Effect of charge density fluctuations within a droplet on dielectric polarization of ionic microemulsions

A statistical model of the dielectric polarization of ionic water-in-oil microemulsions is proposed. The model makes it possible to describe the effect of temperature and dispersed phase content on the static dielectric permittivity behavior of the microemulsions at a region far below percolation. With the help of this model, the microemulsions formed with the surfactant, sodium bis(2-ethylhexyl) sulfosuccinate (AOT), have been analyzed. The studied systems are considered to consist of nanometer-sized spherical non-interacting water droplets of equal size with negatively charged head groups , staying at the interface and positive counterions Na⁺, distributed in the electrical diffuse double layer of the droplet interior. It can be conjectured that two different mechanisms, that provide an increase of the static dielectric permittivity as a function of temperature, may take place. These may be attributed either to the aggregation of droplets or the temperature growth of polarizability of non-interacting and therefore non-aggregating droplets dispersed in oil. The results support the hypothesis that the experimental temperature behavior of dielectric polarization far below the percolation region is only due to the polarization of a single droplet and not to an aggregation. The droplet polarizability is proportional to the fluctuation mean-square dipole moment of a droplet. It is shown that this mean-square dipole moment and the corresponding value of the dielectric increment, depend upon the equilibrium distribution of counterions within a diffuse double layer. The density distribution of ions is determined by the degree of the dissociation of the ionic surfactant. The dissociation of the ionic surfactant in the system has been analyzed numerically. The relationship between the constant of dissociation and the experimental dielectric permittivity has been ascertained.     

Extending Pummerer reaction chemistry. Synthesis studies in the phakellin alkaloid area

The syntheses of (+/-)-dibromophakellstatin and, from this species, (+/-)-dibromophakellin are described. Oxidative cyclization of a phenylthiolated dihydrooroidin derivative triggered by a Pummerer reaction constitutes the key step in this biomimetic approach to this family of marine alkaloids.     

Treatment of TDS data for the lumped-capacitance method. Frequency-domain treatment

A method of determining the complex dielectric permittivity from the quantities directly measured by the lumped-capacitance method is described. The approach is based on the relation between the charge Q(t) of the measuring condenser filled with a dielectric, an applied voltage V(t) and the dielectric response function Φ(t).     

An optimal poincaré inequality for convex domains of non-negative curvature

Archive for Rational Mechanics and Analysis -     

Calculation of the backscattering-channeling surface peak

The backscattering-channeling spectrum exhibits a surface peak corresponding to the interaction of the beam with the first monolayer(s) of the crystal. Using Monte Carlo techniques we have calculated the surface peak intensity for a wide range of experimental conditions. We show that the results can be expressed as a function of one parameter, thus permitting the generation of an almost universal curve for surface peak estimates. The results are in reasonable agreement with experiment. A method of background subtraction from the surface peak is also discussed.     

Orbit structure and countable sections for actions of continuous groups

It is shown that if a second countable locally compact group G acts nonsingularly on an analytic measure space (S, μ), then there is a Borel subset E ? S such that EG is conull in S and each sG ∩ E is countable. It follows that the measure groupoid constructed from the equivalence relation s ～ sg on E may be simply described in terms of the measure groupoid made from the action of some countable group. Some simplifications are made in Mackey's theory of measure groupoids. A natural notion of “approximate finiteness” (AF) is introduced for nonsingular actions of G, and results are developed parallel to those for countable groups; several classes of examples arising naturally are shown to be AF. Results on “skew product” group actions are obtained, generalizing the countable case, and partially answering a question of Mackey. We also show that a group-measure space factor obtained from a continuous group action is isomorphic (as a von Neumann algebra) to one obtained from a discrete group action.     

The Wightman axioms and the mass gap for weakly coupled (ϕ4)3 quantum field theories

We show that for sufficiently small coupling constant λ the λ?⁴ model in three spacetime dimensions exists and satisfies all the Wightman axioms. We also establish the existence of a nonzero mass gap.     

Effect of CO2 laser-pulse mode quality on multiple photon absorption in SF6

Unimolecular multiple-photon absorption cross sections are measured for SF₆ for both single and multiple longitudinal mode CO₂ laser pulses at three different frequencies over a four-decade range of energy fluence. The results indicate only a weak dependence on pulse mode structure for fluence levels corresponding to as little as 0.02 photons absorbed per molecule.     

Diffusion-filtration model of methane escape from a coal seam

Methane desorption from a coal seam is theoretically investigated using a model including both the diffusion of methane in coal lumps and its filtration through net-shaped pores and cracks. The methane density distribution along the seam at an arbitrary time instant is found. Explicit dependences of the amount of the methane escaped from the seam on the lump size, open and closed porosity, viscosity and solubility of methane, and pressure and temperature in the seam are determined. An effective diffusion coefficient in lumps containing methane-filled closed pores is found. In the case of hindered diffusion, the methane can be subdivided into the “fast” and “slow” fractions.     

Rapid tarnishing of silver nanoparticles in ambient laboratory air

Silver has useful surface-plasmon-resonance properties for many potential applications. However, chemical activity in silver nanoparticles exposed to laboratory air can make interpretation of optical scattering and extinction spectra problematic. We have measured the shift of the plasmon polariton wavelength of arrays of silver nanoparticles with increasing exposure to ambient laboratory air. The resonance peak wavelength shifts 65 nm in 36 h (1.8 nm/h). We show by scanning Auger spectroscopy that the shift is due to contamination from sulfur, most likely chemisorbed on the surface. The rate of corrosion product growth on the nanoparticles is estimated to be 3 nm per day, 7.5 times higher than that of bulk Ag under the same conditions.