Studies of layered uranium(VI) compounds. I. High proton conductivity in polycrystalline hydrogen uranyl phosphate tetrahydrate

We have found that hydrogen uranyl phosphate tetrahydrate HUO₂PO₄·4H₂O has a high proton conductivity. The ac conductivity was 0.4 ohm^?1 m^?1 at 290°K measured parallel to the faces of sintered disks of the compound. The activation energy was found to be 31 ± 3 kJ mole^?1. The values of conductivity were between 3 and 10 times lower when measured perpendicular to the disk faces due to preferred orientation of the plate-like crystals. Both the powder and sintered disks are stable in air and insoluble in phosphoric acid solution of pH 2.5. Experiments are described which enable possible grain boundary contributions to the conductivity to be determined in such hydrates. The extrinsic grain boundary contribution to the conductivity was found to be small from experiments in which the pH in a solution cell was varied. The abnormally high bulk H⁺ conductivity thus inferred is attributed primarily to the high concentration of H⁺, which exists as H₃O⁺ in the interlamellar hydrogen-bonded network. A Grotthus-type mechanism of conduction is proposed which involves intermolecular transfer steps (hopping) and intramolecular transfer steps, in comparable numbers, the former facilitated by the high concentration of H₃O⁺ ions in the structure, and the latter most likely facilitated by the high concentration of H-bond vacancies.     

The Effect of Biological Substrates on the Ultrafast Excited-state Dynamics of Zinc Phthalocyanine Tetrasulfonate in Solution

Zinc phthalocyanine tetrasulfonate (ZnPcS₄), a potential photosensitizer for photodynamic therapy (PDT), has been studied using femtosecond laser spectroscopy. The excited-state dynamics in water have been found to be fast (<80 ps) and dominated by intermolecular aggregation. Since the proposed mechanism for PDT is energy transfer from the triplet excited state of the photosensitizer to triplet O₂ creating singlet O₂, the short lifetime is expected to be unfavorable for producing singlet O₂. This leads to the suggestion that the presence of biological substrates may have an effect on the excited-state dynamics. To test this hypothesis, the lifetimes of the ex-cited states of ZnPcS₄ have been directly measured in the presence of a model membrane, n-hexadecyltrimethylammonium bromide (CTAB). The excited-state dynamics of ZnPcS₄ in buffer solutions and with human serum albumin (HSA) have also been measured. The presence of HSA and CTAB increases the excited-state lifetime significantly relative to that observed in water. The longer lifetime of ZnPcS4 in CTAB (>1 ns) indicates that the micellar surface favors monomer formation. By increasing the excited-state lifetime, the surface substantially in-creases the photosensitizing potential of ZnPcS₄.     

Rheology and stability of oil-in-water nanoemulsions stabilised by anionic surfactant and gelatin 2) addition of homologous series of sugar-based co-surfactants

The oil-in-water emulsions used in silver-halide photographic coatings are stabilised with anionic surfactants and made in the presence of excess gelatin, which acts as an electrosteric stabilising agent and continuous phase viscosifier. The oil droplet sizes are close to 100 nm but the adsorbed gelatin increases the effective volume of the droplets significantly. These nanoemulsions are manufactured and coated at temperatures in excess of 40 degrees C, where gelatin adopts a random coil structure. At oil concentrations above 15% by volume, the emulsions are viscoelastic liquids with a high low-shear viscosity and strong shear-thinning. The viscosity and shear-thinning can be decreased by reducing the adsorption of gelatin, which can be achieved by addition of nonionic surfactants. This is a rheological study of the effects of adding novel, nonionic sugar-based surfactants on the rheology of photographic nanoemulsions, with additional measurements of static and dynamic surface tension. These surfactants have two sugar (gluconamide) heads and either one or two alkyl tails. Homologous series of each type of sugar surfactant were investigated over a wide range of alkyl tail length. The optimum surfactant choice for commercial applications depends not only on rheological effects but also on ease of synthesis, purification and dissolution, and of course, cost. The dynamic surface tension of the emulsion containing the anionic-nonionic surfactant mixture must also be compatible with the multilayer coating process.     

Rheology and stability of oil-in-water nanoemulsions stabilised by anionic surfactant and gelatin 1) addition of nonionic, cationic and ethoxylated-cationic co-surfactants

Oil-in-water emulsions stabilised by anionic surfactant and gelatin provide the bulk of photographic coating fluids. Their rheology is of crucial importance to the fluids' performance in coating and their concentration in drying. Gelatin complexes with non-adsorbed micelles and adsorbs to the oil-surfactant-water interface, which effects an increase in the viscosity of the continuous phase and the volume of the nano-sized oil droplets, respectively. The consequences of these interactions are high viscosity and strong shear thinning. Here, the effects on the emulsion rheology of a series of bulk, commercially available surfactants were studied. These co-surfactants were chosen so as to weaken the interactions between gelatin and the anionic surfactant and hence reduce viscosity and thinning thus enabling the emulsions to be concentrated. The co-surfactants had polar head groups of three types: simple nonionic based on polyethylenenoxide, simple cationic based on a quaternary alkyltrimethyl ammonium, and combined nonionic-cationic based on a quaternised bis-ethoxylated primary amine. This last type proved the most effective at reducing the low-shear viscosity of the emulsion and reducing the shear thinning, although, at high concentrations the polyethoxylated cationic surfactants induced flocculation and coalescence of the oil droplets.     

Controlling porosity within colloidal heteroaggregates

Heteroaggregates of cationic poly(2-vinylpyridine) microgels and anionic polystyrene latex particles have been made by mixing dilute, aqueous suspensions. The growth of the heteroaggregates was arrested by the addition of anionic silica particles that adsorbed to the free surface of the cationic microgel particles. The resulting heteroaggregates were then concentrated by vacuum filtration, freeze-dried, and characterized by mercury porosimetry and electron microscopy. The inclusion of soft, deformable microgels resulted in heteroaggregates with higher porosity than obtained with heteroaggregates of anionic and cationic latex particles. Control of the pore volumes within the freeze-dried filter cakes was demonstrated by two approaches. In the first approach, heteroaggregation at a constant KCl concentration of 0.01 mM was arrested at different times after mixing the latex and microgel particles, thereby limiting the size of the aggregates. The porosity of the resulting filter cake increased from 61 to 65 vol % as the aggregation time increased from 15 to 120 s. In the second technique, the aggregation time prior to arrest was maintained at 120 s while the KCl concentration was varied between 0.01 and 10 mM. The pore volume of the aggregates decreased from 65 to 57 vol % as the electrolyte concentration increased, a trend explained in terms of the effect of the Debye length on the aggregation process.     

Proton conductivity and phase relationship in solid KOH between 248 and 406°C

Existing evidence indicates that between 248°C and the melting point at 406°C, KOH is a rotator phase. We have shown that, as might be expected, this results in enhanced proton conductivity, and a value of 2×10⁻³ ohms⁻¹ cm⁻¹ was found at 350°C, which is the highest reported for proton conducting solid electrolytes in this temperature range. Excess protons are provided by water molecules residing on the normal OH^- sites, and charge compensation is provided by CO²⁻₃ ions in the solid solution of KOH(≈ 1 m/o K₂CO₃, 1.3m/o H₂O). The activation energy for proton hopping between adjacent H₂O and OH⁻ species probably accounts for most of the observed activation energy of 53±3 kJ mol⁻¹. From TGA studies the isobars at 0.05 and 10 Torr were established for KOH-rich compositions in the KOH---H₂O system, and it was shown that the rotator phase of KOH is stable between these vapour pressures.     

The unitary representation theory of GL(n) of an infinite discrete field

IfK is an infinite field and ifG=GL(n, K) with the discrete topology, then all principal-series representations ofG are irreducible, and any two such with the same central character ψ are weakly equivalent to one another and to the ψ-regular representation. In addition, every irreducible unitary representation ofG which is not one-dimensional weakly contains a representation of the principal series. We deduce that every maximal ideal ofC*(G) is either of codimension 1 or else a kernel of a principal-series representation. In particular, except in the exceptional case whereK is an infinite algebraic extension of a finite field, the reducedC*-algebra of PGL(n, K) is simple, as was also shown by de la Harpe in many cases. Partially supported by NSF Grant DMS-85-06130. It is a pleasure also to acknowledge the hospitality of the Institute for Advanced Studies, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel, from January to August, 1988. Partially supported by NSF Grants DMS-84-00900 and DMS-87-00551. Much of this work was done while visiting at, and partially supported by, the Department of Mathematics and Computer Science, Bar-Ilan University, 52100 Ramat Gan, Israel.