Showing papers in "Journal of the Chemical Society, Faraday Transactions in 1992"
TL;DR: In this article, the temperature and pressure dependence of the relative permittivity of the solvent, H2O, with respect to a modified version of the Uematsu-Franck equation was investigated.
Abstract: Within the framework of the revised HKF (H. C. Helgeson, D. H. Kirkham and G. C. Flowers, Am. J. Sci., 1981, 281, 1249) equations of state (J. C. Tanger IV and H. C. Helgeson, Am. J. Sci., 1988, 288, 19), prediction of the standard partial molal thermodynamic properties of aqueous ions and electrolytes at high pressures and temperatures requires values of the effective electrostatic radii of the ions (re), as well as provision for the temperature and pressure dependence of the relative permittivity of the solvent, H2O. Values of the relative permittivity of H2O, together with the Born functions needed to compute the standard partial molal Gibbs free energy, enthalpy, entropy, heat capacity and volume of solvation were calculated as a function of temperature and density from a modified version of the Uematsu–Franck equation (M. Uematsu and E. U. Franck, J. Phys. Chem. Ref. Data, 1980, 9, 1291). The temperature/pressure dependence of re is described in terms of a solvent function designated by g, which was evaluated in the present study at temperatures and pressures to 1000 °C and 5 kbar by regressing experimental standard partial molal heat capacities and volumes of NaCl reported in the literature together with published dissociation constants for NaClo at supercritical temperatures and pressures using the revised HKF equations of state for aqueous species. The calculated values of re decrease substantially with increasing temperature at constant pressure ⩽2 kbar, and with decreasing pressure at constant temperature 400 °C. The equations and parameters summarized below permit calculation of the standard partial molal properties of aqueous species from the revised HKF equations of state over a much more extensive range of temperature than was previously possible.
461 citations
TL;DR: In this paper, the vibrational spectrum of the OH groups on the external and internal surfaces of H-ZSM5 and Na-Z SM5 samples of different crystallite dimensions has been investigated.
Abstract: Nanosized ZSM5 zeolites with microcrystal dimensions in the 20–120 nm range have been characterized by means of IR spectroscopy and HRTEM microscopy. The vibrational spectrum of the OH groups on the external and internal surfaces of H-ZSM5 and Na-ZSM5 samples of different crystallite dimensions has been investigated. For the sake of comparison the spectra of silicalite samples containing different concentrations of sodium and aluminium are also shown. For this purpose high-purity silicalite samples were prepared following a novel synthesis route.Carbon monoxide (a very weak Lewis base) was used to probe the acidity present on the external and internal surfaces of the zeolites through formation of 1 : 1 adducts with silanols (both internal and external), Bronsted-acid groups (both framework and extraframework), Na+ ions, and Lewis Al3+ centres (in extraframework and framework positions). The IR-active CO stretching modes of the complexes are shifted to higher wavenumber with respect to the free molecule; the positive shift can be used to estimate the acid strength. CO that was physically adsorbed in the zeolite channels has also been investigated.
321 citations
TL;DR: The characteristics of hydrotalcite, following different stages of calcination, have been studied by means of thermogravimetric (TG), X-ray diffraction (XRD), IR, 1H and 27Al NMR techniques as discussed by the authors.
Abstract: Calcined hydrotalcites have been used extensively as catalysts for base-catalysed reactions. The calcination procedure in critical in determining the behaviour of the final material. The characteristics of the hydrotalcite, following the different stages of calcination, have been studied by means of thermogravimetric (TG), X-ray diffraction (XRD), IR, 1H and 27Al NMR techniques. We have shown that dehydroxylation begins within layers and then in a second stage occurs between adjacent layers, causing collapse of the structure. This process is accompanied by a change from octahedral to tetrahedral coordination of the aluminium. Decarbonation leads to the formation of micropores of radius <1.75 nm. Dehydroxylation and decarbonation are reversible processes, and their rates depend on the calcination temperature.
287 citations
TL;DR: The mean first-nearest-neighbour coordination number text-decoration:overlineN1 of atoms in a cluster is an important parameter characteristic of cluster size and geometry as mentioned in this paper.
Abstract: The mean first-nearest-neighbour coordination number text-decoration:overlineN1 of atoms in a cluster is an important parameter characteristic of cluster size and geometry For metal clusters and catalysts, its value is directly measurable by EXAFS spectroscopy Literature expressions for text-decoration:overlineN1 in clusters are not accurate; ideal values have previously had to be worked out numerically by counting atomic site types New, rigorous analytical formulae for text-decoration:overlineN1 as a function of cluster edge length have now been derived for icosahedral and cuboctahedral geometries For clusters of the same size, text-decoration:overlineN1(icos) always exceeds text-decoration:overlineN1(cuboct), by a factor which is greatest for small clusters The value of text-decoration:overlineN1 has recently been proposed to play an important role in the non-metal–metal transition of mercury and other metal-atom clusters; it follows that an icosahedral cluster might be ‘more metallic’ than a cuboctahedral one with the same number of atoms, a consequence which should readily be testable experimentally The use of the text-decoration:overlineN1 parameter also gives new insight into cluster growth mechanisms
266 citations
TL;DR: In this article, the spontaneous precipitation of calcium carbonate from aqueous solutions which are very supersaturated with respect to calcite, both in the absence of inhibitors, and in the presence of triphosphate as inhibitors, was studied.
Abstract: This paper reports a study of the spontaneous precipitation of calcium carbonate from aqueous solutions which are very supersaturated with respect to calcite, both in the absence of inhibitors, and in the presence of triphosphate as inhibitors. The sequence of events during precipitation is governed strongly by the initial supersaturation, the temperature, and the presence or absence of triphosphate.At high supersaturation, the first-formed phase is an amorphous calcium carbonate. It is observed only above a well defined ionic activity product, is homogeneously nucleated, and its formation is not inhibited by triphosphate. Over the temperature range 289–333 K, this amorphous phase has a solubility product, K1, defined by the equation: log K1=(1247.0/T)– 10.224, At concentrations insufficient to produce the amorphous phase, the first-formed solid nucleates heterogeneously. In the absence of triphosphate it is calcite. In the presence of triphosphate, the first solid is calcium carbonate hexahydrate at lower temperatures, and a vaterite/calcite mixture at higher temperatures. CaCO3·6H2O is formed only at temperatures below ca. 25 °C, and decomposes to vaterite. These crystalline phases also result from the decomposition of the amorphous CaCO3 at higher initial supersaturation.In the course of the work, the solubility product for CaCO3·6H2O (K2) was measured in the range 277–312 K as: log K2=(1696/T)+ 0.9336 The quantitative basis of the study was consistent with a re-evaluation of the CaCO03 pair constant K3, as: log K3= 7.722 –(1347.2/T). These results show that at temperatures below 25 °C the binding of CO2–3 to calcium is significantly greater than earlier publications have suggested: the implication for the distribution of carbonate species in the oceans has yet to be fully assessed.
261 citations
TL;DR: In this paper, a continuous polycrystalline films of ZSM-5 (MFI-type) crystals have been grown on porous ceramic (clay) supports, and the films are thermomechanically stable upon calcination at 400 °C in air to remove template ions (tetrapropylammonium), during which process the porous support seems to have a stabilizing effect.
Abstract: Continuous polycrystalline films of ZSM-5 (MFI-type) crystals have been grown on porous ceramic (clay) supports. The films are thermomechanically stable upon calcination at 400 °C in air to remove template ions (tetrapropylammonium), during which process the porous support seems to have a stabilizing effect. When the hydrothermal conditions induce substantial aluminium leaching from the support, large analcime crystals are grown on the support. Gas permeation experiments with both pure gases (permanent gases, alkanes and difluorodichloromethane) and mixtures thereof have been performed on the MFI composite to investigate the separation potential of this new type of membrane. Expected selectivities as a result of large differences in diffusivity are found to be strongly reduced by differences in sorption of the same order of magnitude, and the reduced mobility of weakly adsorbing (fast moving) molecules caused by the slower moving species. However, from the observed low permeation rates it is expected that, owing to the presence of a porous support with relatively low porosity, the molecular sieving effect is reduced as well.
257 citations
TL;DR: In this paper, the influence of the precursor on the metal fraction exposed (MFE) and on the nature of adsorbed CO species is clearly shown, and the bridged entities are more sensitive to the MFE value than the linear ones.
Abstract: Palladium–alumina catalysts prepared from various precursors and by different preparation methods have been studied after calcination by O2 and reduction by H2, using IR spectroscopy of adsorbed CO, EPR and diffuse reflectance measurements. In addition to Pd2+ ions, the unreduced samples contain Pd3+ and Pd+ species. Pd2+ and Pd– entities are still present on catalysts reduced at 573 K. The influence of the precursor on the metal fraction exposed (MFE) and on the nature of adsorbed CO species is clearly shown. Bridged entities are more sensitive to the MFE value than the linear ones. At 100 Torr [1 Torr ≈(101 325/760) Pa], the bridged entities are compressed, which leads to a CO-(surface Pd) ratio of near unity. The MFE increases when the precursors are taken in the following order nitrate < chloride (impregnated)≈ acetylacetonate < chloride (exchanged). The sintering brought about by increasing the reduction temperature seems to affect the largest metal particles rather than the small ones which strongly interact with the carrier.
211 citations
TL;DR: In this article, the authors investigated the second-order transition in the differential scanning calorimetry (DSC) thermogram for sucrose and glycerol solutions as a function of moisture content.
Abstract: Differential scanning calorimetry (DSC) thermograms have been recorded for sucrose and glycerol solutions as a function of moisture content. Simple second-order transitions were observed at the glass transition for the higher concentration samples which did not form ice. More complicated thermograms were observed from the lower solution concentrations which formed ice. The origin of these transitions in the DSC thermograms from frozen solutions is discussed, together with the methods used to calculate the amount of ice in the freeze-concentrated solutions. The glass-transition temperatures (Tg) and the ice-melting temperatures (Tm) were used to construct the supplemented phase diagrams for both of these solutes. The maximum freeze concentration (C′g) for sucrose is determined to be 81.2% sucrose, and is shown by a novel experimental approach to occur at a temperature (T′g) of –40 °C.
209 citations
TL;DR: In this paper, a sample of triblock copolym Synperonic F127 was purified and the micellisation and gelation properties of aqueous solutions of purified and unpurified copolymers were investigated by surface tension measurement, static and dynamic light scattering, differential scanning calorimetry and NMR spectroscopy.
Abstract: A sample of triblock copoly(oxyethylene/oxypropylene/oxyethylene) Synperonic F127 was purified. The micellisation and gelation properties of aqueous solutions of purified and unpurified copolymers were investigated by surface tension measurement, static and dynamic light scattering, differential scanning calorimetry and NMR spectroscopy. Generally, the results obtained for the two samples were similar: an exception was the surface tension. Endothermic standard enthalpies of micellisation were obtained over a wide concentration range, with corresponding endothermic standard enthalpies of gelation in the high concentration range. Considered on an equivalent basis, i.e. kJ mol–1(chains), gelation was found to be an almost athermal process compared to micellisation. Based on the presented evidence, particularly that from DSC, and considering other recent studies, it was concluded that the thermal gelation of F127 (i.e. gelation on raising the temperature) resulted essentially from the packing of spherical micelles. A small thermal event at the gelation point was ascribed to a disorder–order discontinuity.
189 citations
TL;DR: In this article, the effects of high-temperature reduction of Rh/CeO2 catalyst on the hydrogenation of CO, CO2, acetone and ethene, and the hydrogenolysis of ethane, in transient and continuous conditions, have been investigated.
Abstract: The effects of the high-temperature reduction of Rh/CeO2 catalyst on the hydrogenation of CO, CO2, acetone and ethene, and on the hydrogenolysis of ethane, in transient and continuous conditions, have been investigated. The high-temperature reduction (HTR) at 773 K induced a transient Rh–CeO2 interaction in the catalyst which enhances the rate of CO, CO2 and acetone hydrogenation. Temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) show the reduction of Ce4+ to Ce3+ after HTR in the near surface layers. We suggest that the oxygen vacancies on the support (i.e. presence of Ce3+) can interact with the CO moiety promoting its activation.
169 citations
TL;DR: In this article, the diffuse reflectance spectra (UV-VIS) of bulk and silica-supported CeO2(⩽1.10% Ce) are presented, which exhibit distinct spectral features assigned to surface cerium oxygen species.
Abstract: The diffuse reflectance spectra (UV–VIS) of bulk and silica-supported CeO2(⩽1.10% Ce) are presented. Very small supported cerium oxide particles (10–15 A) are obtained, which exhibit distinct spectral features assigned to surface cerium oxygen species.
TL;DR: In this paper, the effect of change in counterion charge and size on the micro-emulsion properties was investigated, and the ion replacement, assessed by UV-VIS spectrophotometry, was ca. 100% efficient.
Abstract: The sodium salt of the di-chained anionic surfactant bis-2-ethylhexylsulfosuccinate [Aerosol-OT or Na(AOT)] stabilises essentially monodisperse, spherical water-in-oil microemulsion (w/o) droplets in alkanes over a wide range of pressure, temperature and composition. In order to investigate the effect of change in counterion charge and size on the microemulsion properties, we have replaced the Na+ counterion by doubly charged ions. The surfactant is then M2+(AOT)2·nH2O: M is from the series Mg2+, Ca2+, Co2+, Ni2+, Cu2+ and Zn2+, and n is the number of water ligands associated with the surfactant molecule. The value of n was determined by FTIR and depends on M2+, but can be between 2 and 8 per molecule of M2+(AOT)2 depending on the nature of M. The ion replacement, assessed by UV–VIS spectrophotometry, is ca. 100% efficient. The effect of temperature on the phase stability of the single-phase M2+(AOT)2 water-in-oil (w/o) microemulsion systems is negligible, in contrast to that observed for the corresponding Na(AOT) system. The structure and properties of the microemulsion are found to be dependent on the counterion identity. Small-angle neutron scattering (SANS) and viscosity measurements provide evidence for the existence of rod-shaped aggregates for Co2+, Ni2+, Cu2+ and Zn2+ at low water constants given by w=[H2O]/[AOT]≈ 5, whilst for Mg2+ and Ca2+ spherical aggregates are present as for Na+. On further addition of water at constant surfactant concentration (w > 10) with Co2+, Ni2+, Cu2+ and Zn2+ the aggregates undergo a shape change, and a more spherical structure is favoured. The results may be explained in terms of the interaction of the different counterions with the SO–3 head group of the surfactant.
TL;DR: In this article, the role of adsorbed proteins and free polysaccharide in the stability of oil-in-water emulsions with respect to coalescence, creaming, and flocculation is reviewed with emphasis on the role played by proteins.
Abstract: Factors affecting the stability of oil-in-water emulsions with respect to coalescence, creaming and flocculation are critically reviewed with emphasis on the role of adsorbed protein and free polysaccharide. Crucial differences in structure between adsorbed layers of a disordered protein (β-casein) and a globular protein are described with particular reference to recent results obtained by specular neutron reflectance. Based on parallel experiments in emulsions and at planar oil/water interfaces, information is now becoming available on the structure and composition of layers adsorbed from mixtures of proteins or mixtures of protein + surfactant. Some of the general features of competitive and cooperative adsorption in mixed systems containing protein can be described by a new lattice-based Monte Carlo computer simulation model. In this regard, the statistical description of how covalent linkage to polysaccharide affects the surface and emulsifying behaviour of protein is especially noteworthy.
TL;DR: In this article, it was shown that the basic sites detected by pyrrole chemisorption in zeolites are Lewis bases, which is in agreement with the view that the most common basic sites are the framework oxygens adjacent to the cations.
Abstract: The basicity of alkali-metal cation X, Y, mordenite and ZSM-5 can be characterized by the infrared NH stretching frequency of chemisorbed pyrrole. It was shown that the basic sites detected by pyrrole chemisorption in zeolites are Lewis bases. This is in agreement with the view that the basic sites in zeolites are the framework oxygens adjacent to the cations. Moreover the coexistence of two NH-stretching bands in the spectrum of pyrrole chemisorbed in a zeolite containing two kinds of cations indicates that the basicity of these sites is determined mainly by the local environment rather than by the bulk zeolite composition. The basicity in higher Si zeolites becomes weaker both in basic strength and the density of basic sites, as indicated by the high NH-stretching frequency and the weak band intensity. A pyrrole species associated with acid sites is also detected.
TL;DR: In this article, the coefficients of intracrystalline self-diffusion of the n-alkanes from propane to n-hexane adsorbed in zeolite ZSM-5 were studied by means of the pulsed field gradient (PFG) NMR method with field gradient amplitudes up to 25 T m −1 over a temperature range from −20 to +380 °C.
Abstract: The coefficients of intracrystalline self-diffusion of the n-alkanes from propane to n-alkanes from propane to n-hexane adsorbed in zeolite ZSM-5 are studied by means of the pulsed field gradient (PFG) NMR method with field gradient amplitudes up to 25 T m–1 over a temperature range from –20 to +380 °C. The diffusivities are found to decrease monotonically with increasing chain lengths. They are in satisfactory agreement with the results of quasielastic neutron scattering and MD simulations as well as with non-equilibrium measurements by the frequency response and single-step adsorption technique. However, in comparison with zero-length-column (ZLC) desorption and permeation studies, these data, though exhibiting the same trends of chain length dependence, are ca. two orders of magnitude larger. As expected, the diffusivities are intermediate between those for zeolite NaX and NaCaA, exhibiting, however, the smallest activation energies. This indicates that the differences in the potential energies of the adsorbate molecules on their intracrystalline trajectories are in general smaller than in zeolites NaCaA and NaX.
TL;DR: In this paper, the effect of acid treatment of montmorillonite clay on its activity as a support for ZnCl2 alkylation catalyst was examined, and the maximum activity was associated with long acid treatment times.
Abstract: The effect of acid treatment of montmorillonite clay on its activity as a support for ZnCl2 alkylation catalyst is examined. Maximum activity is associated with long acid treatment times. Structural characterisation, using X-ray diffraction (XRD), 29Si magic-angle-spinning nuclear magnetic resonance (MASNMR) and elemental analysis, suggests that there is little residual clay structure in the most active supports. A selection of commercially available high-porosity silicas are shown to be more active supports than the acid clays under study. Porosimetry measurements indicate that the presence of mesopores may be an important requirement for an effective ZnCl2 catalyst support.
TL;DR: In this article, the thermal stability of SO2 adsorption on different metal oxides has been studied using various techniques (thermogravimetry, temperature-programmed desorption, IR spectroscopy).
Abstract: SO2 adsorption on different metal oxides (MgO, CeO2, ZrO2, MgAl2O4, TiO2-anatase, TiO2-rutile, Al2O3 and Na–Al2O3) has been studied using various techniques (thermogravimetry, temperature-programmed desorption, IR spectroscopy). Several types of species are formed and the results are discussed in terms of their thermal stability. Weakly adsorbed species can result from the coordination of SO2 on Lewis acid sites such as Al2O3 or TiO2. However, such interactions are quite weak. On more basic oxides, such as Na–Al2O3, SO2 acts as an electron acceptor and adsorbs on either weakly basic O2– sites or on the basic OH– groups. In this latter case, the formation of hydrogen sulfite species is suggested. More strongly adsorbed species occur on all the oxides. These are characterized by strong absorption bands between 1100 and 800 cm–1, and are due to sulfite species. Several types generally occur. Gravimetric measurements allow us to compare the thermal stability and the amount of the different types of adsorbed SO2 species. The quantity of SO2 irreversibly adsorbed after evacuation at T < 370 K gives rise to an evaluation of the number of basic sites and therefore to a scale of basicity of the different oxides.The results are compared with those obtained using other probe molecules such as carbon dioxide or hexafluoroisopropanol. A special case is that of ceria for which it is observed that heating under vacuum transforms sulfite species into sulfates, preventing the use of TPD for the determination of its relative basicity.
TL;DR: In this article, the Coulombic behavior at the collapsed diatomic limit for vanishingly small interatomic separations, R→∞, has been investigated for 13 chemically stable diatomics and the Ar2 van der Waals molecule.
Abstract: The extended Hartree–Fock approximate correlation energy potential model recently reported for diatomics has been improved to account for the Coulombic behaviour at the collapsed diatomic limit for vanishingly small interatomic separations, R→ 0. Also discussed is how to impose the appropriate inverse exponential dependence on R into the extended Hartree–Fock energy contribution as R→∞. Test results are presented for the ground states of 13 chemically stable diatomics and the Ar2 van der Waals molecule. For Ar2, the potential-energy function is obtained from a multiproperty fit including spectroscopic, scattering, and second virial coefficient data. For the remaining diatomics, the calibration procedure uses only spectroscopic Rydberg–Klein–Rees (RKR) data. The new Ar2 potential is then used to calculate the thermophysical properties of gaseous argon over the temperature range 102⩽T/K⩽ 105. In all cases, the potential-energy function of the present work has been found to be very accurate.
TL;DR: In this paper, a new equation for facile computation of ET(30) values of binary solvent mixture from their composition (mole fraction, molality or molarity) is derived.
Abstract: A new equation for facile computation of ET(30) values of binary solvent mixture from their composition (mole fraction, molality or molarity) is derived. The equation is based on one parameter which takes into account the preferential solvation of the ET indicator by the solvents of the mixture. The equation has been successfully applied to 52 binary solvents. The results show that the studied binary solvents show four different behaviours which can be explained in terms of solvent–solvent interactions. The proposed equation allows a simple definition of the solvent polarity as a function of composition.
TL;DR: In this paper, trifluoroacetic acid is added to CH2Cl2 solutions of 1, 10-phenanthroline (1), 2, 9-diphenyl-1, 10phenanthropyl (2) and 2,9-dianisyl- 1,10-phennthroline(3) to cause strong changes in the absorption and fluorescence spectra at room temperature.
Abstract: Addition of trifluoroacetic acid to CH2Cl2 solutions of 1, 10-phenanthroline (1), 2,9-diphenyl-1, 10-phenanthroline (2) and 2,9-dianisyl-1, 10-phenanthroline (3) causes strong changes in the absorption and fluorescence spectra at room temperature. The low-energy absorption bands move to the red, while isosbestic points are maintained. The intensity of the structured, short-lived ππ* fluorescence band decreases and a new, unstructured longer-lived fluorescence band arises at longer wavelength. The number of equivalents of acid needed to obtain 50% protonation in the ground state increases in the series 3 < 2 < 1. The spectral changes can be fully reversed by addition of base. The proton affinity in the singlet excited state is higher than that in the ground state (ΔpKa≈ 14 units for 3), but the protonation and deprotonation processes are too slow to compete with the excited-state decay. In a rigid CH2Cl2 matrix at 77 K the shift of the fluorescence band upon protonation is smaller than that observed at room temperature. The effect of protonation on the phosphorescence band, which can be observed in the rigid CH2Cl2 matrix at 77 K, is very small.
TL;DR: In this paper, the effect of adsorbing submonolayers of bismuth and tin onto single-crystal, polycrystalline and dispersed platinum electrodes on the oxidation of formic acid and methanol was investigated.
Abstract: The effect of adsorbing submonolayers of bismuth and tin onto single-crystal, polycrystalline and dispersed platinum electrodes on the oxidation of formic acid and methanol was investigated. While the oxidation of formic acid on Pt(111) was found to be enhanced by adsorbed sub-monolayers of bismuth, the oxidation of methanol was inhibited by all adsorbed coverages of both tin and bismuth on all electrodes. The adsorbed tin was found to be more stable on the dispersed Pt electrodes than on the smooth electrodes.
TL;DR: In this article, the rate constants for the reactions OH + O−2 and OH + HO2 have been determined in water up to 200 °C by pulse radiolysis of O2-saturated solutons at pH 7.9 (O−2) and pH 2 (HO2) as measured at room temperature.
Abstract: Rate constants for the reactions OH + O–2 and OH + HO2 have been determined in water up to 200 °C by pulse radiolysis of O2-saturated solutons at pH 7.9 (O–2) and pH 2 (HO2) as measured at room temperature. The rate constants were obtained by using the computer code FACSIMILE to simulate the measured absorbance–time profiles. Good agreement between simulation and experiment could be achieved only by including HO–3 and H2O3 as long-lived weakly absorbing products, and using values of the molar absorption coefficients of O–2 and HO2 that have been redetermined in this work to be ca. 15% lower than the recommended values in the literature. The temperature dependence of each rate constant is less than that expected for diffusion-controlled reactions, with the effect being more marked for OH + HO2.
TL;DR: In this article, the sulfate radical in concentrated sulfuric acid and hydrogensulfate solutions has been directly demonstrated in two formation processes: a fast one completed in the duration of the electron pulse, and a slow one occurring over a microsecond time range.
Abstract: In the pulse radiolysis of concentrated sulfuric acid solutions, the absorption spectrum and the molar absorption coefficient (1600 dm3 mol–1 cm–1) of the sulfate radical are unchanged up to 10 mol dm–3 H2SO4, suggesting that the sulfate radical exists in the dissociated form (SO–4). Two formation processes for the sulfate radical have been directly demonstrated in sulfuric acid and hydrogensulfate solutions: a fast one completed in the duration of the electron pulse, and a slow one occurring over a microsecond time range. For sulfate solutions only the fast formation process is observed. In sulfuric acid solutions the slow formation process is OH + HSO–4→ H2O + SO–4(4.7 × 105 dm3 mol–1 s–1) and OH + H2SO4→ HSO4+ H2O → SO–4+ H3O+(1.4 × 107 dm3 mol–1 s–1), and the fast formation process is the direct action of radiation on sulfuric acid with a G value of (2.7 ± 0.4)× 10–2 molecule eV–1. The yields of (OH + SO–4) and H can be quantified as: G(OH + SO–4)= 2.9fw+ 2.7fs and G(H)= 3.7fw+ 2.7fs. The yields of SO–4 have also been evaluated and the decay kinetics and reactions of the sulfate radical studied.
TL;DR: In this paper, a silica gel has been modified with γ-aminopropyltriethoxysilane under varying conditions, controlling the influence of water in the different modification stages.
Abstract: Silica gel has been modified with γ-aminopropyltriethoxysilane under varying conditions, controlling the influence of water in the different modification stages. Diffuse reflectance infrared Fourier-transform (DRIFT) spectra revealed the influence of surface water in the reaction stage and of air humidity in the curing stage. These results were confirmed and refined by 29Si and 13C cross-polarisation magic-angle-spinning nuclear magnetic resonance (CPMASNMR) spectroscopy. Combining the results of both techniques, four modification structures present on the silica surface are proposed, depending on the conditions used.
TL;DR: In this paper, a simple symmetry rule for fullerene isomer distributions was applied to show that the 13C NMR pattern of C84 in uniquely consistent with a 2 : 1 thermodynamic mixture of isoenergetic D2 and D2d symmetry isomers.
Abstract: Application of a simple symmetry rule for fullerene isomer distributions reveals that the 13C NMR pattern of C84 in uniquely consistent with a 2 : 1 thermodynamic mixture of isoenergetic D2 and D2d symmetry isomers.
TL;DR: In this article, the adsorption of DCl, CCl4, CFCl3, CF2Cl2 and CF3Cl on thin D2O ice films at 110 K was studied using reflection-absorption infrared spectroscopy (RAIRS).
Abstract: The adsorption of DCl, CCl4, CFCl3, CF2Cl2 and CF3Cl on thin D2O ice films at 110 K has been studied using reflection–absorption infrared spectroscopy (RAIRS). These compounds interact to varying degrees with the ice surface via the OD bonds dangling into the vacuum. DCI was found to dissociate ionically, evidenced by a strong band attributable to D3O+ and the absence of any absorption band for molecular DCl.
TL;DR: In this article, Babchin et al. modified the De Lacey and white model to include the inertial terms in the Navier-Stokes and colloidal-particle force balance equations, which are extremely important in determining the electrophoretic mobility for frequencies above ca. 105 Hz.
Abstract: The theory developed by De Lacey and White (E. H. B De Lacey and L. R. White, J. Chem. Soc, Faraday Trans 2, 1981, 77, 2007) to calculate the electrophoretic mobility of a solid, spherical colloidal particle subjected to an oscillating electric field is modified to include the inertial terms in the Navier–Stokes and colloidal-particle force balance equations. These inertial terms are extremely important in determining the mobility for frequencies above ca. 105 Hz, particularly at large particle size.The magnitude and phase of the electrophoretic mobility are displayed as functions of zeta potential, frequency, particle size and electrolyte concentration. The numerical results are compared with the De Lacey and white model and the approximate theories developed by O'Brien (R. W. O'Brien, J. Fluid Mech., 1988, 190, 71) and Babchin et al. (A. J. Babchin, R. S. Chow and R. P. Sawatsky, Adv. Colloid Interface Sci., 1989, 30, 111).A program for calculating the electrophoretic mobility and other variable-frequency transport properties is available from the authors.
TL;DR: In this paper, a low-angle scattering technique was used to interpret the significant discrepancies between theoretical predictions and experimental results of slow coagulation kinetics, and the variations in values of all usual parameters in the accepted theoretical models were tested to eliminate the disagreements, but the task proved unsuccessful.
Abstract: Coagulation rates in dispersions of uniform spherical particles of surfactant-free sulfate latices, silica and cerium (hydrous) oxide were measured as a function of the ionic strength by a low-angle scattering technique. The results were used to interpret the significant discrepancies between theoretical predictions and experimental results of slow coagulation kinetics. The variations in values of all usual parameters in the accepted theoretical models were tested in order to eliminate the disagreements, but the task proved unsuccessful. However, the assumption of the segregated surface charge on the smooth spherical particles has led to the concordance between the measured and calculated stability ratios. In the model employed, the heterocoagulation theory is required even though one deals with systems of identical particles.
TL;DR: In this article, the dissolution kinetics of two samples of commercial ground Fontainebleau sand consisting of > 99.6% quartz are studied at 25 °C in constant ionic strength conditions and in the pH range 4.5-10.5.
Abstract: The dissolution kinetics of two samples of commercial ground Fontainebleau sand consisting of > 99.6% quartz are studied at 25 °C in constant ionic strength conditions and in the pH range 4.5–10.5. The strong pH dependence of the reaction rate is attributed to changes in the surface speciation of quartz. The ionization of the surface silanol groups is investigated using potentiometric methods and the results are compared with literature data using a simple surface speciation model that accounts for the effects of the electrical double layer on the hydrogen ion activity. The surface speciation model produces titration curves in good agreement with the experimental data and offers an interpretation of the relationship between the density of ionized groups, [SiO–(s)], and pH.The rate constant, k+, for the dissolution reaction: SiO2(solid)+ 2H2O → Si(OH)4(aq) is determined from 25 experiments as: log k+=– 14.547 ± 0.162 + 0.361 ± 0.022 pH at 25 °C, ionic strength of 0.010 mol dm–3 and pH in the range 4.5–10.5. The constant, k+, is in units of mol m–2 s–1. The results are compared with the available literature data at 25 °C. Although there is general agreement between the experimental data and the surface speciation model concerning the pH dependence of the dissolution rates, the variation between the absolute values of the rate constants from different quartz samples is large. The pH dependence observed in this work and that of Brady and Walther, is consistent with a pK for the dissociation of surface silanol groups and surface density of terminal silanol groups of ca. 7.5 and 11 nm–2, respectively. A slope of ca. 0.33 obtained from the regression of pH and log k+ is predicted from the surface model. Similar gradients have been observed in other studies using quartz and also for a range of other silicate minerals for solution pH 7.
TL;DR: In this paper, the effects of surface conduction behind the slip plane on the electrokinetic transport properties of colloids were quantified and the thin-double-layer theory of Fixman was extended for dilute sols of spherical particles.
Abstract: To quantify the effects of surface conduction behind the slip plane on the electrokinetic transport properties of colloids, we have extended the thin-double-layer theory of Fixman for dilute sols of spherical particles. The computations show that it causes the static conductivity and the low-frequency dielectric response of the sol to increase and the mobility of the particles to decrease. Hence, the occurrence of surface conduction significantly changes the relationships between the different electrokinetic properties. Therefore, for a rigorous interpretation of experimental results taking possible effects of surface conduction adequately into account, it is imperative to collect data with more than one electrokinetic technique. For polystyrene latices, a comparison between theory and experiment has been made. The mobility and static conductivity data can be well reconciled if surface conduction is accounted for. The corresponding estimates of surface charge densities agree very well with values obtained by independent measurements. However, the extreme dielectric behaviour of latex colloids cannot be attributed to surface conduction.