Effect of macromolecular length on anisotropic scattering of light in an electric field

Light scattering by monodisperse solutions of rigid rod-like anisotropic macromolecules, with linear dimensions l of the order of incident wavelength λ, oriented in an external d.c. electric field, →E has been analysed. The relative variations δV^E_v, δH^E_v, δV^E_h, δH^E_h of the scattered light components are discussed for the three values [l/λ] = 1, 0·5, 2, and various reorientation parameters, p = [μE/kT] of the permanent dipole moment μ and q = [(₃ − ₁)E²/2kT] of the moment induced by the principal polarizabilities ₁ = ₂ ≠ ₃. The saturation orientation field strength has been calculated for certain macromolecules with the aim of determining their optical anisotropy numerically.     

State of hydration of the proton and the transition between molten-salt and aqueous-solution double-layer and kinetic behaviour

The influence of the state of hydration of the proton, from H₃O⁺ in the room-temperature melt, H₃O⁺. CF₃SO⁻₃, to 1 M aqueous in CF₃SO₃H solution in water, on the interfacial capacitance behaviour of the Au electrode is reported. The results allow the transition from “molten-salt” to “aqueous-solution” double-layer capacitance behaviour to be evaluated.

At positive potentials, near the potential for onset of surface oxidation of Au by OH, the interfacial capacitance is dominated by a pseudo-capacitance associated with chemisorption of the anion CF₃SO⁻₃, with charge transfer. At negative potentials in the range in which cathodic H₂ evolution occurs, the capacitance varies from ca 23 to ca 50 μF cm⁻², with the maximum arising around a degree of proton hydration corresponding to H₇O⁺₃.

Cathodic H₂ evolution kinetics have been studied from the same proton sources and show characteristic dependences of log i_o and the heat of activation on the hydration-state of H⁺. The Tafel slopes decrease with increasing temperature, giving a further example of non-conventional behaviour of the transfer coefficient.     

Synthesis and crystal structure of carbonate cancrinite Na8[AlSiO4]6CO3(H2O)3.4, grown under low-temperature hydrothermal conditions

The synthesis of cancrinite in the system Na₂O–SiO₂–Al₂O₃–Na₂CO₃–H₂O was studied under low-temperature hydrothermal conditions in the 353 K<T<473 K interval. The aim was to reveal the suitable range for the crystallization of pure-phase carbonate cancrinite with the ideal composition Na₈[AlSiO₄]₆CO₃(H₂O)₂ without cocrystallization of sodalite or intermediate disordered phases between cancrinite and sodalite. It was found that cancrinite formation reacts very sensitive on the temperature within the autoclaves whereas the concentration of reactants and the alkalinity of the hydrothermal solution have a much lower influence on the phase formation. Thus the temperature of crystallization of carbonate cancrinite without any by-products should not remain below 473 K. At the lower reaction temperature of 353 K the formation of a disordered intermediate phase between the cancrinite and the sodalite structure has been obtained in every case, independent of the template concentrations and the base. Some problems to detect this in a typical powder product mixture are discussed. Besides the ²⁹Si and ²⁷Al MAS NMR characterization of the products, the crystal structure refinement of pure carbonate cancrinite of ideal composition Na₈[AlSiO₄]₆CO₃(H₂O)_3.4, has been carried out from X-ray powder data using the Rietveld method: P6₃, a=1271.3(1) pm, c=518.6(1) pm, R_WP=0.073, R_F=0.016 for 347 structure factors and 45 variable positional parameters.     

The effects of nitrogenation on the electrochemical properties of nanocrystalline diamond films

The effect of the nitrogenation on the electrochemical properties of nanocrystalline diamond films produced by microwave plasma CVD in CH₄–Ar–H₂–N₂ gas mixtures was studied systematically, using cyclic voltammetry and electrochemical impedance spectroscopy measurements, for the first time. Differential capacitance, kinetic parameters of reactions in [Fe(CN)₆]^3-/4-redox system and potential window were found to be sensitive to the nitrogen concentration in the process gas. With its increase (from 0 to 25%), a transition of the NCD film behavior from “poor conductor” to metal-like character takes place. The heavily N-doped nanocrystalline diamond films have satisfactory electrochemical properties to be used as electrodes.     

Raman spectroscopic studies on the sulfation of cerium oxide

In the present study, we have examined sulfation of cerium oxide via impregnation of (NH₄)₂SO₄, followed by heating in the temperature range of 220–720°C, using Raman Spectroscopy. Based on the SO and SO stretching frequencies in the range of 900–1400 cm⁻¹, a wide range of surface oxysulfur species and bulk cerium-oxy-sulfur species are identified. At 220°C, a mixture of (NH₄)₂SO₄ crystals, SO²⁻_4(aq) and HSO¹⁻_r(aq) is found to have formed on ceria's surface, whereas complete conversion of (NH₄)₂SO₄ to SO²⁻_4(aq) and HSO¹⁻_4(aq) occurs at 280°C. At 350°C, formation of a mixture of surface pyrosulfate S₂O²⁻_7(surf.0, consisting of two SO oscillators and a bulk type compound identified as Ce(IV)(SO₄)_x(SO₃)_2−x (0 < x < 2) have been observed. Upon introduction of moisture, the former transforms to HSO¹⁻_4(surf.), whereas the latter remains unchanged. At 400°C, only the bulk type compound can be observed. At 450°C, only Ce₂(SO₄)₃ is generated and remains stable until 650°C. Further increase in the temperature to 720°C results in the formation of CeOSO₄. The present study not only provides a more thorough understanding of the sulfation of cerium oxide at a molecular level, but also demonstrates that Raman spectroscopy is a highly effective technique to characterize sulfation of metal oxides.     

Physico-Chemical Criteria for Maximum Adhesion. Part II: A New Comprehensive Thermodynamic Analysis

In this paper, two parameters defined as the relative work of adhesion [W_A/γ_L] and the relative interfacial energy [γ_SL/γ_L] have been examined for their assumed usefulness in correlating the thermodynamic properties of the components of the system substrate/ adhesive with its practical performance (strength). It is shown that the minimum value of [γ_SL/γ_L] relevant to conditions for the maximum adhesion becomes zero only for those systems (relatively rare) for which interaction factor Φ₀ is equal to 1.0.

Several transition points were identified for boundary conditions acquired at θ = 0° and θ = 90° which can be used to predict the properties and performance of an adhesive joint. These transition points are: a_MIN—energy modulus of the system (E. M. S.), relevant to the minimum interfacial energy; a_S—E. M. S. where self-spreading of adhesive occurs; a_CRIT—E. M. S. relevant to conditions under which the thermodynamic work of adhesion becomes negative and the system exhibits a tendency for self-delaminating or has “zero-strength”; a_CF—E. M. S. beyond which the geometry of the interface at any interfacial void or boundary of the joint may be regarded as a crack tip.

It is shown that only in those systems for which Φ₀ = 1.0 can a minimum contact angle of 0° indicate a condition for the maximum strength. If Φ₀ is known, the optimum contact angle can be estimated and hence the optimum surface energy of the substrate (adjusted by surface treatment, etc.) for the maximum adhesion.     

(CH3CH[NH3]CH2NH3)1/2·ZnPO4, a zincophosphate analogue of aluminosilicate zeolite thomsonite

The synthesis and structure of (CH₃CH[NH₃]CH₂NH₃)_1/2·ZnPO₄, an organically templated zincophosphate (ZnPO) analogue of aluminosilicate zeolite thomsonite (THO), are described. The ZnPO framework is built up from an alternating, vertex-sharing, network of ZnO₄ and PO₄ groups (d_av(Zn–O)=1.944 (8) Å, d_av(P–O)=1.535 (9) Å, θ_av(Zn–O–P)=130.5°) involving distinctive 4=1 secondary building units. The 1,2-diammonium propane cations are highly disordered in the [0 0 1] 8-ring channels. Crystal data: (CH₃CH[NH₃]CH₂NH₃)_1/2·ZnPO₄, M_r=198.42, orthorhombic, space group Pncn (no. 52), a=14.119 (6) Å, b=14.136 (5) Å, c=12.985 (5) Å, V=2591 (3) ų, Z=10, R(F)=0.057, R_w(F)=0.061 (for a twinned crystal).     

Kinetics of the selective catalytic reduction of NO by NH3 on a Cu-faujasite catalyst

The kinetics of the selective catalytic reduction (SCR) of NO by NH₃ in the presence of O₂ has been studied on a 5.5% Cu-faujasite (Cu-FAU) catalyst. Cu-FAU was composed of cationic and oxocationic Cu species. The SCR was studied in a gas phase-flowing reactor operating at atmospheric pressure. The reaction conditions explored were: 458<T_R<513 K, 2503 (ppm) < 4000, 12 (%) < 4. The kinetic orders were 0.8–1 with respect to NO, 0.5–1 with respect to O₂, and essentially 0 with respect to NH₃. Based on these kinetic partial orders of reactions and elementary chemistry, a wide variety of mechanisms were explored, and different rate laws were derived. The best fit between the measured and calculated rates for the SCR of NO by NH₃ was obtained with a rate law derived from a redox Mars and van Krevelen mechanism. The catalytic cycle is described by a sequence of three reactions: (i) Cu^I is oxidized by O₂ to “Cu^II-oxo”, (ii) “Cu^II-oxo” reacts with NO to yield “Cu^II-N_xO_y”, and (iii) finally “Cu^II-N_xO_y” is reduced by NH₃ to give N₂, H₂O, and the regeneration of Cu^I (closing of the catalytic cycle). The rate constants of the three steps have been determined at 458, 483, and 513 K. It is shown that Cu^I or “Cu^II-oxo” species constitute the rate-determining active center.     

Catalytic behavior of La–Sr–Ce–Fe–O mixed oxidic/perovskitic systems for the NO+CO and NO+CH4+O2 (lean-NOx) reactions

Mixed oxides of the general formula La_0.5Sr_xCe_yFeO_z were prepared by using the nitrate method and characterized by XRD and Mössbauer techniques. The crystal phases detected were perovskites LaFeO₃ and SrFeO_3−x and oxides -Fe₂O₃ and CeO₂ depending on x and y values. The low surface area ceramic materials have been tested for the NO+CO and NO+CH₄+O₂ (“lean-NO_x”) reactions in the temperature range 250–550°C. A noticeable enhancement in NO conversion was achieved by the substitution of La³⁺ cation at A-site with divalent Sr⁺² and tetravalent Ce⁺⁴ cations. Comparison of the activity of the present and other perovskite-type materials has pointed out that the ability of the La_0.5Sr_xCe_yFeO_z materials to reduce NO by CO or by CH₄ under “lean-NO_x” conditions is very satisfying. In particular, for the NO+CO reaction estimation of turnover frequencies (TOFs, s⁻¹) at 300°C (based on NO chemisorption) revealed values comparable to Rh/-Al₂O₃ catalyst. This is an important result considering the current tendency for replacing the very active but expensive Rh and Pt metals. It was found that there is a direct correlation between the percentage of crystal phases containing iron in La_0.5Sr_xCe_yFeO_z solids and their catalytic activity. O₂ TPD (temperature-programmed desorption) and NO TPD studies confirmed that the catalytic activity for both tested reactions is related to the defect positions in the lattice of the catalysts (e.g., oxygen vacancies, cationic defects). Additionally, a remarkable oscillatory behavior during O₂ TPD studies was observed for the La_0.5Sr_0.2Ce_0.3FeO_z and La_0.5Sr_0.5FeO_z solids.     

Surface enhanced Raman spectroelectrochemical studies of the corrosion films on iron and chromium in aqueous solution environments

“In situ” laser Raman spectra of the corrosion films on iron have been observed in aerated 5 M KOH and 0.15 M NaCl solutions via surface enhancement by the electrodeposition of a silver overlayer. Essentially the same spectra are observed in the two solutions as a function of applied potential in spite of a breakdown of passivity on iron in the chloride solution. Fe(OH)₂ and Fe₃O₄ are found in the prepassive potential region while FeOOH is present in the passive region. A film which is very difficult to reduce appears to be always present on the electrode surface even at hydrogen evolution potentials; this film is believed to be -FeOOH. Surface enhanced Raman spectra of the corrosion films on chromium have also been obtained in NaCl solution for the first time. The passive film has a composition that corresponds most closely to an amorphous form of Cr₂O₃, with some Cr(OH)₃ also present. The film is converted in the transpassive region to a higher oxide form, presumably CrO²⁻₄. Reversible reduction of this species to Cr₂O₃ is indicated.     

Crystal structures of fully dehydrated fully Sr-exchanged zeolite X and of its ammonia sorption complex

The crystal structures of fully dehydrated Sr₄₆–X [Sr₄₆Si₁₀₀Al₉₂O₃₈₄; a=25.214(7) Å] and of its ammonia sorption complex, Sr₄₆–X·102NH₃ [Sr₄₆Si₁₀₀Al₉₂O₃₈₄·102NH₃; a=25.127(7) Å], have been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd at 21(1)°C. The Sr₄₆–X crystal was prepared by ion exchange in a flowing stream of aqueous 0.05 M Sr(ClO₄)₂ for 5 days followed by dehydration at 360°C and 2×10⁻⁶ Torr for 2 days. To prepare the ammonia sorption complex, another dehydrated Sr₄₆–X crystal was exposed to 230 Torr of zeolitically dried ammonia gas for 1 h followed by evacuation for 12 h at 21(1)°C and 5×10⁻⁴ Torr. The structures were refined to the final error indices, R₁=0.043 and R_w=0.039 with 466 reflections, and R₁=0.049 and R_w=0.044 with 382 reflections, for which I>3σ(I). In dehydrated Sr₄₆–X, all Sr²⁺ ions are located at two crystallographic sites. 16 Sr²⁺ ions are at the centers of the double six-rings, filling that site (site I, Sr–O=2.592(6) Å). The remaining 30 Sr²⁺ ions are in the supercage (site II); each extends 0.56 Å into the supercage from the plane of its three nearest oxygen atoms (Sr–O=2.469(6) Å). In the structure of Sr₄₆–X·102NH₃, the Sr²⁺ ions are located at three crystallographic sites: 12 are found at site I [Sr–O=2.652(10) Å]; four in the sodalite units (site I′) each coordinated to three framework oxygen atoms at 2.654(9) Å and also to three ammonia molecules at 2.76(8) Å. The remaining 30 Sr²⁺ ions lie at site II. Each extends 1.12 Å into the supercage where it coordinates to three framework oxygen atoms at 2.584(7) Å and also to three ammonia molecules at 2.774(24) Å.     

Aerosol growth studies — IV. Phase transformation of mixed salt aerosols in a moist atmosphere

The phase transformation and subsequent droplet growth of the mixed salt aerosols NaCl—KCl and (NH₄)₂SO₄—H₂SO₄ were investigated in a continuous-flow apparatus at 25 and 30°C as a function of relative humidity. Monodisperse salt aerosols (d = ≈ 0.5 μm, OG = 1.07–1.13) were prepared and mixed with N₂ carrier gas at controlled humidities. The particle-size distribution of the aerosol before and after growth by water vapor condensation was continuously monitored with an optical particle counter. It was found that mixed salt aerosols were characterized by stage-wise growth when the relative humidity in the atmosphere was increased. The onset of growth took place at a specific deliquescence humidity determined by the water activity at the eutonic composition. Thus, mixed NaCl—KCl aerosols deliquesce at 73.8 ± 0.5% r.h. regardless of initial compositions. For sulfate aerosols containing 0.75 to 0.95 mole fraction (NH₄)₂SO₄ (the balance being H₂SO₄), the onset of growth occurs at 69.0 ± 0.5% r.h.. In the composition range of 0.5 to 0.75, a deliquescence humidity of 39.0 ± 0.5% is noted. Below 0.5 mole fraction, however, the mixed-sulfate aerosols are expected to exhibit hygroscopic properties on the basis of thermodynamic considerations.     

A CRITICAL EVALUATION OF THREE EQUATIONS OF STATE FOR DENSE GASES AGAINST OBSERVED DATA

The van der Waals, the Benedict-Webb-Rubin, and a virial equation of state predictions for dense gases are evaluated against observed “integral” properties (p, ρ, T) and “slope” properties like the speed of sound [(∂p/∂p)^1/2₃], isothermal compressibility coefficient [(∂ In ρ /∂p)_T], and few other thermodynamic properties directly derived from the observed data. The documented procedure thus constitutes a stringent test methodology to evaluate the applicability of these equations of state to dense gases which may also be followed for appraising any other equation of state. The graphical comparisons between the predictions and the observed data are presented and discussed critically     

Effect of ammonium polyacrylate on rheology of anatase TiO2 nanoparticles dispersed in silicon alkoxide sols

Ammonium polyacrylate (NH₄PA) was introduced into powdered mixtures consisting of anatase-structured TiO₂ nanoparticles and silicon alkoxide precursors at the sol level, and the rheological behavior of the mixtures was examined under various solid loadings (φ=0.05–0.13 in volumetric ratios), shear rates (  s⁻¹) and NH₄PA concentrations. The alkoxide precursors were mixtures of tetraethyl orthosilicate (TEOS, Si(OC₂H₅)₄), ethyl alcohol (C₂H₅OH), H₂O and HCl in a constant [H₂O]/[TEOS] ratio of 11. The nanoparticle–sol mixtures generally exhibited a pseudoplastic flow behavior over the shear-rate regime examined. The NH₄PA appeared to serve as an effective surfactant which facilitates the suspension flow by reducing the flow resistance at low NH₄PA concentrations. At φ=0.10, a viscosity reduction ca. 85% was found at  s⁻¹ when the NH₄PA concentration was held at 2.5 wt.% of the solids. As the NH₄PA exceeded a critical level, e.g., [NH₄PA]≥3.0 wt.%, the NH₄PA acted as a catalyst which quickly turned the TiO₂–silica sol mixtures (φ=0.10) into a gelled structure, resulted in a pronounced increase of mixture viscosity. The maximum solids concentration (φ_m) of the mixtures was experimentally determined from a derivative of relative viscosity, i.e., (1−η_r^−1/2)–φ dependence. The estimated φ_m increased from 0.127 to 0.165 when NH₄PA of 0.5 wt.% was introduced into the TiO₂–silica sol mixtures.     

Oxidation of methane in a wide range of pressures and effect of inert gases

The analysis of methane oxidation in a wide range of pressures (0.05–10 atm) demonstrates that the efficiency of CH₃ radicals recombination is of great importance for high selectivity of C₂ hydrocarbons formation. The relative efficiency of different “colliders” assisting the stabilization of exited C₂H*₆ molecules increases in the series: solid surface> Ar> He. The increase of the overall reaction rate upon increasing pressure of the inert gases in the case of catalysts having a higher surface area and more developed pore structure is likely due to the contribution of the surface-induced chain reaction in the volume of pores.     

Rate constants, activation free enthalpies and activation entropies of the electrochemical reduction of 1,4-diazines in DMF

The standard exchange current densities of nine 1,4-diazines at gold electrodes were measured in DMF in the temperature range of +25 up to −59°C. From the resulting free enthalphy of activation of pyrazine as a function of temperature an activation entropy of −1.5 k is calculated. It is compared with the activation entropy of −1.1 k following from the theory of Marcus. The relative change of the free enthalpy of activation with temperature is only a property of the temperature dependences of refractive index and dielectric constant of the solvent. It is used to obtain the free enthalpies of activation for the standard temperature of 298 K.

The experimental values for the compounds are compared with theoretical values, calculated from the inner and outer reorganization energies λ_i and λ₀. For λ_i the bond lengths and force constants were obtained from Hückel calculations, for λ_o different approximations according to Marcus and Peover were used. The calculated and the measured values coincide within less than 10%.

The relation between the free solvation enthalpy and the outer reorganization energy is discussed.     

Synthesis, structure and thermal properties of a novel 3D aluminophosphate UiO-26

The synthesis of a novel 3D aluminophosphate is described. The thermal properties of the material were investigated, and the existence of three high-temperature variants was revealed. The crystal structures of the as-synthesized material (UiO-26-as) and the material existing around 250°C (UiO-26-250) were solved from powder X-ray diffraction data. UiO-26-as with the composition [Al₄O(PO₄)₄(H₂O)]²⁻[NH₃(CH₂)₃NH₃]²⁺ crystallizes in the monoclinic space group P2₁/c (no. 14) with a=19.1912(5), b=9.3470(2), c=9.6375(2) Å and β=92.709(2)°. It exhibits a 3D open framework consisting of connections by PO₄ tetrahedra with AlO₄ tetrahedra, AlO₅ trigonal bipyramids and AlO₅(H₂O) octahedra forming two types of layers stacked along [1 0 0] and connected by Al–O–P bondings. The structure possesses a 1D 10-ring channel system running along [0 0 1], in which doubly protonated 1,3-diaminopropane molecules are located. UiO-26-250 with the composition [Al₄O(PO₄)₄]²⁻[NH₃(CH₂)₃NH₃]²⁺ crystallizes in the monoclinic space group P2₁/c with a=19.2491(4), b=9.27497(20), c=9.70189(20) Å and β=93.7929(17)°. The transformation to UiO-26-250 involves removal of the water molecule which originally is coordinated to aluminum. The rest of the structure remains virtually unchanged. The crystal structures of the two other variants existing around 400 (UiO-26-400) and 600°C (UiO-26-600) remain unknown.     

SINTERING KINETICS AND TRANSPORT PROPERTY EVOLUTION OF LARGE MULTI-PARTICLE AGGREGATES

Ultrafine (“nano”-) particles produced from highly supersaturated vapors or liquids are usually aggregated, often containing thousands of small 'primary' particles bound together in tenuous structures characterized by mass fractal dimensions less than 3. Such aggregates have large initial surface area but are metastable with respect to more compact configurations. Available restructuring mechanisms include surface energy driven coalescence, which, in the case of viscous flow at high gas temperatures, is ultimately able to obliterate all evidence of the original (“primary”) particles. We here exploit the notion that, provided an aggregate is sufficiently large, it can be treated like a spatially non-uniform porous medium, undergoing finite-rate surface energy driven viscous flow sintering leading to final collapse to a single dense sphere. For this purpose, after a D_ƒ ≌s const stage of sintering [associated with a corresponding increase in mean apparent primary particle ('grain') size], we use an extension of the sintering rate models of Mackenzie and Shuttleworth (1949) and Scherer (1977), treating the material of the restructuring aggregate to be a Newtonian viscous fluid. We predict and report here the time-dependent increase in fractal dimension, D_ƒ, and associated decreases in: aggregate outer (maximum) radius, mobility radius, and changes in accessible surface area with dimension-less time [real time in multiples of the characteristic sintering time, μ (R₁)_t=0/σ cr, where u is the material's viscosity (R_l)_t=0 is the effective initial grain radius and a the material surface tension]. In these units, we find that the total required coalescence time does not increase with N as sensitively as N^1/3 an important observation for processes involving very large aggregates. With validation and the indicated extensions, our pseudo-continuum methods are efficient enough to be used for estimating the morphological- and transport property-evolution of entire populations of restructuring aggregates, perhaps characterized by some non-separable probability density function pdf(N, D_ƒ,R₁,) locally, in non-isothermal combustion-synthesis reactors.     

Reptation in polymer blends

Forward recoil spectrometry (FRES) was used to measure the tracer diffusion coefficients D*_PS and D*_PXE of deuterated polystyrene (d-PS) and deuterated poly(xylenyl ether) (d-PXE) chains in high molecular weight protonated blends of these polymers. The D*s were shown to be independent of matrix molecular weights and to decrease as M⁻², where M is the tracer molecular weight, suggesting that the tracer diffusion of both species occurs by reptation. These D*s were used to determine the monomeric friction coefficients ζ_0,PS and ζ_0,PXE of the individual PS and PXE macromolecules as a function of ф, the volume fraction of PS in the PS:PXE blend. Since ζ_0,PSζ_0,PXE at each ф, the rate at which a PS molecule reptates is much greater than that of a PXE molecule, even though both chains are diffusing in identical surroundings. Part of this difference may be due to the difficulty of backbone bond rotation of the PXE molecule. However, even when measured at a constant temperature increment above the glass transition temperature, ζ_0,PS and ζ_0,PXE were observed to be markedly composition dependent. In addition the ratio ζ_0,PS/ζ_0,PXE varied from a maximum of 4 × 10⁻² near ф=0.85 to a minimum of 5 × 10⁻⁵ for ф=0.0. These results show that intramolecular barriers do not solely determine the ζ₀s of the components in this blend. Clearly, the interactions between the diffusing chains and the matrix chains also influence ζ₀.