Thermodynamics of adsorption (as a function of temperature and pressure) of N2 and CO2 on graphites

Summary A quantitative treatment, coupled with critical qualitative considerations, pertaining to the study of thermodynamics of adsorption of N₂ and CO₂ on different graphite samples, through the evaluation of the film pressure, as a function of temperature and pressure, is presented. The . determination of the adsorbent specific surface areas is affected through the graphic manipulation of the Langmuir adsorption equation utilizing data adopted from literature, and the values are then computed by employing the Gibbs adsorption equation by making use of the areas under them-lnP curves. The results deduced are discussed specifically in terms of as related to the nature of the adsorbents, the magnitude of the calculated thermodynamic functions and the specific surface areas for the individual adsorbents.

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Zusammenfassung Die Arbeit bringt eine thermodynamische Studie der Adsorption von N₂ and CO₂ an verschiedenen Graphitpräparaten. Grundlage der Betrachtung ist der Filmdruck als Funktion der Temperatur und des Druckes. Die Bestimmung der spezifischen Oberfläche des Adsorbens wird beeinflußt durch die graphische Behandlung der Langmuirschen Adsorptionsgleichung. Es wurden Literaturwerte adoptiert und-Werte nach der Gibbsschen Adsorptionsgleichung berechnet. Die abgeleiteten Ergebnisse werden diskutiert und zur Natur des Adsorbens, zur Größenordnung der berechneten thermodynamischen Funktionen und zur spezifischen Oberfläche der einzelnen Adsorbentien in Beziehung gesetzt.

     

Isobaric thermal expansivity of the binary system 1-hexanol + n-hexane as a function of temperature and pressure

The isobaric thermal expansivity against temperature and pressure for the system 1-hexanol + n-hexane was directly determined by means of a calorimetric method. From these data, the excess isobaric thermal expansivity is calculated at representative temperatures and pressures. The obtained results for this excess quantity are qualitatively discussed by applying well-known arguments often used for explaining the thermodynamic behavior of alcohol + alkane mixtures. In order to check the consistency of these data with those of literature, the derivative of excess molar volume against temperature and that of excess isobaric molar heat capacity against pressure are calculated and compared with those obtained from literature data. Very good coherence between both data sources is obtained.     

Subsolidus phase diagram of the binary system (n-C n H2n+1NH3)2CoCl4

The perovskite-type layered compounds decylammonium tetrachlorocobaltate (n-C₁₀H₂₁NH₃)₂CoCl₄ and dodecylammonium tetrachlorocobaltate (n-C₁₂H₂₅NH₃)₂CoCl₄ exist the solid–solid phase transition in the temperatures range 330–380 K. These laminar materials contain bilayers sandwiched between metal halide layers. The experimental subsolidus binary phase diagram of (n-C₁₀H₂₁NH₃)₂CoCl₄–(n-C₁₂H₂₅NH₃)₂CoCl₄ has been established over the whole composition range by differential thermal analysis and X-ray diffraction. In the phase diagram, one intermediate compound (n-C₁₀H₂₁NH₃) (n-C₁₂H₂₅NH₃)CoCl₄ at $ W_{{{\text{C}}_{ 1 0} {\text{Co}}}} \% = 50. 9 2 $ and two eutectoid invariants points at $ W_{{{\text{C}}_{ 10} {\text{Co}}}} \% = 2 9. 8 3 $ and $ W_{{{\text{C}}_{ 10} {\text{Co}}}} \% = 7 7. 9 8 $ were observed, two eutectoids temperatures are about 343 ± 1 and 340 ± 1 K. There are three noticeable solid solution ranges (α, β, γ) at the left and right boundary and middle of the phase diagram.     

Calculation of the rate constant for state-selected recombination of H+O2(nu) as a function of temperature and pressure

Classical trajectory calculations using the MERCURY/VENUS code have been carried out on the H+O(2) reactive system using the DMBE-IV potential energy surface. The vibrational quantum number and the temperature were selected over the ranges nu=0 to 15, and T=300 to 10 000 K, respectively. All other variables were averaged. Rate constants were determined for the energy transfer process, H+O(2)(nu)-->H+O(2)(nu(")), for the bimolecular exchange process, H+O(2)(nu)-->OH(nu('))+O, and for the dissociative process, H+O(2)(nu)-->H+O+O. The dissociative process appears to be a mere extension of the process of transferring large amounts of energy. State-to-state rate constants are given for the exchange reaction, and they are in reasonable agreement with previous results, while the energy transfer and dissociative rate constants have never been reported previously. The lifetime distributions of the HO(2) complex, calculated as a function of v and temperature, were used as a basis for determining the relative contributions of various vibrational states of O(2) to the thermal rate coefficients for recombination at various pressures. This novel approach, based on the complex's ability to survive until it collides in a secondary process with an inert gas, is used here for the first time. Complete falloff curves for the recombination of H+O(2) are also calculated over a wide range of temperatures and pressures. The combination of the two separate studies results in pressure- and temperature-dependent rate constants for H+O(2)(nu)(+Ar) right arrow over left arrow HO(2)(+Ar). It is found that, unlike the exchange reaction, vibrational and rotational-translational energy are liabilities in promoting recombination.     

Specific volume of polymers as a function of temperature and pressure

Summary The specific volume of pure and plasticized polyvinylchloride and of polymethylmethacrylate was studied with a dilatometer as function of temperature (–80 to +150 °C) and hydrostatic pressure (1 to 1000 at.). Beside the glass transition also two types of secondary transitions are observed with PVC and PMMA which correspond to dynamic secondary dispersion ranges.Below the glass transition the specific volumina depend on how they were reached.The transition temperatures of the samples were determined at elevated pressures with decreasing temperatures. Glass transition and secondary transitions cause stepwise increases of the thermal expansivities and the isothermal compressibilities. Above the glass transition the compressibilities are rising in proportion to the temperature.With PVC all transition temperatures are shifted by about 0,013 to 0,015 °C/at as the pressure is raised. With PMMA the transitions at –7 °C and at +62 °C are not affected by hydrostatic pressure while the glass transition is shifted by 0,018 °C/at.Addition of plasticizer to PVC shifts the glass transition to lower and the secondary transition to higher temperatures.There is no agreement between the measured shifting factorsdT _g/dp and those calculated according toEhrenfest's law/=dT _g/dp.

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Zusammenfassung Das spezifische Volumen von reinem und weichgemachtem Polyvinylchlorid und von Polymethylmethacrylat wurde mit Hilfe eines Dilatometers als Funktion der Temperatur (–80 bis +150 °C) und des hydrostatischen Drucks (1 bis 1000 at) gemessen. Außer dem Glasübergang wurden bei PVC und PMMA noch weitere sekundäre Übergangsbereiche gefunden, die dynamischen Nebendispersionsgebieten entsprechen.Unterhalb der Glastemperatur hängen die spezifischen Volumina stark von dem Wege ab, auf dem sie erreicht wurden.Die Übergangstemperaturen der Proben wurden bei erhöhtem Druck mit fallender Temperatur bestimmt. Der Glasübergang und die sekundären Übergänge sind von stufenweisen Änderungen der thermischen Ausdehnungskoeffizienten und der isothermen Kompressibilitäten begleitet. Oberhalb des Glasübergangs steigen die Kompressibilitäten proportional zur Temperatur an.Bei PVC werden alle Übergangstemperaturen durch steigenden Druck um etwa 0,013 bis 0,015 °C/at verschoben. Bei PMMA werden die Übergangsgebiete bei –7 °C und bei +62 °C durch hydrostatischen Druck nicht beeinflußt, während der Glasübergang um etwa 0,018 °C/at verschoben wird.Der Zusatz von Weichmacher zu PVC verschiebt den Glasübergang zu tieferen und den sekundären Übergang zu höheren Temperaturen.Die gemessenen VerschiebungsfaktorendT _g/dp stimmen nicht mit den nach derEhrenfestschen Beziehung berechneten überein.

     

Specific volume of polysulfone as a function of temperature and pressure

The pressure–volume–temperature (PVT) properties of a commercial polysulfone derived from bisphenol A and 4,4′-dichlorodiphenylsulfone are studied experimentally and theoretically in the temperature range 30–370°C and for pressures to 2000 kg/cm². PVT surfaces are determined for an annealed glass, formed under zero pressure, and for the melt. Two glass-transition lines must be distinguished: T(P) which is the intersection of the glass and melt PVT surfaces, and T_g(P), which is obtained by pressurizing the melt isothermally. The application of Ehrenfest-type equations to these transitions are discussed. The Prigogine–Defay ratio r = Δ_kΔC_p/TV(Δα)² at P = 0 is found to be equal to 0.95 (±20%), using ΔC_p data determined on identical samples. The melt data is compared with the Simha–Somcynski hole theory, using the reducing parameters V^* = 0.788 cm³/g, T^* = 12,560°K, P^* = 10,875 bar. The hole fraction appearing in the theory is found to be constant along T(P), but the glass PVT relationship cannot be reproduced by using the Simha–Somcynsky theory together with the assumption that the hole fraction remains constant in the glass. At P = 0 the hole fraction must be allowed to decrease with decreasing temperature, but at a slower rate than in the melt.     

Polymer thin film properties as a function of temperature and pressure

We report on evanescent wave optical measurements of the glass transition temperature, T_g, of spin-cast PMMA films as a function of film thickness and molecular weight. It was found that for films of high molecular weight PMMA (M_n > 100,000 g mol⁻¹) a strong T_g depression occurs for films that are thinner than 100 nm in case they are deposited on hydrophobic substrates. This strong T_g depression of up to 25°C decreases if similarly thick films of PMMA of low molecular weights are investigated and vanishes completely for PMMA with M_n < 12,000 g mol⁻¹. For films made of these materials T_g is found to be identical to that of the bulk even for films as thin as 5 nm. The results might be interpreted in terms of free volume considerations. To check this assumption we also designed and built a pressure cell that can be used together with the evanscent wave optical techniques for similar measurement, but with the additional option to do the measurements at different pressures up to ca. 100 MPa to further vary the free volume of these polymer films in constrained geometry. Some first results obtained with this setup are also described.     

Photon correlation spectroscopy of polystyrene as a function of temperature and pressure

Photon correlation spectroscopy is employed to study the slowly relaxing density and anisotropy fluctuations in bulk atactic polystyrene as a function of temperature from 100 to 160°C and pressure from 1 to 1330 bar. The light-scattering relaxation function is well described by the empirical function ?(t) = exp[?(t/τ)^β], where for polystyrene β = 0.34. The average relaxation time is determined at each temperature and pressure according to 〈τ〉 = (τ/β)Γ(1/β) where Γ(x) is the gamma function. The data can be described by the empirical relation 〈τ〉 = 〈τ〉₀ exp[(A + BP)/R(T ? T₀)] where R is the gas constant and T₀ is the ideal glass transition temperature. The empirical constant A/R is in good agreement with that determined from the viscosity or the dielectric relaxation data (1934 K). The empirical constant B can be interpreted as the activation volume for the fundamental unit involved in the relaxation and is found to be comparable to one styrene subunit (100 mL/mol). The quantity B appears to be a weak function of temperature. The use of pressure as a tool in the study of light scattering near the glass transition now has been established.     

Theoretical investigation (DFT and MP2) of the intermolecular proton transfer in the supersystems uracil-(H2O) n and uracil-(CH3OH) n (n = 1, 2)

Twelve binary and eight ternary H-bonded systems between uracil and water/methanol were investigated at the B3LYP and MP2 theoretical levels using 6 − 31 + G(d) basis functions. The binary and ternary systems that contain the hydroxo-uracil tautomer H-bonded with water and methanol were found to be the most stable complexes. The calculated energy barriers of the intermolecular proton exchange showed that the methanol molecule provokes larger reduction of the energy barrier of the intermolecular proton exchange reactions than the water molecule. Correspondence: Vassil B. Delchev, Department of Physical Chemistry, University of Plovdiv, Plovdiv, Bulgaria     

Synthesis, Raman spectra and crystal structures of [Cu(XeF2)n](SbF6)2 (n=2, 4)

Pure [Cu(XeF2)2](SbF6)2 was prepared by the reaction of Cu(SbF 6) 2 with a stoichiometric amount of XeF2 in anhydrous hydrogen fluoride (aHF) at ambient temperature. The reaction between Cu(SbF6)2 and XeF2 (1:4 molar ratio) in aHF yielded [Cu(XeF2)4](SbF6)2 contaminated with traces of Xe 2F 3SbF6 and CuF2. The 6-fold coordination of Cu(2+) in [Cu(XeF2)2](SbF6)2 includes two fluorine atoms from two XeF2 ligands and four fluorine atoms provided by four [SbF6](-) anions. The neighboring [Cu(XeF 2)2](2+) moieties are connected via two [SbF6] units, with the bridging fluorine atoms in cis positions, into infinite [Cu(eta(1)-XeF2)2](cis-eta(2)-SbF 6)2[Cu(eta(1)-XeF 2)2] chains. Because of the high electron affinity of Cu(2+), coordinated XeF2 shows the highest distortion (Xe-Fb=210.2(5) pm, Xe-Ft=190.6(5) pm) observed so far among all known [M(x+)(XeF2)n](A)x (A=BF4, PF6, etc.) complexes. The four equatorial coordination sites of the Cu(2+) ion in [Cu(XeF 2) 4](SbF6)2 are occupied by four XeF 2 ligands. Two fluorine atoms belonging to two [SbF6] units complete the Cu (2+) coordination environment. The neighboring [Cu(XeF2)4](2+) species are linked via one [SbF6] unit, with bridging fluorine atoms in trans positions, into linear infinite [Cu(eta(1)-XeF2)4](trans-eta(2)-SbF6)[Cu(eta(1)-XeF2)4] chains. To compensate for the remaining positive charge, crystallographically independent [SbF6](-) anions are located between the chains and are fixed in the crystal space by weak Xe...F(Sb) interactions.     

Theoretical investigation of the temperature dependence of the fifth-order Raman response function of fluid and liquid xenon

The temperature dependence of the fifth-order Raman response function, R(5)(t1,t2), is calculated for fluid xenon by employing a recently developed time-correlation function (TCF) theory. The TCF theory expresses the two-dimensional (2D) Raman quantum response function in terms of a two-time, computationally tractable, classical TCF. The theory was shown to be in excellent agreement with existing exact classical MD calculations for liquid xenon as well as reproducing line shape characteristics predicted by earlier theoretical work. It is applied here to investigate the temperature dependence of the fifth-order Raman response function in fluid xenon. In general, the characteristic line shapes are preserved over the temperature range investigated (for the reduced temperature points T* = 0.5, 1.0, and 2.0); differences in the signal decay times and a large decline in intensity with decreasing temperature (and associated anharmonicity) are observed. In addition, there are some signature features that were not observed in earlier results for T* = 1. The most dramatic difference in line shape is observed for the polarization condition, xxzzxx, that shows a vibrational echo peak. In contrast, the fully polarized signal changes mainly in magnitude.     

Thermodynamic properties of aqueous polyacrylamide solutions as a function of pressure and temperature

The viscosities of dilute aqueous solutions of polyacrylamide were measured at temperatures from 20 to 60.4° and pressures up to 150 MPa using a falling-body viscometer. The viscosity coefficient, ν, increases exponentially with increasing pressure at a given temperature and concentration. The rate of increase of the apparent energy of activation. E≠, with pressure becomes more rapid as the concentration of the solutions increases. Intrinsic viscosity, [ν], increases with increasing pressure at a given temperature but almost levels off at pressures above 100 MPa while the Huggins constant, k_H, decreases. The Flory-Huggins interaction parameter, _X1, decreases at a greater rate with increasing pressure as the temperature decreases indicating that the effect of pressure on improving the compatability between the polymer and solvent is greater near the θ-temperature. The second virial coefficient, A₂, was calculated from the intrinsic viscosity data and compared with the results obtained by light scattering technique.     

Ab initio investigation of vibrational spectra of water-(CO2)n complexes (n = 1, 2)

In this paper, we have calculated using the ab initio method the IR vibrational spectra of complexes of CO2 formed with water (sp3 O-donating atom). Binding energies and structures of the CO2-H2O and water-(CO2)2 complexes have been determined at the second-order level of the Moller-Plesset perturbation theory (MP2) using Dunning's basis sets. The results are presented and critically discussed in terms of the nature of the water-CO2 interactions, electron donor acceptor (EDA) and weak O...H-O interactions. For water-(CO2)2 trimer, it is also shown that the contribution to the interaction energy of the irreducible three-bodies remains relatively negligible. We have analyzed the frequency shifts and the IR and Raman intensity variations under the complex formation. We have particularly emphasized the splitting of the 2 bending mode of CO2 and stretching modes of water, which have been revealed as the most pertinent probes to assess the nature of the forces involved in the different complexes. Finally, because water can play the role of Lewis base and acid as well, we found that weak O...H-O interactions can cooperate with EDA interactions in trimer, leading to very specific spectral signatures that are further discussed.     

Ab initio investigation of water clusters (H2O)n (n = 2–34)

Various properties of typical structures of water clusters in the n = 2–34 size regime with the change of cluster size have been systematically explored. Full optimizations are carried out for the structures presented in this article at the Hartree–Fock (HF) level using the 6‐31G(d) basis set by taking into account the positions of all atoms within the cluster. The influence of the HF level on the results has been reflected by the comparison between the binding energies of (H₂O)_n (n = 2–6, 8, 11, 13, 20) calculated at the HF level and those obtained from high‐level ab initio calculations at the second‐order Møller–Plesset (MP2) perturbation theory and the coupled cluster method including singles and doubles with perturbative triples (CCSD(T)) levels. HF is inaccurate when compared with MP2 and CCSD(T), but it is more practical and allows us to study larger systems. The computed properties characterizing water clusters (H₂O)_n (n = 2–34) include optimal structures, structural parameters, binding energies, hydrogen bonds, charge distributions, dipole moments, and so on. When the cluster size increases, trends of the above various properties have been presented to provide important reference for understanding and describing the nature of the hydrogen bond. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Density functional theory investigation of the geometric and electronic structures of [UO2(H2O)m(OH)n](2 - n) (n + m = 5)

Gradient corrected density functional theory has been used to calculate the geometric and electronic structures of the family of molecules [UO2(H2O)m(OH)n](2 - n) (n + m = 5). Comparisons are made with previous experimental and theoretical structural and spectroscopic data. r(U-O(yl)) is found to lengthen as water molecules are replaced by hydroxides in the equatorial plane, and the nu(sym) and nu(asym) uranyl vibrational wavenumbers decrease correspondingly. GGA functionals (BP86, PW91 and PBE) are generally found to perform better for the cationic complexes than for the anions. The inclusion of solvent effects using continuum models leads to spurious low frequency imaginary vibrational modes and overall poorer agreement with experimental data for nu(sym) and nu(asym). Analysis of the molecular orbital structure is performed in order to trace the origin of the lengthening and weakening of the U-O(yl) bond as waters are replaced by hydroxides. No evidence is found to support previous suggestions of a competition for U 6d atomic orbitals in U-O(yl) and U-O(hydroxide)pi bonding. Rather, the lengthening and weakening of U-O(yl) is attributed to reduced ionic bonding generated in part by the sigma-donating ability of the hydroxide ligands.     

Dielectric investigation of a binary system of self-organized diols

Dielectric measurements on a binary system of two liquid crystalline diols were carried out. These show a SmA and a cubic phase. In the middle concentration range of the system, a columnar phase is induced. The self assembly of the molecules giving different liquid crystalline phases is clearly seen in the dielectric spectrum, and therefore details of this process may be discussed. All the samples show a high frequency dielectric absorption which is related to the dynamics of the network of hydrogen bonds. In the columnar phase an additional low frequency mechanism is seen which may be caused by the internal dynamics in the columns.     

Structural and vibrational characterization of the organic semiconductor tetracene as a function of pressure and temperature

Neutron powder diffraction and inelastic neutron scattering measurements were performed on crystalline tetracene, a molecular semiconductor of triclinic crystal structure that adopts a herringbone layered motif, as a function of pressure up to 358 MPa. In combination with theoretical and simulated computations, these measurements permit detailed characterization of the structural and vibrational changes of tetracene as a function of pressure. Powder diffraction at 295 K reveals anisotropic modification of the crystal structure with increasing pressure. Particularly, the unit cell parameters associated with the two-dimensional herringbone layers of the solid state structure displayed continuous change at all measured pressures, whereas perpendicular to the herringbone layers the structure remains relatively unchanged. The measured compressibilities along the [1 0 0], [0 1 0], and [0 0 1] crystal axes are −3.8 × 10⁻⁴, −1.9 × 10⁻⁴, and −3.4 × 10⁻⁴ Å/MPa, respectively. Inelastic neutron scattering spectra were collected at several pressures in the 25–75 and 0–25 meV energy ranges using a filter analyzer and a Fermi chopper time-of-flight spectrometer, respectively. Assignment of the spectral peaks to specific intramolecular vibrational modes has been accomplished using ab initio density functional theory calculations and the low energy lattice phonon modes were interpreted from the results of molecular dynamics simulations at 1 atm and 358 MPa. Anisotropic behavior parallel to that observed in the structural measurements is also apparent in both the intramolecular and lattice phonon vibrational dynamics. Intramolecular vibrations having atomic displacements entirely within the plane of the molecule’s aromatic ring remain unchanged with increasing pressure while vibrations with atomic displacements perpendicular to the molecular plane shift to higher energy. The lattice phonons display a similar anisotropy with increasing pressure. Phonon modes propagated within the herringbone layer are significantly shifted to higher energy with increasing pressure relative to the modes with displacements primarily perpendicular to the layers. Overall, both the planar internal geometry and the layered arrangement of the tetracene molecules significantly influence the observed structural and vibrational behavior with increasing pressure.     

Size matters! Fragmentation chemistry of [Cu(L)n]2+ complexes of diacylglycerophosphocholines as a function of coordination number (n = 2-7)

[Cu(L)(n)](2+) complexes of 1,2-dihexanoyl-sn-glycero-3-phosphocholine (L = D6PC) are formed upon electrospray ionization mass spectrometry (ESI-MS) of an 8 mM solution of D6PC with 4 mM CuCl(2) in 10 mM ammonium acetate buffer, pH 6.1. The collision-induced dissociation (CID) reactions of the [Cu(L)(n)](2+) complexes were examined in a linear ion trap mass spectrometer. A rich fragmentation chemistry was observed, including: loss of a neutral ligand; intermolecular ligand-ligand S(N)2 methylation; metal ion induced ligand fragmentation via carboxylate abstraction; and phosphate abstraction. The dominant reaction channel depends on the size (n) of the complex. Thus loss of neutral ligand(s) is the sole reaction channel for n = 5-7. At n = 4, S(N)2 methylation and carboxylate abstraction start to compete with neutral ligand loss. At n = 2 the carboxylate abstraction and phosphate abstraction reactions dominate the CID spectrum. The carboxylate abstraction and phosphate abstraction reactions are likely to be driven via neighboring group pathways. PM3 calculations, carried out to compare competing neighboring pathways based on the relative stabilities of the product ions, suggest a preference for five-membered ring formation for ligand fragmentation involving both carboxylate and phosphate abstraction.     

Thermodynamics of adsorption (as a function of temperature and pressure) of N2, C02, Ar,CH4 and C2H4 on charcoals

Summary A quantitative treatment pertaining to the evaluation of the Gibbs function for the adsorption study of Ar, N₂, CO₂, CH₄ and C₂H₄ on wood-charcoal and charcoal is given. The specific surface areas were determined from the Langmuir adsorption equation; Gibbs function,, was estimated from the areas under them-lnP curves (from data from literature). Related thermodynamic functions are calculated in order to validate a correlation between the quantitatively deduced results with the known qualitative surface aspects in relation to a transition from mobile to a partly localized state for N₂ arid CH₄.

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Zusammenfassung Es wird eine quantitative Ermittlung der Gibbsschen Funktion für die Adsorption von Ar, N₂, CO₂, CH₄ und C₂H₄ an Holzkohle und Kohle gegeben. Die spezifischen Oberflächen wurden aus der Langmuir-Adsorptionsgleichung ermittelt, und die Gibbssche Funktion wurde aus den Flächen unter denm-InP Kurven entnommen (nach Literaturdaten). Um Zusammenhänge zwischen den quantitativ abgeleiteten Ergebnissen und Oberflächenaspekten in Bezug auf einen Übergang von einem mobilen in einen weniger mobilen Zustand von adsorbiertem Stickstoff und Methan zu erlangen, wurden weitere thermodynamische Funktionen berechnet.

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With 11 figures and 6 tables