Combined inelastic neutron scattering and solid-state DFT study of dynamics of hydrogen atoms in trioctahedral 1M phlogopite

Inelastic neutron scattering (INS) was used to study the vibrational dynamics of the hydrogen atoms in natural trioctahedral phlogopite, K_0.93Na_0.03(Mg_2.47Fe_0.22Al_0.16Fe_0.04Tl_0.06)[Si_2.84Al_1.16]O₁₀OH_1.71F_0.28Cl_0.01, within the 50–1,000?cm^?1 energy range. The INS spectra collected using direct geometry spectrometer SEQUOIA (ORNL) were interpreted by means of the solid-state DFT calculations covering both normal mode analysis and molecular dynamics. To optimize the structure and to calculate the vibrational modes under harmonic approximation, both a hybrid PBE0 and the AM05 functional were used, while the molecular dynamics calculations (60?ps/1?fs) were performed only with the computationally less-demanding AM05 functional. The main contributions to the dominant band within ~750–550?cm^?1 are symmetric and antisymmetric Mg–O–H bending modes, overlapping with the skeletal stretching and bending modes causing weaker secondary movements of H atoms of inner hydroxyl groups. Signatures of the Mg–O–H bending modes appear down to ~400?cm^?1, where a region of octahedra deformation modes starts. These deformations cause just shallow movements of the hydrogen atoms and are mirrored by the modes with close vibrational energies. The region from ~330?cm^?1 down to the low-energy end of the spectrum portrays induced vibrations of the H atoms caused by deformation of individual polyhedra, translational vibrations of the parts of the 2:1 layer relative one to another, and librational and translational vibrations of the layer. The main difference between the INS spectrum of dioctahedral Al-muscovite and trioctahedral Mg-phlogopite is that the Mg–O–H modes are all assigned to in-plane vibrations of the respective hydrogen atoms.     

Quasielastic incoherent neutron scattering study of the rotational dynamics of the water molecules in analcime

The rotational motion of the water molecules in analcime has been investigated by quasielastic incoherent neutron scattering. Reorientational times vary from 65 ± 20 ps at 260 K to 26 ± 3 ps at 370 K, and the temperature dependence suggests an activation energy for reorientations of 780 ± 200 K. Measurements of the temperature dependence of the intensity of the elastic incoherent scattering indicate that there is no cooperative ordering at low temperatures, and that instead there is a gradual freezing-in of the motion of individual water molecules.     

Lattice dynamics and inelastic neutron scattering from forsterite,Mg2SiO4: Phonon dispersion relation,density of states and specific heat

Magnesium-rich olivine (Mg_0.9Fe_0.1)₂SiO₄ is considered to be a major constituent of the Earth's upper mantle. Because of its major geophysical importance, the temperature and pressure dependence of its crystal structure, elastic and dielectric constants, long-wavelength phonon modes and specific heat have been measured using a variety of experimental techniques. Theoretical study of lattice dynamics provides a means of analyzing and understanding a host of such experimental data in a unified manner. A detailed study of the lattice dynamics of forsterite, Mg₂SiO₄, has been made using a crystal potential function consisting of Coulombic and short-range terms. Quasiharmonic lattice dynamical calculations based on a rigid molecular-ion model have provided theoretical estimates of elastic constants, long-wavelength modes, phonon dispersion relation for external modes along the three high symmetry directions in the Brillouin zone, total and partial density of states and inelastic neutron scattering cross-sections. The neutron cross-sections were used as guides for the coherent inelastic neutron scattering experiment on a large single crystal using a triple axis spectrometer in the constant Q mode. The observed and predicted phonon dispersion relation show excellent agreement. The inelastically scattered neutron spectra from a powder sample have been analyzed on the basis of a phonon density of states calculated from a rigid-ion model, which includes both external and internal modes. The experimental data from a powder sample show good agreement with the calculated spectra, which include a multiphonon contribution in the incoherent approximation. The computed phonon densities of states are used to calculate the specific heat as a function of temperature using both the rigid molecular-ion and rigid ion models. These results are in very good agreement with the calorimetric measurement of the specific heat. The interatomic potential developed here can be used with some confidence to study physical properties of forsterite as a function of pressure and temperature.     

Determination of acoustic phonon dispersion curves in layer silicates by inelastic neutron scattering and computer simulation techniques

Inelastic neutron scattering and computer modelling techniques have been used to study acoustic phonons in several layer silicate minerals. Experimental measurements have been made on four layer silicate minerals; namely samples of muscovite, Fe-bearing muscovite, margarite and chlorite. The longitudinal acoustic modes propagating along the [0, 0, ξ] direction of muscovite and Fe-bearing muscovite were found to be the same, within experimental error. The longitudinal and transverse acoustic modes propagating along [0, 0, ξ] of muscovite have been calculated using computer simulation techniques based on lattice dynamics. The experimental and calculated longitudinal modes of muscovite are in excellent agreement, thereby demonstrating the complementary nature of both techniques. The shape of both experimental and calculated dispersion curves was found to be approximately sinusoidal, indicating that interatomic forces act principally between nearest-neighbour atoms.     

Method of radial sounding and elemental analysis of formations during well logging using neutron inelastic scattering gamma time-of-flight spectrometry

This paper describes a well logging method and device designed to determine radial inhomogeneities in the elemental content of the borehole environment with high spatial resolution. The sounding factor that determines the spatial resolution is the time elapsed from the moment of neutron emission from the device to the moment the device records the gamma rays from neutron inelastic scattering (inelastic gamma rays, IGRs) in the formation. The time interval characterizes the distance to the point of origin of a gamma ray, and the energy of a gamma ray passing through the formation without interaction determines the chemical element involved in inelastic scattering.Simulations have shown that at each time, the density of inelastic scattering is very well localized in space owing to the small number of fast-neutron scatterings: on average, one to two events. It is the compact localization of inelastic scattering events that provides high radial resolution (and, if necessary, high azimuth resolution) during fast-neutron sounding of formations and measurement of unsteady IGR fluxes. Recording of IGR distributions over time also provides increasing sounding depth because powerful IGR fluxes from nearby regions reach the detector at short times and do not overlap the weaker IGR fluxes from distant regions because the latter reach the detector later.To evaluate the radial resolution of the method, we calculated the response of the sonde for typical models of a borehole environment which include a borehole, an iron casing, cement, an invaded zone, and an uninvaded rock. The boundaries of spatial inhomogeneities and the elemental content in the regions between these boundaries were determined from time dependences of unscattered spectral lines in IGR spectra for the elements Ca, Si, C, O, and Fe. The results of the numerical simulation indicate a high sensitivity of the measurements to the radial boundaries and an adequate spatial resolution: about 1 cm at a 0.1 ns time sampling of logs. The interfaces between the radial zones are clearly marked in the time distributions by steep fronts with a length of 0.1 ns (at a collimation angle of the source of about 30°) to 0.15–0.4 ns (at an angle of 90°).A method of solution was formulated for the inverse problem consisting of determining the boundaries of the radial zones and the elemental content in these zones. The problem is solved using a qualitative model of the borehole environment, for example, a “borehole–casing–cement–invaded zone–uninvaded rock” model. The method is based on searching for approximating model curves to measured time distributions of unscattered IGR fluxes jointly for all components of the model. The search is conducted by spatial optimization of the sought parameters—the distances {rS} from the neutron source to the boundaries of the zones and the concentrations {C} of specified chemical compounds in these zones. The initial approximations for the sought parameters {rS} and {C} are calculated by linear inversion of logs, which proves to be very accurate because the contribution of singly scattered neutrons to the inelastic scattering density at small times (10 ns) is, on average, 50–90%.Model curves are calculated by numerical simulation of the transport of neutrons and gamma rays. An appropriate calculation method is the Monte Carlo technique. Since the multiplicity of neutron scattering is low and, for gamma rays, only the unscattered component is of interest, the numerical simulation is a fast process.The practical implementation of the method requires the use of advanced developments in the design of neutron generators, spectral gamma-ray detectors, and fast analyzers for recording subnanosecond processes. Use of associated-particle neutron generators, Ge semiconductor detectors with electron cooling or LaBr3 (Ce) and BaF2 based fast scintillator blocks of high energy resolution will allow the application of the proposed method to logging measurements.     

Accretion of neutral hydrogen and helium onto a neutron star. The X-ray and gamma-ray radiation of cool neutron stars

The accretion of neutral gas (hydrogen and helium) onto a neutron star is studied. The gas is gravitationally captured into the magnetosphere of the star, where it is ionized by thermal radiation from the stellar surface and accelerated by the electric field at the light cylinder and in a tube of open magnetic lines. Particles accelerated at light cylinder generate gamma-ray, some particles move to the star and heat its polar regions, resulting in the emission of X-rays. Our calculations of the model parameters of the X-ray and gamma-ray radiation indicate that the radiation intensities should be sufficient to be observed.     

Light scattering and extinction in a highly turbid coastal inlet

Multifactor regression analysis was used to test for relationships between chemical, physical and optical properties of the water column in the organically rich, highly turbid waters of Florida's Fort Pierce Inlet. Optical measurements were made at three visible light wavelengths (445 nm, 542 nm and 630 nm). Scattering by suspended particulate material was found to be the primary optical mechanism controlling downwelling irradiance at all three wavelengths. Larger particles showed constant scattering efficiencies of 2 when their diameters exceeded 3 to 5 microns, depending upon the wavelength used for observation. Selective absorption had a definite effect on the transmission of radiant energy in the 445 nm wavelength range. High correlation between extinction at 445 nm and the cross-sectional area of the suspended particulate material indicates particulate, rather than dissolved materials, are the major water column constituents that selectively absorb short wavelength radiant energy in this inlet. Spectral distribution of the downwelling radiant energy field was found to shift dramatically over a period of several months. These shifts in downwelling spectral irradiance were attributed to seasonal and/or event related shifts in concentrations of selectively absorbing compounds within the water column.     

Molecular dynamics study and normal mode analysis of diffuse scattering in quartz

X-ray scattering intensities in quartz were investigated in molecular dynamics (MD) simulation at different temperatures with the aid of normal mode analyses. The MD-simulated structure produced diffuse streaks most remarkably along a hexagon joining symmetry-equivalent points of index 400 in the hk0 plane, extending in ± c ^*. In addition, most Bragg spots were found to be associated by radial diffuse scattering in the six a ^* directions. The normal mode analysis and calculations of first-order scattering intensities showed that the lowest-lying branch in G-M is of transversal acoustic characters, and is responsible for the diffuse scattering radiating from Bragg reflections in the a ^*-directions. The transversal acoustic characters of atomic displacements in these modes provide the scattering in phase at points on the fourth hexagon. Phonon dispersion relations of the MD crystal were examined in view of symmetry-adapted atomic displacement patterns, and a possibility was suggested for two curves in G-M, optic soft branch and transversal acoustic branch polarized in the z axis (the second-lowest branch), to exchange the eigenvectors in the middle range of q (0.2–0.3).     

Equations of state of CaSiO3 Perovskite: a molecular dynamics study

The molar volumes and bulk moduli of CaSiO₃ perovskite are calculated in the temperature range from 300 to 2,800 K and the pressure range from 0 to 143 GPa using molecular dynamics simulations that employ the breathing shell model for oxygen and the quantum correction in addition to the conventional pairwise interatomic potential models. The performance of five equations of state, i.e., the Keane, the generalized-Rydberg, the Holzapfel, the Stacey–Rydberg, and the third-order Birch–Murnaghan equations of state are examined using these data. The third-order Birch–Murnaghan equation of state is found to have a clear tendency to overestimate the bulk modulus at very high pressures. The Stacey–Rydberg equation of state degrades slightly at very high pressures along the low-temperature isotherms. In comparison, the Keane and the Holzapfel equations of state remain accurate in the whole temperature and pressure range considered in the present study. K ₀′ derived from the Holzapfel equation of state also agrees best with that calculated independently from molecular dynamics simulations. The adiabatic bulk moduli of CaSiO₃ perovskite along lower mantle geotherms are further calculated using the Keane and the Mie-Grüneisen–Debye equations of state. They are found to be constantly higher than those of the PREM by ~5%, and also very similar to those of the MgSiO₃ perovskite. Our results support the view that CaSiO₃ perovskite remains invisible in the Earth’s lower mantle.     

A variable-temperature neutron diffraction study of ussingite; a strong asymmetric hydrogen bond in an aluminosilicate framework

A powder neutron diffraction study of ussingite, Na₂AlSi₃O₈(OH), over the temperature range 4–850 K has been undertaken. The strong hydrogen bond that exists in this mineral has been accurately determined with the O–H distance at 1.070(8) Å and an O(donor)–O(acceptor) separation of 2.481(5) Å at 4 K, The distribution of hydrogen along the O–O direction remains asymmetric between 4 and 850 K with the H atom being fully ordered at a single site, rather than partially disordered over two sites of a double-potential well, as in serandite. A gradual increase in the bonded O–H distance at higher temperatures is observed, indicative of a broadening of the potential well in which the hydrogen atom is sited. Below 50 K, the material shows negative thermal expansion, likely to be associated with reduced bending motion of the O–H bond.     

The structure of monomeric and dimeric uranyl adsorption complexes on gibbsite: A combined DFT and EXAFS study

We investigated the structure of uranyl sorption complexes on gibbsite (pH 5.6-9.7) by two independent methods, density functional theory (DFT) calculations and extended X-ray absorption fine structure (EXAFS) spectroscopy at the U-L_III edge. To model the gibbsite surface with DFT, we tested two Al (hydr)oxide clusters, a dimer and a hexamer. Based on polarization, structure, and relaxation energies during geometry optimization, the hexamer cluster was found to be the more appropriate model. An additional advantage of the hexamer model is that it represents both edges and basal faces of gibbsite. The DFT calculations of (monomeric) uranyl sorption complexes show an energetic preference for the corner-sharing versus the edge-sharing configuration on gibbsite edges. The energy difference is so small, however, that possibly both surface species may coexist. In contrast to the edge sites, sorption to basal sites was energetically not favorable. EXAFS spectroscopy revealed in all investigated samples the same interatomic distances of the uranyl coordination environment (R_U-Oax ≈ 1.80 Å, R_U-Oeq ≈ 2.40 Å), and towards the gibbsite surface (R_U-O ≈ 2.87 Å, R_U-Al ≈ 3.38 Å). In addition, two U-U distances were observed, 3.92 Å at pH 9.7 and 4.30 Å at pH 5.6, both with coordination numbers of ∼1. The short U-U distance is close to that of the aqueous uranyl hydroxo dimer, UO₂(OH)₂, reported as 3.875 Å in the literature, but significantly longer than that of aqueous trimers (3.81-3.82 Å), suggesting sorption of uranyl dimers at alkaline pH. The longer U-U distance (4.30 Å) at acidic pH, however, is not in line with known aqueous uranyl polymer complexes. Based on the EXAFS findings we further refined dimeric surface complexes with DFT. We propose two structural models: in the acidic region, the observed long U-U distance can be explained with a distortion of the uranyl dimer to form both a corner-sharing and an edge-sharing linkage to neighboring Al octahedra, leading to R_U-U = 4.150 Å. In the alkaline region, a corner-sharing uranyl dimer complex is the most favorable. The U-O path at ∼2.87 Å in the EXAFS spectra arises from the oxygen atom linking two Al cations in corner-sharing arrangement. The adsorption structures obtained by DFT calculations are in good agreement with the structural parameters from EXAFS analysis: U-Al (3.394 Å), U-U (3.949 Å), and U-O (2.823 Å) for the alkaline pH model, and U-Al (3.279 Å), U-U (4.150 Å), and U-O (2.743 Å) for the acidic pH model. This work shows that by combining EXAFS and DFT, consistent structural models for uranyl sorption complexes can be obtained, which are relevant to predict the migration behavior of uranium at nuclear facilities.     

The dynamics of a river bend: a study in flow and sedimentary processes

Comprehensive field measurements of flow and sedimentary processes have been made with the aid of stable scaffolding bridges spaced along the length of a bend of the River South Esk, Scotland. At river stages between about two-thirds full and bankfull, channel width, mean depth and mean flow velocity at a cross-section vary little in the streamwise direction. Flow resistance reaches a maximum at these stages, and the bed topography is stable and in equilibrium with flow and bedload transport. Stable flow geometry is thus related in some way to energy conservation, and to maximization of flow resistance. Detailed observations over a large range of river stages of mean velocity distributions, secondary circulation, water surface configuration, bed shear stress and resistance to flow, bed configurations and bed load transport rates agree with much (but not all) of the comparable published experimental studies and selected theoretical work. Generalized physical models of flow and sediment transport in natural curved channels (Engelund, 1974; Bridge, 1977) are demonstrated to be sound in basis and can simulate the bend studied very well. Although there is a pressing need for further development of these models, the results lend confidence to their use in simulating ancient river sedimentation. Sediment deposited on point bars is the result mainly of bedload transport over a range of near-bankfull stages. The areal distribution of grain-size characteristics and bed configurations at these stages give rise, with lateral deposition, to vertical facies sequences that vary substantially in the streamwise direction.     

Self-diffusion of water and its dependence on temperature and ionic strength in highly compacted montmorillonite, illite and kaolinite

The effect of temperature and ionic strength on the diffusion of HTO parallel to the direction of compaction through 5 highly compacted clay minerals (bulk dry density, ρ_b,d = 1.90 ± 0.05 Mg/m³), namely montmorillonite (Na- and Ca-form), illite (Na- and Ca-form), and kaolinite, was studied. The diffusion experiments were carried out at temperatures between 0 °C and 60 °C and at ionic strengths of 0.01 M and 1 M NaCl for the Na-form clays and kaolinite, and of 0.005 M and 0.5 M CaCl₂ for the Ca-form. The ionic strength had an insignificant influence on the values of the effective diffusion coefficient (variation by less than 10%) for the clays under study at this degree of compaction. The effective diffusion coefficients followed the order Na-montmorillonite < Ca-montmorillonite < Ca-illite < Na-illite  kaolinite. It is thought that the differences between Na- and Ca-montmorillonite originate from the larger size particles, and thus the lower tortuosity of the latter; whereas the differences between Na- and Ca-illite are related to the different degree of solvation of the Na and Ca cations. The activation energies were successfully calculated using the Arrhenius law. Swelling clays (Na- and Ca-montmorillonite) had slightly larger activation energy values (20 kJ/mol) compared to bulk water (17 kJ/mol); Ca-illite (16 kJ/mol), Na-illite (13 kJ/mol) and kaolinite (14.4 kJ/mol) lower values than that of bulk water. The low activation energies of the last three clays may be related to weaker H-bonds between water and the clay surfaces compared to those in bulk water.     

新鲜生活垃圾压缩与直剪联合测定试验研究

在环境土工实验室人工配制了新鲜生活垃圾,采用大型压缩与直剪联合测定仪器,对96个新鲜垃圾试样进行了压缩变形、剪应力及剪应变联合测定试验,研究了垃圾的压缩变形对强度的影响及规律。试样的初始孔隙比分别为2.1、2.5、2.9,试样竖向压力分别为25、50、100、200 kPa,每级压力作用下的时间分别为0、0.25、0.5、1、2、6、12、24 h。试验研究表明,（1）不同压缩时间下剪应力与剪应变的关系符合双曲线模型,模型参数a在0.028～0.144之间,参数b在0.012～0.024之间;（2）不同竖向压力及不同剪应变限值下抗剪强度与竖向压缩应变（压缩时间）的关系可以拟合成多项式的形式,得到了参数的拟合值及相关系数,相关系数在0.63～0.98之间;（3）不同初始孔隙比的抗剪强度与竖向压力的关系符合库仑定律,得到了不同剪应变限值的抗剪强度参数,黏聚力在11.1～34.2 kPa之间,内摩擦角在 11.2°～30.6°之间。     

The influence of pressure on the structure and dynamics of hydrogen bonds in zoisite and clinozoisite

Density functional theory calculations have been used to study the pressure-induced changes of the hydrogen bond of Fe-free orthozoisite and clinozoisite and the concomitant shifts of the OH-stretching frequencies. Two independent parameter-free lattice dynamical calculations have been employed. One was based on a plane-wave basis set in conjunction with norm-conserving pseudopotentials and a density functional perturbation theory approach, while the other used a localised basis set and a finite displacement algorithm for the lattice dynamical calculations. Both models confirm the unusually large pressure-induced red-shift found experimentally (−33.89 cm⁻¹/GPa) in orthozoisite, while the pressure-induced shifts in clinozoisite are much smaller (−5 to −9 cm⁻¹/GPa). The atomistic model calculations show that in orthozoisite the nearly linear O–H⋯O arrangement is compressed by about 8% on a pressure increase to 10 GPa, while concomitantly the O–H distance is significantly elongated (by 2.5% at 10 GPa). In clinozoisite, the O–H⋯O arrangement is kinked at ambient conditions and remains kinked at high pressures, while the O-H distance is elongated by only 0.5% at 10 GPa. The current calculations confirm that correlations between the distances and dynamics of hydrogen bonds, which have been established at ambient conditions, cannot be used to infer hydrogen positions at high pressures.     

Thermal expansion and decomposition of jarosite: a high-temperature neutron diffraction study

The structure of deuterated jarosite, KFe₃(SO₄)₂(OD)₆, was investigated using time-of-flight neutron diffraction up to its dehydroxylation temperature. Rietveld analysis reveals that with increasing temperature, its c dimension expands at a rate ~10 times greater than that for a. This anisotropy of thermal expansion is due to rapid increase in the thickness of the (001) sheet of [Fe(O,OH)₆] octahedra and [SO₄] tetrahedra with increasing temperature. Fitting of the measured cell volumes yields a coefficient of thermal expansion, α = α₀ + α₁ T, where α₀ = 1.01 × 10⁻⁴ K⁻¹ and α₁ = −1.15 × 10⁻⁷ K⁻². On heating, the hydrogen bonds, O1···D–O3, through which the (001) octahedral–tetrahedral sheets are held together, become weakened, as reflected by an increase in the D···O1 distance and a concomitant decrease in the O3–D distance with increasing temperature. On further heating to 575 K, jarosite starts to decompose into nanocrystalline yavapaiite and hematite (as well as water vapor), a direct result of the breaking of the hydrogen bonds that hold the jarosite structure together.     

27Al, 29Si,and 23Na MAS NMR study of an Al,Si ordered alkali feldspar solid solution series

Solid-state ²⁷Al, ²⁹Si and ²³Na MAS NMR spectra have been obtained for an Al,Si ordered low albite to low microcline ion exchange series for which unit-cell parameters and ²⁹Si NMR data have previously been reported. ²⁷Al δ_i vary continuously with composition from 63.4 (±0.5) ppm for albite to 58.9 (±0.5) ppm for microcline, and parallel the ²⁹Si chemical shifts assigned to the T2m-site. The ²⁷Al and ²⁹Si chemical shifts for this series correlate well with composition-dependent lattice parameters, most notably cell volume and the angle [201]¹b. The linewidths of the ²⁹Si and ²⁷Al resonances indicate a significant amount of structural disorder in the intermediate compositions due to Na, K substitution. The 1 σ width of the distribution of average Si-O-T angles for each T-site is estimated to be about 1° for the Or33 sample. The average ²³Na δ_i varies monotonically from -8.5 (±1) ppm for albite to -24.3 (±1)ppm for Or83. Similarly, the average ²³Na nuclear quadrupole coupling constant decreases from 2.60 to 1.15 (±0.05) MHz and the asymmetry parameter of the electric field gradient increases from 0.25 to 0.6 with increasing K-content from albite to Or83. The observed variations in the quadrupole coupling parameters are consistent with simple electrostatic calculations. Higher resolution ²³Na spectra of the intermediate compositions obtained at 11.7 T indicate the presence of an inhomogeneous linebroadening which is related to the distribution of Na-environments. A model based on a random distribution of local compositions does not simulate the spectra, suggesting that the distribution of Na is skewed toward Na-rich clusters. Observation of the ²³Na NMR lineshape of Or49 after short periods of heat treatment indicate that ²³Na NMR is very sensitive to the changes in the Na, K distribution accompanying the early stages of exsolution. Reversible changes occur after heating at 530° C for 3 h, whereas heating at 600° C produces no changes, possibly bracketing the position of the coherent spinodal for Al, Si ordered alkali feldspars at this composition.     

Comparative study of tracer diffusion of HTO, Na and Cl in compacted kaolinite, illite and montmorillonite

The through-diffusion of HTO, ²²Na⁺ and ³⁶Cl⁻ in kaolinite, homo-ionic Na-illite and homo-ionic Na-montmorillonite was measured at a high degree of compaction as a function of the salt concentration in the ‘external solution’, i.e. in the solution in contact with the clay sample. The clays were chosen for this study because of their differences in the number and nature of ion exchange sites leading to different proportions of interlayer-, inter-particle and free pore water. It was found that the diffusive mass transfer of Na⁺ in Na-montmorillonite and Na-illite increased with decreasing external salt concentration, while the opposite trend was observed for the diffusion of Cl⁻. These trends are more pronounced in the case of Na-montmorillonite than in Na-illite, while almost no salt effect was observed for kaolinite. Similarly no salt effect was observed for the diffusion of HTO through all of the clays tested. These observations are in agreement with a conceptual model where it is assumed that cations diffuse preferentially in the interlayer or diffuse double-layer porosity, while anions are almost completely excluded from these regions. In the case of Na⁺ diffusion, the salt effects can be explained by an influence on the concentration gradient of diffusing cations, while in the case of Cl⁻ the external salt concentration has an effect on the accessible porosity. Effective diffusion coefficients of Cl⁻ fulfil the same relationship to porosity as those of the uncharged HTO, when using accessible porosities for such a comparison. Furthermore it is shown that pore diffusion coefficients for the three tracers are fairly well correlated with the respective diffusion coefficients in bulk water, if the effective diffusion coefficients for Na⁺ are derived from calculated tracer concentration gradients in the interlayer or diffuse double-layer porosities.     

Characterization of the CO2 fluid adsorption in coal as a function of pressure using neutron scattering techniques (SANS and USANS)

Small angle neutron scattering techniques have been applied to investigate the phase behavior of CO₂ injected into coal and possible changes in the coal pore structure that may result from this injection. Three coals were selected for this study: the Seelyville coal from the Illinois Basin (R_o = 0.53%), Baralaba coal from the Bowen Basin (R_o = 0.67%), and Bulli 4 coal from the Sydney Basin (R_o = 1.42%). The coals were selected from different depths to represent the range of the underground CO₂ conditions (from subcritical to supercritical) which may be realized in the deep subsurface environment. The experiments were conducted in a high pressure cell and CO₂ was injected under a range of pressure conditions, including those corresponding to in-situ hydrostatic subsurface conditions for each coal. Our experiments indicate that the porous matrix of all coals remains essentially unchanged after exposure to CO₂ at pressures up to 200 bar (1 bar = 10⁵ Pa). Each coal responds differently to the CO₂ exposure and this response appears to be different in pores of various sizes within the same coal. For the Seelyville coal at reservoir conditions (16 °C, 50 bar), CO₂ condenses from a gas into liquid, which leads to increased average fluid density in the pores (ρ_pore) with sizes (r) 1 × 10⁵ ≥ r ≥ 1 × 10⁴ Å (ρ_pore ≈ 0.489 g/cm³) as well as in small pores with size between 30 and 300 Å (ρ_pore ≈ 0.671 g/cm³). These values are by a factor of three to four higher than the density of bulk CO₂ (ρ_CO2) under similar thermodynamic conditions (ρ_CO2 ≈ 0.15 g/cm³). At the same time, in the intermediate size pores with r ≈ 1000 Å the average fluid density is similar to the density of bulk fluid, which indicates that adsorption does not occur in these pores. At in situ conditions for the Baralaba coal (35 ^OC, 100 bar), the average fluid density of CO₂ in all pores is lower than that of the bulk fluid (ρ_pore / ρ_CO2 ≈ 0.6). Neutron scattering from the Bulli 4 coal did not show any significant variation with pressure, a phenomenon which we assign to the extremely small amount of porosity of this coal in the pore size range between 35 and 100,000 Å.