Critical dynamics of dimers: implications for the glass transition

The Adam-Gibbs view of the glass transition relates the relaxation time to the configurational entropy, which goes continuously to zero at the so-called Kauzmann temperature. We examine this scenario in the context of a dimer model with an entropy-vanishing phase transition and stochastic loop dynamics. We propose a coarse-grained master equation for the order parameter dynamics which is used to compute the time-dependent autocorrelation function and the associated relaxation time. Using a combination of exact results, scaling arguments, and numerical diagonalizations of the master equation, we find nonexponential relaxation and a Vogel-Fulcher divergence of the relaxation time in the vicinity of the phase transition. Since in the dimer model the entropy stays finite all the way to the phase transition point and then jumps discontinuously to zero, we demonstrate a clear departure from the Adam-Gibbs scenario. Dimer coverings are the "inherent structures" of the canonical frustrated system, the triangular Ising antiferromagnet. Therefore, our results provide a new scenario for the glass transition in supercooled liquids in terms of inherent structure dynamics.     

Functionalization of Si(111) surfaces with alkyl chains terminated by electrochemically polymerizable thienyl units

A Si(111) surface has been derivatized with a thiophene-terminated alkyl monolayer which was subsequently photoanodically oxidized in the presence of thiophene to yield a strongly adherent and smooth conducting film.     

Reaction Mechanisms in Both a CCl2F2/O2/Ar and a CCl2F2/H2/Ar RF Plasma Environment

Decomposition of dichlorodifluoromethane (CCl₂F₂ or CFC-12) in aradiofrequency (RF) plasma system is demonstrated. The CCl₂F₂decomposition fractions _{CCl 2 F 2} and mole fractionsof detected products in the effluent gas stream of CCl₂F₂/O₂/Ar andCCl₂F₂/H₂/Ar plasma, respectively, have been determined. The experimentalparameters including input power wattage, O₂/CCl₂F₂ or H₂/CCl₂F₂ ratio,operational pressure, and CCl₂F₂ feeding concentration wereinvestigated. The main carbonaceous product in the CCl₂F₂/O₂/Arplasma system was CO₂, while that in the CCl₂F₂/H₂/Ar plasma systemwas CH₄ and C₂H₂. Furthermore, the possible reaction pathways werebuilt-up and elucidated in this study. The results of the experimentsshowed that the highly electronegative chlorine and fluorine wouldeasily separate from the CCl₂F₂ molecule and combine with the addedreaction gas. This led to the reactions terminated with the CO₂,CH₄, and C₂H₂ formation, because of their high bonding strength. Theaddition of hydrogen would form a preferential pathway for the HCland HF formations, which were thermodynamically stable diatomicspecies that would limit the production of CCl₃F, CClF₃, CF₄, andCCl₄. In addition, the HCl and HF could be removed by neutral orscrubber method. Hence, a hydrogen-based RF plasma system provideda better alternative to decompose CCl₂F₂.     

Quadrupole ion trap mass spectrometry of fullerenes

The fullerenes C₆₀ and C₇₀ can be ionized by desorption from a liquid matrix upon bombardment by Cs⁺ ions of 7 keV kinetic energy. The resulting radical cations, when activated in the ion trap by collisions with Xe target, in the presence of helium, undergo extensive dissociation by loss of multiple C₂ units. Large internal energies are deposited into these molecular ions and the dissociation efficiency is in excess of 60%.     

Mechanistic aspects of oxidation of N-substituted 5H-dibenz[c,e]azepines by aldehyde oxidase

5H-Dibenz[c,e]azepine ( 2 ) and its N-ethyl and N-(2-ethoxyethyl) analogues 3 and 4 were prepared and evaluated as substrates for aldehyde oxidase. Quaternization of 2 with ethyl iodide furnished 3 , while 4 was prepared by lithium aluminum hydride reduction of N-(2-ethoxy)ethyldiphenimide followed by mercuric acetate oxidation of the resultant amine 6 . The rates of oxidation of 2 and 3 were similar, suggesting a lack of selectivity by the enzyme for the respective imine and iminium functional groups in these compounds. The rate of oxidation of 3 decreased with increasing pH while the extent of “hydration” of this substrate increased over a similar pH range, signifying a preference by the enzyme for 3 over its carbinolamine equilibrium partner. Experiments with deuterium labelled analogues of 2 and 3 indicated that azomethine hydrogen loss from these substrates during enzymatic oxidation was not rate determining. Thus 5H-dibenz[c,e]azepine-5,5,7-d₃ ( 7 ), prepared by lithium aluminum deuteride reduction of diphenimide ( 5 ), and its N-ethyl analogue 8 , had respective enzymatic oxidation rates which did not differ from those of their non-deuterated counterparts.     

Models for the biological production of glycerol and biosurfactants from potato-processing industry residuals

Applied Biochemistry and Biotechnology -     

Spectroelectrochemical responsiveness of a freestanding, boron-doped diamond, optically transparent electrode toward ferrocene

The spectroelectrochemical responsiveness of an optically transparent diamond electrode (OTE) toward ferrocene was investigated. A freestanding, mechanically polished, boron-doped diamond disk (0.38 mm thick and 8 mm in diameter) served as the OTE for UV-Vis transmission spectroelectrochemical measurements. A specially designed, thin-layer spectroelectrochemical cell was constructed for the measurements in which ferrocene was electrooxidized to ferricinium ion via a one-electron redox reaction. The oxidation reaction product was spectroscopically monitored in the ultraviolet (λ=252, 285 nm) region of the electromagnetic spectrum. Well-defined and highly stable spectroelectrochemical responses were observed for this type of diamond OTE. The results indicate that diamond is a useful OTE material for spectroelectrochemically studying nonaqueous redox reactions.     

Colorimetric detection of uranium(VI) on building surfaces after enrichment by solid phase extraction

A method for detecting and quantifying uranium(VI) levels on building materials that include concrete, Plexiglas, glass and steel surfaces is presented. Uranium(VI) was extracted from building material surfaces using a pH 2.2 buffer rinse and, subsequently complexed by an organic chelating agent, arsenazo III. The application of a uranium-chelating molecule, arsenazo III, allows for concentration enhancement using C₁₈ solid phase extraction and colorimetric detection of the uranium complex using ultraviolet-visible spectroscopy at 654 nm. The method has a detection limit (based on 3σ) of 40 ng/L (5 ng/cm²) and an overall extraction efficiency greater than 80% for each surface type (concrete, Plexiglas, glass, steel). Methods to prevent interference by metal ions commonly found on building materials are discussed.     

Introduction

Journal of Nanoparticle Research -