A low diffusion particle method for simulating compressible inviscid flows

A new particle method is presented for the numerical simulation of compressible inviscid gas flows, through procedures which involve relatively small modifications to an existing direct simulation Monte Carlo (DSMC) algorithm. Implementation steps are outlined for simulations involving various grid geometries and for gas mixtures comprising an arbitrary number of species. The proposed method is compared with other numerical schemes through a series of one-dimensional and two-dimensional test cases, and is shown to provide a significant reduction in both artificial diffusion and statistical scatter effects relative to existing DSMC-based equilibrium particle methods.     

Synthesis and micellar characterization of short block length methoxy poly(ethylene glycol)-block-poly(caprolactone) diblock copolymers

A series of short block length methoxy poly(ethylene glycol)-block-poly(caprolactone) diblock copolymers was synthesized and characterized in order to assess the potential of these copolymers as a micellar drug-delivery system. Varying the caprolactone:MePEG weight ratio in the reaction mixture allowed the synthesis of diblock copolymers with a MePEG molecular weight of 750 g/mol and PCL block lengths of 2, 5 or 10 repeat units. Phase diagrams of aqueous solutions of the copolymers were constructed which displayed characteristic cloud points and Krafft points. As the degree of polymerization of PCL increased, critical micelle concentration (CMC) values decreased from 6.97 x 10(-1) to 3.38 x 10(-3) g/l, partition equilibrium coefficients (Kv) increased from 1.09 x 10(4) to 22.2 x 10(4),and hydrodynamic diameters increased from 12.2 to 19.5 nm. The micelle morphology was determined to be spherical by transmission electron microscopy.     

Barriers to energy migration in mixed-ligand zinc(II) complexes

Spectroscopic investigations of Zn(4-Cl-PhS)₂(dmphen) (4-Cl-PhS = anion of 4-chlorobenzenethiol; DMPHEN = 2,9-dimethyl-1,10-phenanthroline) in the p2₁/N and p2₁/C crystal phases and in polymethylmethacrylate (PMMA) are reported. The results demonstrate the existence of an intramolecular barrier to energy migration between 3ππ* and ligand-to-ligand charge-transfer levels.     

Molecular similarity: The introduction of flexible fitting

Summary A method of comparing molecules on a quantitative basis which includes the possibility of compounds using their flexibility to achieve matching is described and applied to a series of hypoglycemic and hypolipidemic agents.     

Self-assembled growth of nanostructural Ge islands on bromine-passivated Si(111) surfaces at room temperature

We have deposited relatively thick (∼60 nm) Ge layers on Br-passivated Si(111) substrates by thermal evaporation under high vacuum conditions at room temperature. Ge has grown in a layer-plus-island mode although it is different from the Stranski-Krastanov growth mode observed in epitaxial growth. Both the islands and the layer are nanocrystalline. This appears to be a consequence of reduction of surface free energy of the Si(111) substrate by Br-passivation. The size distribution of the Ge nanoislands has been determined. The Br-Si(111) substrates were prepared by a liquid treatment, which may not produce exactly reproducible surfaces. Nevertheless, some basic features of the nanostructural island growth are reasonably reproducible, while there are variations in the details of the island size distribution.     

Excitations in a dense baryon-antibaryon system

The Self-Consistent Field approximation has been applied to a distribution of spin 1/2 baryons and antibaryons interacting through a classical neutral pseudoscalar meson field. Excitations in the distribution due to density fluctuations are described by an equation relating the meson field frequency to the nucleon coupling strength. We have evaluated at zero temperature and zero momentum transfer and find that the raito/M _n is very close to the ratiom /M _n, whereM _n is the nucleon mass andM the meson mass. For this limiting case,T=q=0, the excitation frequency depends only upon terms characteristic of the particle-antiparticle interaction. We therefore interpret the ⁰ meson as the lowest excitation of a bound nucleon-antinucleon pair.The authors thank Dr. J. Fischer for a number of sugestions which have substantially improved the clarity of the paper.     

Externally tunable dynamic confinement effect in organosilica sol-gels

The feasibility of using the pores of organosilica sol-gels for dynamic confinement of a luminescent molecule is demonstrated in this paper. The porous organosilica sol-gels act as reversible thermoresponsive materials which exhibit reduced pore volume at higher temperature and enlarged pores at lower temperature, which causes dynamic alterations in the nanoenvironment of encapsulated entities. A particularly remarkable feature of this system is that the temperature-dependent confinement effect provided by the gels is reversible and can be efficiently modulated by simply changing the temperature.     

Heterogeneous proton-bound dimers with a high dipole moment monomer: how could we experimentally observe these anomalous ionic hydrogen bonds?

Electronic structure calculations (CBS-QB3 and G3MP2) have been used to predict a suitable method to experimentally observe the anomalous structure which is predicted to exist in a proton-bound dimer with a high dipole moment monomer. The enthalpy associated with forming the proton-bound dimer from its protonated and neutral monomers is shown to be linearly related to the difference in proton affinities which has been observed experimentally. However, unlike previous experimental studies, the linear correlation is not predicted to depend strongly, if at all, on whether the basic sites are C=O, C=N, or O(H) n-donor bases. Thermochemical measurements, then, are probably not the best method to distinguish between the structures of heterogeneous proton-bound dimers. It has been shown that a suitable method to experimentally observe the anomalous structure of proton-bound dimers containing a high dipole moment monomer (or very polar monomer) is by spectroscopic measurement. The O-H+-O asymmetric stretch is probably not the best infrared band to try to correlate with structure. The best band to observe is one which is in a region of the spectrum not masked by other absorptions and is also sensitive to the proximity of the binding proton. For example, it is shown that the methanol-free O-H stretch is very sensitive to the O-H+ bond distance for a series of heterogeneous proton-bound dimers containing methanol. It is predicted that the free O-H stretch of the methanol/acetonitrile proton-bound dimer is more closely related to the O-H stretch in protonated methanol than the O-H stretch in neutral methanol. Observations of these bands should confirm that the proton is closer to methanol in the methanol/acetonitrile proton-bound dimer despite acetonitrile having a higher proton affinity.