Showing papers in "Journal of the Chemical Society, Faraday Transactions in 1987"

The kinetics of solubilisate exchange between water droplets of a water-in-oil microemulsion

Abstract: Exchange rates of aqueous solubilisates between water droplets in a water-in-oil microemulsion stabilised by sodium bis(2-ethyl-hexyl) sulphosuccinate (AOT) have been measured as a function of droplet size, temperature and the chain length of the oil The effects of additives (eg alcohols) on the exchange kinetics have also been investigated Exchange rates were measured using very fast chemical reactions as indicators for exchange Three types of reaction were investigated: proton transfer, metal–ligand complexation and electron transfer Similar exchange rates were found for all three reactions The indicator reactions involve the exchange of reactant ions of differing size and charge type; exchange rates were, however, independent of the ion transferred, but dependent on droplet size and temperature For AOT as dispersant, exchange occurs with a second-order rate constant of 106–108 dm3 mol–1 s–1, two to four orders of magnitude slower than the droplet encounter rate as predicted from simple diffusion theory The apparent activation enthalpy is high (and increases with droplet size) but is largely compensated by a positive activation entropy Exchange, on balance, is a relatively facile process which typically takes place on a millisecond timescale (depending on the droplet concentration)The exchange mechanism involves transient water droplet coalescence and separation This is the dynamic process whereby the equilibrium properties of the microemulsion, eg droplet size and polydispersity, are maintained There is a correlation between the exchange rate constants and the stability of the single-phase microemulsion This relationship between the kinetic and equilibrium properties is discussed in terms of the ‘natural curvature’ of the surfactant interface and inter-droplet interactions

The consolidation of concentrated suspensions. Part 1.—The theory of sedimentation

Abstract: The concentration or consolidation of suspensions of fine particles under the influence of a gravitational field has been analysed. The rate and extent of consolidation depends upon a balance of three forces, the gravitational driving force, the viscous drag force associated with flow of liquid in the sediment and a particle or network stress developed as a result of direct particle–particle interactions. In the case of colloidally stable suspensions, this particle stress is the osmotic pressure of the particles; in the case of flocculated or coagulated suspensions, it is the elastic stress developed in the network of particles. A constitutive equation is suggested for irreversibly flocculated suspensions undergoing consolidation which embodies the concept of a concentration-dependent yield stress Py(ϕ). This is then used to analyse the sedimentation behaviour of flocculated sediments and to derive expressions for the initial sedimentation rate. The initial rate of change of sediment height with time in a uniform gravitational or centrifugal field is given approximately by: [graphic ommitted] where B=Δρgϕ0H0/Py(ϕ0), u0 is the sedimentation rate of an isolated particle, ϕ0 is the initial (uniform) volume fraction of solids, r(ϕ0) is a dimensionless hydrodynamic interaction parameter, Δρ is the difference in density between solid and liquid, g is the gravitational or centrifugal acceleration and H0 is the initial sediment height. The theory accounts correctly for the equilibrium consolidation behaviour of strongly flocculated suspensions, and preliminary experimental data suggest that it is not inconsistent with their dynamic behaviour. The estimation of the yield stress Py(ϕ) from a batch centrifuge experiment is also described.

Tin dioxide gas sensors. Part 1.—Aspects of the surface chemistry revealed by electrical conductance variations

Abstract: Detailed measurements are reported of the dependence of the conductance of porous, sintered pellets of tin dioxide, on temperature, moisture and oxygen partial pressure. Comparison is made with the behaviour of thin layers prepared by radio-frequency sputtering. The effect of introducing a non-equilibrium mixture of a combustible gas in air, on the conductance and conductance activation energy is described. The theoretical basis for current interpretations of the behaviour of this material is reviewed. The results are discussed using a model in which the conductance of the porous pellets is controlled by different sorts of intergrain contact, represented as necks, necks depleted of conduction electrons, and Schottky barriers. The results are best rationalised by postulating that the conductance is in fact controlled by Schottky barriers separating domains or agglomerates, each comprising a rather large number of crystallites. The effects of temperature, moisture, oxygen partial pressure and combustible gases are discussed in terms of their effect on the Schottky barrier height by way of altrations in the area density, charge and occupancy of surface states (chemisorbed oxygen species). The existence of a surface state level located ≳ 1.1 eV below the conduction band edge is deduced. Adsorbed water strongly affects both the conductance and the response to combustible gases. Loss of water from the surface over the temperature range 280–450 °C results in a sigmoidal conductance-temperature relationship in moist air. The effect is to lower the resistance at temperatures below this range to no more than one-tenth of the values observed for dried pellets. A surface transformation O2–↔ OH– is inferred. In dried air, with dried pellets, inflexions in the conductance-temperature behaviour at temperatures below 230 °C are tentatively attributed to the effect of O2–↔ O–↔ O–2 surface transformations. The onset of a conductance response to the presence of a combustible gas coincides with the onset of a surface-catalysed combustion. At higher temperatures an effect of combustible gases is to lower the conductance activation energy for the porous pellets, but not for the sputtered layers. The effect of moisture on the response to CO was to extend the response to lower temperature; on dried pellets in dried air the response disappeared abruptly when the temperature fell below 350 °C.

Promotion of methanol synthesis and the water-gas shift reactions by adsorbed oxygen on supported copper catalysts

Abstract: The surfaces of the copper metal crystallites of working Cu/ZuO/Al2O3 and other copper catalysts are partially oxidised in reaction mixtures for methanol synthesis and the water-gas shift reaction. Work with unsupported polycrystalline copper has confirmed earlier results that copper metal is the active phase in supported copper catalysts. The coverage of adsorbed oxygen, O(a), up to half-monolayer, was determined by reaction with N2O and it was found to be controlled by the overall reaction CO2(g)= CO(g)+ O(a). The free energy of formation of O(a) was calculated to be –240 kJ mol–1 at 513 K. The induction period found in methanol synthesis from CO–CO2–H2 mixtures is consistent with the calculated rate of formation of O(a). The role of O(a) in the methanol synthesis and water-gas shift reactions is both as promoter and reaction intermediate. The dissociative chemisorption of hydrogen on copper is promoted by O(a) but this is not necessary for the reactions. Experiments with unsupported polycrystalline copper have shown that O(a) both increases the extent of physisorption of CO2 and creates new chemisorbed states of CO2, with desorption energies of 109, 113 and 125 kJ mol–1. O(a) is also essential for the dissociative chemisorption of water on copper. A regenerative mechanism for the water-gas shift reaction on copper [involving the formation and reaction of O(a)] has been established by observation of the separate stages. The adsorbed formate intermediate, pivotal in methanol synthesis from carbon dioxide, is irrelevant to the water-gas shift reaction.

Atomic transport properties in UO2 and mixed oxides (U, Pu)O2

Abstract: Atomic diffusion processes in UO2 and in the fast-breeder reactor fuel, (U, Pu)O2 are reviewed. Emphasis is given to the slower-moving species. i.e. U and Pu. Self-diffusion, chemical diffusion, diffusion in a thermal gradient, enhancement of diffusion by radiation and fission and the operative diffusion mechanisms are discussed. The main parameter, besides the temperature, is the oxygen-to-metal ratio (O/M ratio) of the oxide. The experimental results are compared with recent calculations reported elsewhere in this volume. Also treated are effects of the possible λ-transition at ca. 2600 K in UO2 on high-temperature kinetic processes.The present knowledge on the diffusion and mobility of fission products with emphasis on volatile and gaseous elements, and of other actinides with emphasis on their valence states are treated. Gaps in our knowledge are pointed out and the relevance of the available results for oxide fuel during reactor operation is discussed. Whereas much is known for the as-produced ‘virgin’ fuel, more results are urgently needed for oxides with higher burn-ups containing a few per cent fission products.Finally, technological applications of the diffusion results are treated. As an example, important savings in cost, energy and time in fuel sintering were recently achieved based on basic studies of diffusion properties of UO2.

The thermodynamics of solvation of ions. Part 2.—The enthalpy of hydration at 298.15 K

Abstract: So-called ‘absolute’ standard molar enthalpies of hydration of individual ions at 298.15 K have been obtained from the conventional data by means of the widely employed TATB extrathermodynamic assumption, i.e. that ΔhydH°[(C6H5)4As+]=ΔhydH°[(C6H5)4P+]=ΔhydH°[B(C6H5)–4]. This is possible by the evaluation of the standard molar enthalpies of hydration of salts involving the tetraphenyl ions with suitable counter-ions from their lattice enthalpies and heats of solution and appropriate combinations of these data. The resulting value of the standard molar enthalpy of hydration of the hydrogen ion, ΔhydH°[H+]=–1103 ± 7 kJ mol–1, agrees with the values obtained by other reliable methods. The values of ΔhydH° on this basis of numerous ions are tabulated.

The identification and characterisation of mixed oxidation states at oxidised titanium surfaces by analysis of X-ray photoelectron spectra

Abstract: The interaction of oxygen with titanium has been studied by X-ray photoelectron spectroscopy. Methods for analysing composite photoelectron spectra (curve fitting, deconvolution and spectral subtraction) are discussed and examined critically and the advantages of the ‘difference spectra’ approach highlighted. Evidence for variable oxidation states (suboxides) is obtained from an analysis of the Ti(2p) spectra observed during the formation of thin oxide films (ca. 10 A). Angular-dependent studies established that the lower oxidation states Ti2+ and Ti3+ were formed preferentially at the metal–oxide interface, whereas Ti4+ species were dominant at the oxide–gas interface. The distribution of the oxidation states within the thin oxide overlayer may be altered by thermally induced diffusion of Ti0 species from the metal substrate; the latter can occur at low temperature (ca. 200 K) for the thin oxide film regime investigated.

Diffusiophoresis of a rigid sphere through a viscous electrolyte solution

Abstract: Numerical predictions of the migration velocity are presented for slightly non-uniform aqueous solutions of KCl or NaCl over a wide range of zeta potential (ζ) and particle radius divided by Debye length (κa). The characteristic speed of migration is comparable in magnitude to the ensemble mean diffusion velocity of ions forming the concentration gradient. In the absence of a macroscopic electric field induced by the gradient, the particle usually migrates toward higher salt concentration (the chemiphoretic contribution). However, for moderate values of κa, the particle reverses direction when |ζ| becomes sufficiently large. With an induced electric field, competition between electrophoresis and chemiphoresis can result in as many as four reversals in direction over the range of ζ. In some cases, the direction of migration was found to depend on the magnitude of the ionic drag coefficients, which seems to preclude predicting the direction on the basis of equilibrium thermodynamics.

Hemimicelle formation of cationic surfactants at the silica gel–water interface

Abstract: The effects of added inorganic salts (NaCl and NaBr), alcohols (C2H5OH and n-C4H9OH) and urea on the adsorption of alkylpyridinium halides from aqueous solution on silica gel have been investigated at 25 °C. The results show that all the adsorption isotherms exhibit two plateaux either with or without various additives. In the first plateau the surface-active cations are adsorbed as individual ions on the negatively charged silica gel surface through both electrostatic attraction and specific adsorption. Then, at a particular concentration known as the hemimicelle concentration (h.m.c.), the adsorption increases dramatically as hemimicelles form on the adsorbent through association between hydrocarbon chains of the adsorbed molecules. As the concentration increases further, the adsorption reaches the second plateau somewhere above the c.m.c. of the surfactant. Based on the assumption that each adsorbed surface-active cation in the first plateau is an active centre for surface aggregation, the minimum average aggregation number of hemimicelle, nhm, is equal to the ratio between the adsorption amounts of two plateaux, Γ∞2nd and Γ∞1st, or, more generally, between Γcmc and Γhmc, the values of adsorption at the c.m.c. and h.m.c. respectively. The results show that the nhm for alkylpyridinium halides in various media studied in this work is mostly in the range 6–18. The addition of inorganic salts and alcohols will increase and decrease the value of nhm, respectively. The approach for determination of the number of surfactant ions in a hemimicelle from the adsorption isotherms proposed by Chander et al. [S. Chander, D. W. Fuerstenau and D. Stigter, in Adsorption From Solution, ed. R. H. Ottewill, C. H. Rochester and A. L. Smith (Academic Press, London, 1983), p. 197] has been questioned.

Local polarity of solvent mixtures in the field of electronically excited molecules and exciplexes

Abstract: In mixtures of solvents of different dielectric polarities a process of preferential solvation described as ‘dielectric enrichment’ occurs in the solvation shell of dipolar solute molecules. Dielectric enrichment requires the diffusion of solvent molecules, and its extent depends on solvent viscosity and on time; in the case of electronically excited molecules it may be limited by their lifetime. Measurements on short-lived ( 10 ns) fluorescers show that in non-viscous liquid media dielectric enrichment reaches equilibrium in times of 1 ns to over 20 ns depending on the polar solvent mole fraction in the mixture. In the case of exciplexes the process of dielectric enrichment is complicated by the quenching action of polar solvents, which reduces the exciplex lifetime.

High-temperature studies of UO2 and ThO2 using neutron scattering techniques

Abstract: A review of the results of recent neutron diffraction, coherent diffuse and inelastic scattering experiments on UO2 and ThO2 at temperatures between 293 K and 2930 K is given. These provide direct evidence for thermally induced Frenkel oxygen lattice disorder at temperatures > 2000 K and give data on the lattice expansion, ionic potentials and elastic stiffness constants to 2930 K for the first time. The results are important for an understanding of the thermophysical behaviour of UO2 and contribute to the data base of thermodynamic properties of UO2.

Incorporation and stability of iron in molecular-sieve structures. Ferrisilicate analogues of zeolite ZSM-5

Abstract: The ferrisilicate molecular sieve analogues of zeolite ZSM-5 can be prepared from ferrisilicate gels containing low-molecular-weight silica species. Precipitation of rust-red iron hydroxide at the high pH necessary for crystallization and complex formation of the iron with the amine impurities present in the crystal-directing agent are avoided by titrating the basic silica solution into the acidic iron-containing solution followed by introduction of the template. Pure white crystalline materials have been prepared in this manner over an SiO2/Fe2O3 range of 15 to 200. The presence of homogeneously dispersed iron(III) in the framework of the crystals has been confirmed. The stability of the framework iron under thermal and hydrothermal conditions is lower than the corresponding aluminosilicate.

Addition–elimination paths in electron-transfer reactions between radicals and molecules. Oxidation of organic molecules by the OH radical

Abstract: Reactions between the ˙OH radical and molecules Y that ultimately lead to electron transfer from Y to ˙OH have been studied by in-situ radiolysis or photolysis, electron spin resonance and pulse radiolysis techniques with optical and conductance detection. These radical–molecule reactions proceed in aqueous solution via the intermediate formation of covalently bound adducts HO—Y˙. These radicals are able to undergo heterolysis, which may proceed by spontaneous or by catalysed paths. The heterolysis results in a one-electron oxidation of Y, and the overall reaction thus consists in a one-electron transfer from the molecule to ˙OH.[graphic omitted]In the addition step (I), a reducing radical is formed by reaction of the oxidizing ˙OH with Y, which is usually neither an oxidant nor a reductant. In the heterolysis step (II), however, the reducing HOY˙ is converted into the oxidizing Y˙+. This phenomenon, termed redox inversion, is the consequence of the change in oxidation state of Y by two units in going from HOY˙ to Y˙+. Examples of redox processes of this kind are given from the class of substituted benzenes and of N-heterocyclics, and structure–reactivity relations governing the heterolysis of HO—Y˙ are discussed.

Investigation of internal silanol groups as structural defects in ZSM-5-type zeolites

Abstract: A 1H Magic-angle spinning nuclear magnetic resonance investigation of large and uniform crystals of HZSM-5 type zeolites, synthesized either by means of TPA or without organic templates, shows that using TPA leads to a high concentration of defect centres within the ZSM-5 structure. Owing to non-intact Si—O—Si bonds, up to 8% of the lattice Si were found to be present as Si—OH groups. A comparison of the 1H—29Si cross-polarization spectra with the variation of the 1H n.m.r. chemical shift of the Si—OH groups as well as with the results of ion-exchange experiments suggests a vicinal arrangement of the silanol groups. High concentrations of such structural defects within the crystals lead to a slight decrease of crystal sorption capacity for n-hexane and significantly reduce crystal hydrophobicity. Both the intracrystalline self-diffusion and the molecular reorientation of sorbed molecules as measured by means of the 1H n.m.r. pulsed field gradient technique and 13C n.m.r. lineshape analysis, respectively, show an enhanced molecular mobility of guest molecules within the defective ZSM-5 structure

Tin oxide surfaces. Part 17.—An infrared and thermogravimetric analysis of the thermal dehydration of tin(IV) oxide gel

Abstract: The thermal dehydration of high-surface-area (ca. 180 m2 g–1) tin(IV) oxide gel has been studied by thermogravimetric analysis and infrared spectroscopy in the temperature range 293–673 K. The thermogravimetricanalysis data show that two types of physisorbed water are present, one which is largely removed by evacuation at ambient temperature and a more strongly held type which is removed by evacuation at 373–423 K. Further mass loss occurring at higher temperatures is due to the condensation of hydroxyl groups on the surface of the oxide particles. The amount of physisorbed molecular water present on the freshly prepared gel corresponds to a ca. three-monolayer coverage, reducing to ca. 0.74 of a monolayer after evacuation at 295 K, and is totally removed after evacuation at 423 K. The surface concentration of hydroxyl groups is determined to be 19.8 nm–2 at 423–473 K, falling to 16.2 nm–2 at 573 K and 13.0 nm–2 at 673 K. Infrared spectra confirm that physisorbed molecular water [v(OH) 3800–2000 cm–1, δ(OH) 1640 cm–1] is removed by 423 K. Isolated and hydrogen-bonded surface hydroxyl groups give rise to stretching modes at 3640 cm–1(sharp) and 3500 cm–1(very broad), respectively. Surface hydroxyl deformation bands are found at 870, 940, 1175 and 1245 cm–1. The bands at 870 and 950 cm–1 are removed on evacuation by 473 and 523 K, respectively. The deformation bands at 1175 and 1245 cm–1 which remain at 523 K are assigned to isolated surface hydroxyl groups. The band at 770 cm–1, which increases in intensity during dehydration, is assigned to surface Sn—O—Sn linkages formed by the dehydroxylation process. The results are interpreted in terms of hydroxylated exposed [100], [101](both of which contain clusters of three hydroxyl groups attached to each surface tin atom) and [110](which contains both isolated and geminal pairs of hydroxyl groups attached to surface tin atoms) crystal planes of the rutile structure, the former predominating.

Photochemical formation of colloidal metals

Abstract: Colloidal silver and gold have been formed by irradiation of AgClO4 and HAuCl4 solutions with 253.7 nm light in the presence of protective agents such as sodium dodecylsulphate, sodium alginate and colloidal silica. Absorption maxima of colloidal silver and gold have been observed at 390–420 and 540–560 nm, respectively. Colloidal metals have been purified by dialysis and their amounts analysed quantitatively. The quantum yields of metal deposition are 0.053–0.15 for colloidal silver and 0.030–0.039 for colloidal gold. The quantum yield is increased several times by the use of protective agents, with the exception of a AgClO4–sodium alginate solution.

Application of electron spin resonance spectroscopy to the study of the effects of ionising radiation on DNA and DNA complexes

Abstract: Exposure of aqueous solutions of DNA to 60Co γ-rays at 77 K gave radicals whose e.s.r. spectra suggest the presence of guanine cations (G˙+) and thymine anions (T˙–) in about equal yield. Hydroxyl radicals, also detected, were shown to be entirely confined to the pure ice phase, and are not involved in reactions with DNA. The fate of these radicals in the presence and absence of oxygen is discussed, and it is shown by strand-break analysis that there is a significant set of reactions leading to strand-breaks originating both from G˙+ and T˙–. The relatively high proportion of double strand-breaks is explained in terms of pairwise trapping of G˙+ and T˙–. When various electron-affinic compounds are added and the systems frozen, they selectively trap electrons, with consequent loss of T˙–, and its protonated product, ˙TH. Compounds such as nitroimidazoles, which from stable anions, appear to protect DNA, and there is an appreciable fall in the number of strand-breaks. Others, such as iodoacetamide or hydrogen peroxide decrease the yields of T˙– and ˙TH, but the reactive products, ˙OH and H2ĊCONH2, react with DNA so that there is little net change in total damage. Attack by ˙OH is facile, and results in the formation of several carbon-centred radicals which are thought to be formed by hydrogen abstraction from the sugar units. However, there is no major increase in the number of strand-breaks under these conditions. Disulphides also act as efficient electron traps and hence as radio-protecting agents. Sulphydryl compounds, RSH, such as glutathione, do not react initially, but on annealing, the concentration of DNA radicals falls and RSSR– radical anions are detected. The number of strand-breaks is thereby greatly diminished. These results strongly support the theory that RSH compounds are important repair agents for DNA. Compounds such as intercalators and transition-metal complexes which bind directly to DNA are even more efficient at diverting damage to themselves, probably at the electron-transfer level. However, these are not likely to be suitable as radiation protection agents. The properties thought to be desirable for such agents are briefly discussed. Finally, attention is turned to the effect of radiation on DNA–histone complexes and on chromatin. The results are remarkable in that they establish efficient transfer of electrons generated in the proteins to DNA, giving enhanced yields of T˙– and ˙TH. In contrast, the histone hole-centres do not appear to migrate in this way.

Hydrocarbon formation from methylating agents over the zeolite catalyst ZSM-5. Comments on the mechanism of carbon–carbon bond and methane formation

Abstract: The mechanism of hydrocarbon formation from methanol has been studied by reacting methylating agents MeX (X = OH, I, OSO3Me) over the acidic form of the pentasil zeolite ZSM-5 and the sodium form of its conjugate base. At >0.1% conversions the product distributions obtained are similar, indicating that similar mechanisms for carbon–carbon bond formation operate with the three reagents. Hydrocarbon formation (CH4 and C2H4) from Me2SO4 over Na-ZSM-5 was observed and is considered to be strong evidence against the involvement of dimethyloxonium methylide as an intermediate since such a species cannot be formed with this reactant under basic conditions. Further evidence against the involvement of a dimethyloxonium methylide is given using reactivity comparisons of oxygen and sulphur-containing reagents. MeI and Me2SO4 were found to be less reactive than MeOH and at low conversion the primary hydrocarbon products observed, CH4and C2H4, suggest that these are the primary products formed in methanol conversion. Under conditions where conversion increased with reaction time, the methane/alkenes ratio decreased markedly and this is considered to be inconsistent with a mechanism involving a surface carbene intermediate formed in a one-step process from the reagent MeX. Instead it is proposed that the crucial first step in the mechanism is the formation of a surface methoxyl species which is primarily responsible for CH4 formation at low conversions. Deprotonation of the surface methoxyl species generates a surface-bonded oxonium methylide which subsequently reacts to form the initial carbon–carbon bond. This mechanistic proposal is both consistent with and unifies the available experimental data obtained for this reaction to date.

The role of Co in sulfidized Co-Mo hydrodesulfurisation catalysts supported on carbon and alumina

Abstract: The thiophene hydrodesulphurisation activities of cobalt sulphide catalysts supported on activated carbon have been measured in a flow microreactor operating at atmospheric pressure. The cobalt content was varied between 1.3 and 13.3 wt% Co, and the oxidic precursor catalysts were dried by three different procedures. Structural characteristics and the degree of Co dispersion in the oxidic precursor and in the sulphidised state of the catalysts were obtained by X-ray photoelectron spectroscopy. The oxidic cobalt phase present in the precursor catalysts was found to be inhomogeneously dispersed over the carbon carrier surface. Dispersion decreased significantly during catalyst sulphidation. XPS results showed that the cobalt was fully sulphidised, although sulphur was found to be present in excess of the amount corresponding to stoichiometric Co9S8.The catalysts demonstrated very high hydrodesulphurisation activities, which were by far superior to those of corresponding carbon- or alumina-supported molybdenum catalysts. By extrapolation of the inverse activities to zero loading the activities of the optimally dispersed cobalt and molybdenum sulphide were determined. Comparison of the activity of carbon-supported Co–Mo with the activities of the optimally dispersed catalyst supported on carbon demonstrated that the so-called promoter effect in sulphidised Co–Mo catalysts can be explained completely by the exceptionally high catalytic activity of cobalt sites and showed that the role of MoS2 in these catalysts is mainly to function as a support for optimally dispersed cobalt ions.

Refractive index and excess volume for binary liquid mixtures. Part 1.—Analyses of new and old data for binary mixtures

Abstract: The relation between the refractive index and the excess molar volume for binary liquid mixtures has been clarified using the additivity rule. The excess volume is represented by two terms due to the excess refractive index and the refractive index difference. Refractive indices have been measured by a differential refractometer for the isorefractive binary mixtures 1-chlorobutane (BuCl)–butan-2-ol (BuOH), 1-chlorobutane–2-methoxyethanol (MOEt), and isopropyl acetate (IPA)–methylethyl ketone (MEK). For the isorefractive binary systems the excess volume was represented by the excess refractive index term and compared with the observed one without knowing the explicit form of the refractive index equation. Coherent correlations were found between the observed excess volume and refractive index for the systems BuCl–BuOH and BuCl–MOEt. The relation of the excess volume and refractive index for the system IPA–MEK suggested a contradiction to the additivity rule. The analyses of the old data for the system propan-1-ol–1,2-dibromoethane have elucidated a strong dependence of the refractive index difference term on the functional form of the refractive index equation. The binary systems studied are not described by a unique equation for the refractive index.

Inelastic neutron scattering investigation of the lattice dynamics of ThO2 and CeO2

Abstract: The phonon dispersion relations of ThO2 and CeO2 have been measured in the three principal crystallographic directions at 293 K using inelastic neutron scattering. After correction for instrumental resolution effects, the data were fitted by rigid ion and shell models of the interionic forces, and the corresponding parameters determined. The results indicate a general similarity between the lattice dynamics of the two compounds.

Redox reactions with colloidal metal oxides. Comparison of radiation-generated and chemically generated RuO2·2H2O

Abstract: Colloids of RuO2· 2H2O and MnO2 have been produced by chemical and radiolytic methods The colloids slowly aggregate upon standing in water over several weeks but radiolysis gives much smaller particles The colloids are active in catalysing water oxidation under chemical and photochemical conditions although there is some corrosion The chemically formed RuO2· 2H2O colloids are quite efficient catalysts for water reduction to H2 but the radiolytically formed colloids do not promote H2 formation Colloidal MnO2 is a poor catalyst for both H2 and O2 generation and it tends to aggregate under operating conditions The RuO2· 2H2O colloids obtained by gamma radiolysis are selective for oxidation processes They are stable and catalyse O2 formation with high efficiencies Such materials appear to be the best catalysts available for use in model systems for the photodissociation of water

Ethanol promotion by the addition of cerium to rhodium–silica catalysts

Abstract: The selectivity of rhodium-based catalysts is very dependent on the choice of support or the promotors. Cerium oxide is very effective for ethanol promotion when added to Rh/SiO2 catalysts or used as support (70–80% ethanol selectivity). The role of the cerium oxide has been studied by various techniques, namely chemical trapping, Fourier-transform infrared spectroscopy and the use of probe molecules. The results show that when cerium oxide is added to rhodium catalysts, a new absorption band appears at 1725 cm–1 in the infrared spectra which may be attributed to a CO molecule bonded through both the carbon and oxygen atoms. Such an interaction could be responsible for the formation and stabilization of the formyl species which, from chemical trapping experiments, is proposed as a key intermediate. These results and those obtained with probe molecules would be in accordance with a reactional pathway where the formyl species is a building block of the C2-oxygenates and the acetaldehyde a primary product of the synthesis. In addition to its role in the activation of carbon monoxide towards hydrogenation, it is also shown that cerium oxide is responsible for the reduction of acetaldehyde to ethanol.

Electrical and catalytic properties of some oxides with the fluorite or pyrochlore structure. CO oxidation on some compounds derived from Gd2Zr2O7

Abstract: The catalytic properties of some mixed zirconates with the pyrochlore or fluorite structure have been investigated using CO oxidation as the test reaction. The presence of terbium ions, leading to mixed conductivity, and the extent of pyrochlore ordering affect the kinetic behaviour and the catalytic activity of the investigated materials. Bismuth-containing compounds show an increased rate of reoxidation.

The thermodynamics of solvation of ions. Part 4.—Application of the tetraphenylarsonium tetraphenylborate (TATB) extrathermodynamic assumption to the hydration of ions and to properties of hydrated ions

Abstract: The tetraphenylarsonium tetraphenylborate (TATB) assumption, i.e. that thermodynamic and other quantities pertaining to the tetraphenylarsonium cation equal those pertaining to the tetraphenylborate anion, is applied to the standard enthalpy, entropy and Gibbs energy of hydration of ions, the standard partial molar heat capacity and volume of aqueous ions, and their equivalent conductivities and Dole–Jones B-coefficients of viscosity at infinite dilution, all at 298.15 K. The effect of uncertainties due to the non-equality in size of the TATB ions is discussed, and related to the relative magnitude of the quantity for small ions and for the constituent ions of TATB, as a criterion for the acceptance of the assumption.

Neutron scattering from concentric cylinders. Intraparticle interference function and radius of gyration

Abstract: A theoretical model to describe the scattering behaviour of concentric cylinder colloidal particles has been developed. In particular, equations are given for the intraparticle form factor and the radius of gyration determined by neutron scattering. It is shown that both of these quantities are strongly dependent on the coherent scattering length density of the solvent medium. From the analysis, the dimensions and average coherent scattering length density of the colloid particle can be determined.

Structural effects on the adsorption of alcohols on titanium dioxides

Abstract: The infrared spectra of methanol, isopropanol and t-butanol adsorbed on TiO2 anatase and rutile are reported and discussed. The irreversible adsorption at room temperature is completely dissociative in all cases on rutile, while undissociatively coordinated molecules are also formed together with alkoxide groups on anatase. This shows the presence on the anatase surface of particular sites whose presence might explain the different behaviour in some surface phenomena reported in the literature of the two crystal modifications.

Use of nitroxides to measure redox metabolism in cells and tissues

Abstract: The metabolic and physical interactions of the paramagnetic nitroxides with cells and tissues are becoming effective tools for measuring biologically significant processes, including some not measured adequately by other means. We have exploited the physical (magnetic) interactions of oxygen with nitroxides to obtain the first reliable and facile measurements of intra-cellular oxygen, using the effects of oxygen on the relaxation times of nitroxides via Heisenberg exchange. With this technique we have demonstrated a significant gradient between extracellular and intracellular oxygen concentrations in our experimental cell suspension system. Previously, the existence of such a gradient has been debated hotly but not tested directly. We also have determined some of the principles of interactions between nitroxides and cells and have demonstrated the feasibility of using metabolic interactions of nitroxides with cells to measure hypoxia in vivo. The principal metabolism of nitroxides by cells is reversible reduction to the hydroxylamine. The rate of reduction depends on the physical characteristics of the nitroxides. Reduction occurs primarily in the intracellular compartment and therefore only nitroxides that can cross the cell membrane readily (e.g. small molecules that are lipid-soluble) can be reduced readily by cells. Once nitroxides get into cells, the rate of reduction depends on the structure of the nitroxide (e.g. those with the five-membered pyrrolidine ring reduce more slowly than those with the six-membered piperidine ring). For some nitroxides the rate of reduction is up to thirty times faster in severely hypoxic cells. This latter phenomenon makes feasible the use of nitroxides to detect hypoxic areas in vivo by using the nitroxides as ‘contrast agents’ for in vivo n.m.r. studies. In principle, regions of the body with hypoxic areas will have lower concentrations of nitroxides because in these regions the nitroxides become reduced to the non-paramagnetic hydroxylamines which do not affect n.m.r. images (paramagnetic molecules affect n.m.r. images by shortening the relaxation times of water protons – the relaxation times of water are the principal imaging parameters for most current in vivo n.m.r. techniques). Thus it appears feasible to use nitroxides to detect and follow processes in vivo that are associated with hypoxia; these include cancer, ischemia (i.e. drastically reduced blood flow) and inflammation.

Rate constants for the reactions of ions with dipolar polyatomic molecules

Abstract: Calculations of rate constants for the reactions of N+ with NH3 and H2O are presented. A combined rotationally adiabatic capture and centrifugal sudden approximation is used. The results are compared with those obtained in CRESU measurements at 27, 68 and 163 K, and SIFT experiments at 300 K. For the N++ H2O reaction, the agreement between both sets of experimental results and the calculations is excellent. For the N++ NH3 reaction, our calculation agrees well with the SIFT result at 300 K, but the CRESU rate constants fall below the calculated values. Both experimental and theoretical rate constants show a strong negative temperature dependence. The sensitivity of the rate constants to the initial rotational states j and K is examined.

Micelle formation of ionic amphiphiles: Thermochemical test of a thermodynamic model

Abstract: Differential enthalpies of dilution of 28 wt % aqueous sodium dodecyl sulphate solution have been measured using a titration microcalorimeter. In this way titration curves were determined in the concentration range 0–0.040 mol dm–3 at three different temperatures. Theoretical titration curves were calculated using values for concentrations of the amphiphile in the monomer and micellar states derived from the thermodynamic model for the association of ionic amphiphiles proposed by Jonsson and Wennerstrom. The computational procedure is described. The agreement between the calculated and experimental curves is good, which shows that the model can give quantitative predictions of the micellization process. It was observed that the decreasing monomer concentration with increasing amphiphile concentration above the c.m.c. requires the quantitative evaluation of the composition of the reaction solutions in order to derive correct values for the enthalpy of micelle formation of ionic amphiphiles from results of calorimetric measurements.