of scattering from DNA is subtracted out by taking the difference of the signals between the two samples. The measurement was made at a series of temperatures from 270 down to 185 K. The relaxing-cage model was used to analyze the quasielastic spectra. This allowed us to extract a Q-independent average translational relaxation time of water molecules as a function of temperature. We observe clear evidence of a fragile-to-strong dynamic crossover (FSC) at T(L)=222+/-2 K by plotting log versus T. The coincidence of the dynamic transition temperature T(c) of DNA, signaling the onset of anharmonic molecular motion, and the FSC temperature T(L) of the hydration water suggests that the change of mobility of the hydration water molecules across T(L) drives the dynamic transition in DNA.     

Dissolution nature of cesium fluoride by water molecules

The structures, stabilities, thermodynamic quantities, dissociation energies, infrared spectra, and electronic properties of CsF hydrated by water molecules are investigated by using density functional theory, M?ller-Plesset second-order perturbation theory (MP2), coupled cluster theory with singles, doubles, and perturbative triples excitations (CCSD(T)), and ab initio molecular dynamic (AIMD) simulations. It is revealed that at 0 K three water molecules (as a global minimum structure) begin to half-dissociate the Cs-F, and six water molecules (though not a global minimum energy structure) can dissociate it. By the combination of the accurate CCSD(T) conformational energies for Cs(H2O)6 at 0 K with the AIMD thermal energy contribution, it reveals that the half-dissociated structure is the most stable at 0 K, but this structure (which is still the most stable) changes to the dissociated structure above 50 K. The spectra of CsF(H2O)(1-6) from MP2 calculations and the power spectra of CsF(H2O)6 from 50 and 100 K AIMD simulations are also reported.     

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.     

Surface tension and its temperature coefficient of molten tin determined with the sessile drop method at different oxygen partial pressures

The surface tension of molten tin has been determined by the sessile drop method at temperatures ranging from 523 to 1033 K and in the oxygen partial pressure (P(O(2))) range from 2.85 x 10(-19) to 8.56 x 10(-6) MPa, and its dependence on temperature and oxygen partial pressure has been analyzed. At P(O(2))=2.85 x 10(-19) and 1.06 x 10(-15) MPa, the surface tension decreases linearly with the increase of temperature and its temperature coefficients are -0.151 and -0.094 mN m(-1) K(-1), respectively. However, at high P(O(2)) (3.17 x 10(-10), 8.56 x 10(-6) MPa), the surface tension increases with the temperature near the melting point (505 K) and decreases above 723 K. The surface tension decrease with increasing P(O(2)) is much larger near the melting point than at temperatures above 823 K. The contact angle between the molten tin and the alumina substrate is 158-173 degrees, and the wettability is poor.     

Inorganic salt hydrates as cryoporometric probe materials to obtain pore size distribution

The depression of the melting temperature of Zn(NO3)2.6H2O was used to obtain the pore size distributions in controlled pore glasses. Measured by 1H NMR, the average value of the temperature depression DeltaT and the known average pore size yield K=DeltaT.d approximately 116 K.nm as the material-dependent factor for Zn(NO3)2.6H2O in the Gibbs-Thompson equation. The melting temperature is close to room temperature. Hence, this salt hydrate and some related other ones are better materials than water (K approximately 50 K.nm) for cryoporometric studies of systems with hydrophilic pores. The data also provide 46 mN/m for the solid-liquid surface tension of this salt hydrate.     

Characterization of pore structure in a nanoporous low-dielectric-constant thin film by neutron porosimetry and X-ray porosimetry

A small-angle neutron scattering (SANS) porosimetry technique is presented for characterization of pore structure in nanoporous thin films. The technique is applied to characterize a spin-on organosilicate low dielectric constant (low-k) material with a random pore structure. Porosimetry experiments are conducted using a "contrast match" solvent (a mixture of toluene-d8 and toluene-h8) having the same neutron scattering length density as that of the nanoporous film matrix. The film is exposed to contrast match toluene vapor in a carrier gas (air), and pores fill with liquid by capillary condensation. The partial pressure of the solvent vapor is increased stepwise from 0 (pure air) to P0 (saturated solvent vapor) and then decreased stepwise to 0 (pure air). As the solvent partial pressure increases, pores fill with liquid solvent progressively from smallest to largest. SANS measurements quantify the average size of the empty pores (those not filled with contrast match solvent). Analogous porosimetry experiments using specular X-ray reflectivity (SXR) quantify the volume fraction of solvent adsorbed at each step. Combining SXR and SANS data yields information about the pore size distribution and illustrates the size dependence of the filling process. For comparison, the pore size distribution is also calculated by application of the classical Kelvin equation to the SXR data.