Showing papers in "Journal of the Chemical Society, Faraday Transactions in 1986"

Polyaniline, a novel conducting polymer. Morphology and chemistry of its oxidation and reduction in aqueous electrolytes

Abstract: The emeraldine salt form of polyaniline, conducting in the metallic regime, can be synthesized electrochemically as a film exhibiting a well defined fibrillar morphology closely resembling that of polyacetylene. Cyclic voltammograms of chemically synthesized and electrochemically synthesized polyaniline are essentially identical. Probable chemical changes which occur and the compounds which are formed when chemically synthesized poly-aniline is electrochemically oxidized and reduced between –0.2 and 1.0 V vs. SCE in aqueous HCl solutions at pH values ranging from –2.12 (6.0 mol dm–3) to 4.0 have been deduced from cyclic voltametric studies. These are shown to be consistent with previous chemical and conductivity studies of emeraldine base and emeraldine salt forms of polyaniline. It is proposed that the emeraldine salt form of polyaniline has a symmetrical conjugated structure having extensive charge delocalization resulting from a new type of doping of an organic polymer–salt formation rather than oxidation which occurs in the p-doping of all other conducting polymer systems.

Enzyme entrapment in electrically conducting polymers. Immobilisation of glucose oxidase in polypyrrole and its application in amperometric glucose sensors

Abstract: A technique is described for the entrapment of glucose oxidase in a polypyrrole matrix electrochemically deposited on a printed platinum electrode. The enzyme activity incorporated into the polymer was quantified by spectrophotometric assay using a flow-through cell and found to be proportional to the activity in the electropolymerisation medium. Polymer-entrapped glucose oxidase electrodes can be operated as amperometric glucose sensors in the presence or absence of soluble mediators. These electrodes respond rapidly, reaching a steady state within 20–40 s, with the enzymatic response being current limiting. The potential of this new immobilisation technique to the development of biosensors is discussed.

Proton-transfer spectroscopy. Perturbation of the tautomerization potential

Abstract: The phenomena of excited-state intramolecular proton transfer (phototautomerization) are distinguished in molecular mechanism according to four classes. Intrinsic intramolecular proton transfer (e.g. 3-hydroxyflavone) involves ultra-rapid proton transfer across an internal H-bond. Concerted biprotonic transfer (e.g. 7-azaindole dimers and solvates, benzanilide dimers) involves cooperative double proton transfer in a cyclical complex. Static and dynamic catalysis of proton transfer (e.g. lumichrome, adenine, guanine) involves strong catalysis in doubly H-bonded acetic acid complexes, or a proton transport by H-bonded pyridine. Proton-relay transfer (e.g. 7-hydroxyquinoline, 3-hydroxyxanthone) is suggested as involving multiproton-bridged solvates, which may serve as experimental models for proton relays and proton pumps in biological systems.Potential functions for proton-transfer spectroscopy are discussed and the perturbations of these potentials by specific solvation and by spectroscopic solvent-cage effects are discussed on the basis of a Born–Oppenheimer spectroscopic solvent-cage model.

Capillary condensation and adsorption in cylindrical and slit-like pores

Abstract: The nature of adsorption of simple fluids confined in model pores is investigated by means of a density functional approach. For temperatures T corresponding to a partial wetting situation a first-order phase transition (capillary condensation) from dilute ‘gas’ to dense ‘liquid’ occurs at relative pressures p/psat close to those predicted by the macroscopic Kelvin equation, even for radii Rc or wall separations H as small as 10 molecular diameters. In a complete wetting situation, where thick films develop, the Kelvin equation is, in general, not accurate. At fixed T the adsorption Γm(p) exhibits a loop; Γm jumps discontinuously at the first-order transition, but the accompanying metastable portions of the loop could produce hysteresis similar to that observed in adsorption measurements on mesoporous solids. Metastable thick films persist to larger p/psat in slits than in cylinders and this has repercussions for the shape of hysteresis loops. For a given pore size the loop in Γm shrinks with increasing T and disappears at a capillary critical temperature Tcapc( Tcapc condensation no longer occurs and hysteresis of Γm will not be observed. Such behaviour is found in experiments. A prewetting (thick–thin film) transition can occur for confined fluids. The transition is shifted to a smaller value of p/psat than that appropriate to prewetting at a single planar wall. Whereas the magnitude of the shift is very small for slits, it is substantial for cylinders and this leads to the possibility of finding a triple point, where ‘liquid’ and thick and thin films coexist, in cylindrical pores whose radii may not be too large for investigation by experiment or computer simulation. Adsorption of supercritical fluids (T > Tc, the bulk critical temperature) in cylinders is mentioned briefly.

Conducting polymer gas sensors

Abstract: Recent results with solid-state semiconductor gas sensors based on organic sensor elements are reviewed. Devices based on metal phthalocyanines show useful responses to NO2. Lead phthalocyanine combines the highest conductivity with the maximum sensitivity to NO2. A thin-film lead phthalocyanine sensor has successfully been used to monitor NOx produced by shot-firing in coal mines. To obtain reasonable conductance and speed of response and recovery, phthalocyanine sensors have been operated at 170°C. Conducting polymer materials, and particularly chemically doped polypyrrole, show responses to toxic gases at ambient temperature. Initial work, using polypyrrole black impregnated filter paper, showed a response to ammonia. More recently, using polypyrrole films electrochemically deposited over electrode arrays, responses to nitrogen dioxide and hydrogen sulphide have also been obtained. Organic-semiconductor gas sensors may have advantages compared to metal-oxide devices in their sensitivity to toxic gases and in their ability to operate at or near room temperature. However, the mechanisms of device function are not yet well understood.

Chemically modified electrodes for the electrocatalytic oxidation of nicotinamide coenzymes

Abstract: Methods for the oxidation of the cofactor, NADH, are discussed and the criteria for a successful mediator are set out. The kinetics of the system are analysed and linear free-energy relationships found for a number of different mediators. The variation of the reaction rate with pH is discussed. The successful oxidation of NADH using a modified electrode with Meldola's blue is described.

Luminescence and redox reactions of the metal-to-ligand charge-transfer excited state of tricarbonylchloro-(polypyridyl)rhenium(I) complexes

Abstract: The metal-to-ligand charge-transfer (MLCT) excited state of tricarbonylchloro(polypyridyl)rhenium(I) complexes, [Re(CO)3(LL)(Cl)], (LL = 2,2-bipyridine, 4,4′-dimethyl-2,2′-bipyridine, 1,10-phenanthroline, 5-chloro-1,10-phenanthroline and 2,2′-bipyrazine) is emissive in solution at room temperature and undergoes facile electron-transfer reactions with a variety of electron donor and acceptor molecules. Laser photolysis studies on three aspects of the excited-state photophysics and photoredox chemistry are described in this work: (a) sensitivity of the room-temperature absorption and emission to variation in the nature of the polypyridyl ligand and solvent; (b) excited-state absorption spectral features and (c) reversible and irreversible ‘reductive’ quenching (using various amines as electron donors) and their relevance to the photocatalytic reduction of CO2 to CO.

Interfacial tension minima in oil–water–surfactant systems. Behaviour of alkane–aqueous NaCl systems containing aerosol OT

Abstract: In heptane and aqueous NaCl systems containing diethylhexyl sodium sulphosuccinate (AOT) very low interfacial tensions, γ, can be attained. For fixed temperature (T) and salt concentration, γ falls as the AOT concentration increases and levels off at a value of γC at a concentration (the critical micelle concentration, c.m.c.) corresponding to the onset of surfactant aggregation in either the aqueous or oil phase, depending on T and salt concentration. For a given T, at low salt concentration AOT resides in the aqueous phase both below and above the c.m.c. At higher salt concentrations surfactant transfers to the oil phase and leaves the aqueous phase close to the c.m.c. expected if no excess of oil were present, but devoid of micelles. In this case the oil phase is shown to be a dilute water-in-oil microemulsion. The transition between behaviour at high and low salt concentrations corresponds to the attainment of a minimum γC. The variation in γC with salt concentration is in part a consequence of the way in which the c.m.c. and the surfactant activity coefficients change with salt concentration. The minimum γC occurs when the apparent degree of dissociation of surfactant in the micelle and at the oil–water interface are equal and close to zero.

Fluid behaviour in narrow pores

Abstract: The behaviour of a Lennard-Jones fluid confined within a straight cylindrical pore has been studied using mean-field theory. The fluid potential parameters were chosen to model argon, and a range of wall–fluid parameters, including values approximating carbon dioxide and graphite walls, was investigated. We calculated the density profile and grand potential of the fluid, and examined the effect of varying the pore radius, pressure, temperature, and strength of the wall–fluid forces on these properties, and especially on the gas–liquid phase transitions that occur. We found that the gas–liquid transition occurs at pressures below the bulk fluid vapour pressure in all cases studied. For a fixed temperature, when the pore radius is decreased the gas–liquid coexistence curve ends in a critical point, as has been observed for fluids between parallel plates. The strength of the wall–fluid forces had a dramatic effect on the phase diagram, changing both the range of pore sizes in which phase transitions occur, and the effect of temperature on them.

Direct measurement of temperature-dependent interactions between non-ionic surfactant layers

Abstract: The force between two surfaces coated with pentaoxyethylene dodecyl ether, C12E5, and immersed in aqueous solution has been measured as a function of surface separation in the temperature range 15–37 °C. The surfaces were prepared by allowing the non-ionic surfactant to adsorb on to hydrophobised mica from the solution. At 15 °C the interaction is repulsive at all separations, but above 20 °C an attractive minimum appears at separations below ca. 4 nm. The attraction increases rapidly with temperature and is identified with the interaction that gives rise to the phase separation (‘clouding’) in micellar solutions of many non-ionic surfactants as well as in the poly-(ethylene oxide)–water system.The net interaction is composed of two large parts, one entropic that is attractive and one enthalpic that is repulsive. The two parts nearly cancel, making the net interaction repulsive at low temperatures but attractive at high. They apparently originate mainly from the hydration interactions between the oxyethylene head groups on one surface with those on the other, and are of the same order of magnitude as in pure poly(ethylene oxide)–water solutions.The thickness of the non-ionic surfactant layers increases with temperature, implying a decrease in surface area per head group. This indicates that the intralayer head-group interaction also becomes more attractive (or less repulsive) with temperature.

Sensors from polymer modified electrodes

Abstract: The construction and operation of ionomer-film modified electrodes are described, including the fundamentals of the ion-exchange processes. Ion-exchange selectivity coefficients for a series of alkylsubstituted pyridinium cations in Nafion have been measured and the large values of these coefficients are attributed to the hydrophobic effect; a free-energy relationship relating the coefficient to the size of the ion has been established. The use of Nafion-coated electrodes for the in vivo measurement of neurotransmitters and for the preconcentration of analytes present in low concentration is discussed. Experiments on the dynamics of the response of ionomer modified electrodes are reported and very high ionic transport rates can be achieved for novel ionically conductive composite polymer membranes. The covalent attachment of redox groups such as ferrocene allows one to control the release of counterions by switching the redox state of the ferrocene.

Zeolites treated with silicon tetrachloride vapour. Part 1.—Preparation and characterisation

Abstract: Crystalline materials with Si/Al ratios ranging from 2.5 to over 100 have been prepared by treating zeolites with silicon tetrachloride vapour at elevated temperatures. Most of the work described involves zeolite Y (synthetic faujasite), but other wide-pore zeolites, such as mordenite, ZSM-5, L, omega (synthetic mazzite) and ferrierite, are also amenable to SiCl4 treatment. We describe the method of preparation of high-silica zeolites and the application of an array of techniques to monitor the degree of crystallinity, unit-cell parameters, the Si/Al ratio and hydrophobicity of the products. Also described are the effects of treatment of wide-pore zeolites with some other inorganic halides.

The viscosity and structure of solutions. Part 1.—A new theory of the Jones–Dole B-coefficient and the related activation parameters: application to aqueous solutions

Abstract: For many years the lowering of the viscosity of water by certain electrolytes has been attributed to the action of their ions in breaking down its special three-dimensional structure. In this paper, the familiar viscosity B-coefficients for the alkali-metal chlorides in aqueous solutions and the related ionic molar contributions to the free energy, enthalpy and entropy of activation, namely Δµ°[graphic omitted]3, Δtext-decoration:overlineH°3 and ΔS°[graphic omitted]3, are re-examined. Particular attention is paid to the nature of the solvent in the transition state.The application of the compensation principle is first explored. If compensation between enthalpic and entropic contributions from solute-induced structural changes occurs in both ground- and transition-state solvents, Δµ°[graphic omitted]3, and hence B cannot by influenced by changes in the structure of the solvent of the type proposed, say, by Frank and Wen. Instead it is suggested that, in aqueous solution, an ion can coordinate more solvent molecules in the more weakly bonded transition-state solvent than in the ground-state solvent. This is particularly important for solutions containing the larger alkali-metal and halide ions, whose enhanced fluidity thus stems not from solvent–solvent bond-breaking in the ground state, but from ion–solvent bond-making in the transition state. Even if the compensation principle is not accepted, this explanation removes structurel changes in the solvent as a unique explanation of the observed trends in B-coefficients.

The physical origin of negative capacitance

Abstract: The phenomenon of negative capacitance, which has been reported in a variety of situations involving electrolytic as well as electronic systems, does not appear to be understood and some tentative interpretations appear to lack plausibility. It is suggested that the physically correct approach lies in the analysis of the corresponding time-domain behaviour under step function bias, which involves a current initially falling and then rising gradually over a period of time before finally decaying to zero. This behaviour, sharply contrasting with the normal dielectric relaxation response, can be envisaged in a variety of situations, some of them involving transport phenomena.

Mechanism of permeation through molecular-sieve carbon membrane. Part 1.—The effect of adsorption and the dependence on pressure

Abstract: The permeation in narrow-pore molecular-sieve carbon membranes (MSCM) is governed by adsorption and activated transport and proceeds exclusively through the ultramicropores. Contribution from the solution–desolution mechanism is unlikely. The dependence of permeabilities on pressure and extent of pore opening has been studied. On the basis of adsorption isotherms the relation between adsorption and permeation is established. It is maintained that the slope of the adsorption isotherm of the penetrant determines the concentration gradient responsible for the flow through the membrane. For weakly adsorbing gases, permeability is independent of pressure, as corresponding to a free molecule in the pore system or to a linear adsorption isotherm. For adsorbing gases permeability decreases with pressure, owing to the decrease in the slope of the type I adsorption isotherm.

The hydration entropies of ions and their effects on the structure of water

Abstract: The conventional entropies of hydration are interpreted in terms of contributions from conversion to the absolute scale, compression from the gas to the solution standard states, long-range electrostatic interactions, immobilization of the solvent near the ion, and water-structural effects. A simple but effective method for the estimation of the entropy of solvent immobilization is presented. Correlations between the entropy of hydration of ions and various measures of the effects of ions on the structure of water, applied in the past mainly ot monoatomic ions, are extended to polyatomic ones.

Interface wandering in adsorbed and bulk phases, pure and impure

Abstract: After brief mention of recent experiments on phase transitions in adsorbed and surface layers which confirm detailed predictions of the theory of planar Ising models, the statistical mechanics of interfaces in d= 2,3, and more bulk dimensions is addressed for both pure and impure systems. The existence of various interface types in two-dimensional ordered layers is explained and the significance of the associated wetting transitions for commensurate melting is mentioned. The general theory is then developed systematically on the basis of the wandering or roughening exponent, ζ(d), which determines the scale, L⊥, of transverse fluctuations of an interface segment of longitudinal length scale, L∥, viaL⊥≃Lζ∥. In terms of ζ, one can understand heuristically wall–interface and interface–interface interactions, and thence obtain the critical exponents describing commensurate–incommensurate transitions, complete wetting behaviour, and critical wetting transitions, including the effects of long-range van der Waals forces. The well known value, ½(3–d), of ζ(d) for thermally driven interface fluctuations and recent results for quenched random media with random-field, ζ=⅓(5–d), or random-bond disorder, then yield a variety of concrete predictions, several already checked by exact model calculations, most representing open experimental challenges.

The partitioning of solutes between water-in-oil microemulsions and conjugate aqueous phases

Abstract: The partitioning of chymotrypsin, pepsin and lysozyme between water-in-oil microemulsion phases stabilised by the anionic surfactant sodium bis-2-ethylhexyl sulphosuccinate (AOT) and conjugate aqueous phases containing NaCl has been measured. For chymotrypsin and pepsin the extent of partitioning as a function of AOT concentration may be described in terms of a partition coefficient between the microemulsion-dispersed water and the bulk water phase. For aqueous phases containing low salt concentrations in equilibrium with microemulsion phases of relatively large droplet sizes, the value of the partition coefficient is sensitive to the overall protein charge. High values of the partition coefficient are observed for positively charged proteins owing to attractive electrostatic interactions with the negatively charged surfactant film. At high salt concentrations, corresponding to small microemulsion droplets, the partitioning is less sensitive to protein charge. Proteins have a higher affinity for the microemulsion-dispersed water than for bulk water even when the overall protein charge is negative. Lysozyme forms a solid-like phase with AOT which dissolves in the oil when sufficient AOT is present for monolayer coverage of the lysozyme surface. Similar partitioning studies with an enzyme substrate provides useful information for the interpretation of enzyme kinetics in microemulsions.

Polysilicate equilibria in concentrated sodium silicate solutions

Abstract: Polysilicate equilibria in concentrated sodium silicate solutions have been studied using potentiometric (glass electrode) and 29Si n.m.r. methods. Experimental data cover the ranges: 11.36 ⩽ pH ⩽ 14.09, 2.4 ⩽[Na+]tot/mol dm–3⩽ 9.6, 2.1 ⩽[Si]tot/mol dm–3⩽ 9.3 with a variation of SiO2/Na2O between 3.3 and 1.0. From available pH and n.m.r. data an equilibrium model is derived describing polymerization and protonation of silicate species in these water-glass solutions. With SiO2/Na2O = 1.0, i.e. in the most alkaline solutions (pH ≳ 13), dimeric as well as cyclic forms of tri- and tetra-meric silicate species predominate. At higher SiO2/Na2O, polymerization leads to the formation of polysilicate species containing branching groups with nuclearities 6–8 dominating. With SiO2/Na2O ≳ 3.3, i.e. close to the precipitation boundary of amorphous silica, cage-like polysilicate species dominate.The average charge per Si atom for the different polysilicate species varied between –2 in the most alkaline solutions down to –0.5 close to the precipitation boundary. The buffering capacity of the different water-glass solutions was found to decrease with increasing SiO2/Na2O.The change in the different formation constants (log βpq) due to variation in total Si and SiO2/Na2O, was found to be linear with respect to the Na+ concentration, i.e. log βpq= log β°pq+k[Na+].Distribution diagrams describing speciation and equilibria in water-glass solutions of different SiO2/Na2O and total Si concentrations are derived using the computer program SOLGASWATER.

Luminescence of porphyrins and metalloporphyrins. Part 11.—Energy transfer in zinc–metal-free porphyrin dimers

Abstract: A series of covalently linked Zn–H2 porphyrin dimers has been synthesized. The linkage consists of a flexible alkoxy chain of variable length. Absorption spectroscopy shows that there is some exciton coupling between the two porphyrin rings, whilst fluorescence spectroscopy shows that the ZnP unit transfers singlet-state excitation energy to the H2P. From time-resolved fluorescence studies it is concluded that the dimers exist in solution in different, non-equilibrating conformations. A partially closed form permits reasonably close approach of the two porphyrin rings so that Forster energy transfer is efficient. In a fully extended form the two porphyrins are held too far apart for energy transfer to compete effectively with non-radiative decay of the ZnP excited singlet state. Triplet energy transfer is not observed, owing to unfavourable orientations for exchange coupling, whilst redox ion intermediates are not seen, even in polar solvent.

Stochastic models of multi-species kinetics in radiation-induced spurs

Abstract: Diffusion-limited kinetics in multi-species radical clusters, typical of those resulting from the passage of high-energy radiation through a liquid, have been analysed using several theories. A Monte Carlo (MC) model, in which the trajectories of the diffusing reactants are modelled by time-discretised random flights and in which reaction occurs when particles undergo pairwise encounters, is proposed as a measure of reality. The random reaction times (RRT) model, an efficient stochastic simulation technique based on the approximation that interparticle distances evolve independently, is generalised to multi-species spurs and validated by comparison with the full Monte Carlo model. Further comparisons with other analytical models are less favourable and demonstrate the importance of a stochastic rate equation in predicting product ratios. The RRT model is also extended to include reactive products in three different ways, each of which provides an acceptable approximation to the observed (MC) kinetics in all systems tested so far.

Amperometric enzyme electrodes

Abstract: The design of a new type of amperometric enzyme electrode, exploiting ferrocene as mediator between enzyme and electrode, is described. Its use is illustrated by application to the determination of glucose through the use of glucose oxidase. Other flavoproteins accept ferrocene as a mediator, as do peroxidases. The synthesis of ferrocene analogues of drugs, e.g. lidocaine and theophylline, permits the development of analytical methods employing components of the immune system.

Diffuse reflectance flash photolysis

Abstract: Examples are given demonstrating recent progress which allows flash photolysis investigations of opaque materials by using diffuse reflectance from analysing sources for transient detection. Experimental details of a nanosecond diffuse reflectance laser flash photolysis system are presented. The limitations associated with the use of integrating spheres for collecting the analysing light are discussed. The decay of transient absorption and time-resolved spectra are reported for triplet states in microcrystals (benzil), adsorbed as a fraction of a monolayer on powdered silica (acridine) and dyed on cotton fabric (aluminium sulphonated phthalocyanine). Photoinduced changes in diffuse reflectance of Co-doped ZnO are also reported and tentatively assigned. The relationship between the measured changes in diffuse reflectance and the distribution of transients below the irradiated surface is discussed and equations which can be applied in two limiting cases are given. Finally, the first observation of a transient absorption on the picosecond timescale from an opaque sample is presented. The potential of the technique for studying the mechanism of heterogeneous photoreactions is stressed.

Photogeneration of hydrogen

Abstract: Water-soluble derivatives of benzophenone are readily reduced to the pinacol or alcohol upon irradiation with near-ultraviolet light in aqueous solution. The hydrogen donor can be a simple organic compound (e.g. methane, alcohol, aldehyde, amine, amide, sulphide, ether, carboxylic acid etc.) but not water. In most cases the photochemistry proceeds rapidly with high quantum yield. If an active Pt colloidal catalyst is added to the solution, the intermediary radicals can be used to reduce water to H2 on the metallic surface. Thus the organic substrate is effectively dehydrogenated under ambient conditions. In some cases the reaction can be used for synthesis of useful organic materials.

Dynamic behaviour of fluorescence quenchers in cetyltrimethylammonium chloride micelles

Abstract: The kinetics of quenching of the fluorescence of pyrene solubilized in cetyltrimethylammonium chloride (CTAC) micelles by three different types of quenchers [pyrene or cetylpyridinium chloride (immobile quenchers), dodecylpyridinium chloride (mobile, hydrophobic quencher) and iodide ion (mobile, hydrophilic quencher)] has been investigated by means of time-resolved fluorescence measurements. The results obtained with the two immobile quenchers have permitted the first determination of the mean micellar aggregation number N of CTAC at high concentration up to 1.2 mol dm–3. These N-values have then been used for a quantitative evaluation of the kinetic and equilibrium quenching parameters for the other two types of quenchers. The results concerning the quenching by the iodide ion are discussed on the basis of the available models proposed for this type of quenching. They appear to favour the electrostatic model of Almgren et al.

Dynamics of concentrated solutions of rigid-chain polymers. Part 1.—Brownian motion of persistent macromolecules in isotropic solution

Abstract: A molecular theory for the dynamics of rigid-chain polymers in concentrated solutions, based on reptation ideas, is presented. It is shown that partial flexibility of the macromolecules plays an important role in the dynamics of the polymer solution. A diffusion equation which describes the Brownian motion of macromolecules is constructed. The relaxation of the Kerr effect is considered and the rotation–diffusion coefficient is calculated. The relaxation of the macromolecular structure factor and time correlation function of polymer density (dynamic form factor) are considered also. In particular it is shown that (i) the relaxation of the Kerr effect must be non-exponential in all cases when the length of macromolecules L is larger than the effective Kuhn segment l; (ii) the relaxation of the structure factor S(q,t) in the region q2Ll≫ 1, L≈l is described by three characteristic times; and (iii) the cooperative diffusion coefficient of macromolecules in concentrated solution is always much larger than the self-diffusion coefficient.

Ordered solution structure of a monodispersed polystyrene latex as studied by the reflection spectrum method

Abstract: Ordered lattice structures, face-centred cubic (f.c.c.) and body-centred cubic (b.c.c.) lattices, are systematically analysed by the reflection spectrum measurements of monodispersed polystyrene latices in deionized solution. The nearest-neighbour interparticle distance (2Dexp) is determined from the wavelength at the reflection peaks. 2Dexp decreases with increasing latex concentration and is smaller than the calculated average values for the f.c.c. or b.c.c. distribution (2D0). The results are consistent with a ‘two-state structure’ which has ordered and disordered regions. The f.c.c. and b.c.c. lattices are stable at high and low concentrations, respectively. The b.c.c. structure often appears at a high solution temperature and in the presence of a small amount of foreign salt. The size of the ordered region is estimated from the width of the reflection pattern and also from microscopic photographs. The number of crystal layers is estimated to be several hundred and depends on latex concentration and solution temperature.

Structural aspects of metal-oxide-pillared sheet silicates. An investigation by magic-angle-spinning nuclear magnetic resonance, fourier-transform infrared spectroscopy and powder X-ray diffractometry

Abstract: Variable-temperature powder X-ray diffractometry, Fourier-transform infrared spectroscopy and 27Al and 29Si magic-angle spinning nuclear magnetic resonance spectroscopy have been used to elucidate both the nature of alumina pillars introduced into sheet aluminosilicate and to monitor changes in structure of the host matrix following the introduction of the pillars. The pillars are neutral oxide columns which are structurally similar to γ-alumina; they are linked through oxygen to aluminium and magnesium atoms within the octahedral layer of the clay sheet. The layer charge of the clay is balanced by protons, released during calcination, residing in the aluminosilicate structure.

Coherent pulse sequence induced control of selectivity of reactions. Exact quantum-mechanical calculations

Abstract: We present a novel approach to the control of selectivity of reaction products. The central idea is that in a two-photon or multiphoton process that is resonant with an excited electronic state, the resonant excited-state potential-energy surface can be used to assist chemistry on the ground-state potential-energy surface. By controlling the delay between a pair of ultrashort (femtosecond) laser pulses, it is possible to control the propagation time on the excited-state potential-energy surface. Different propagation times, in turn, can be used to generate different products. There are many cases for which selectivity of product formation should be possible using this scheme. Our examples show a variety of behaviour ranging from virtually 100 % selectivity to poor selectivity, depending on the nature of the excited-state potential-energy surface. Branching ratios obtained using a swarm of classical trajectories are in good qualitative agreement with full quantum-mechanical calculations.

Interfacial tension minima in oil–water–surfactant systems. Effects of alkane chain length and presence of n-alkanols in systems containing aerosol OT

Abstract: In alkane and aqueous NaCl systems containing Aerosol OT (AOT) the alkane–solution interfacial tension becomes constant at the onset of surfactant aggregation, which can occur in either the aqueous or alkane phase, depending on conditions. The constant tension, γc, can attain ultralow values (ca. 10–3 mN m–1), and has been shown previously to pass through a minimum value with respect to salt concentration and temperature. In the present paper we report an investigation into minima in γc brought about by variation of the chain length of the alkane, and by addition of n-alkanols (‘cosurfactants’) to the system. Results are discussed in terms of the effective molecular geometry of the surfactant and cosurfactant, and penetration of alkane into the surfactant monolayer. A thermodynamic treatment of the effect of cosurfactant demonstrates that minimum γc results when the ratio of surfactant to cosurfactant is equal at the plane alkane–water interface and in mixed aggregates, as expected from the simple geometrical picture.