Temperature compensation in the oscillatory bray reaction

The influence of temperature on the oscillatory frequency of the hydrogen peroxide-iodate ion reaction is found to be two-sided: (i) the period length decreases with increasing temperature in most of the instances studied, (ii) or in some cases an opposite change is observed. A temperature-independent period length (temperature compensation) is also discovered experimentally in a rather wide temperature interval at a narrow concentration range of reactants both in a batch configuration and under flow conditions. A simple model was considered to simulate this behavior. Opposing effects of the composite reactions of the model on the calculated period length with changing temperature are shown to be responsible for temperature compensation or overcompensation.     

Photo-catalytic oxidation of cyclohexane over TiO2: a novel interpretation of temperature dependent performance

The rate of cyclohexane photo-catalytic oxidation to cyclohexanone over anatase TiO(2) was studied at temperatures between 23 and 60 °C by in situ ATR-FTIR spectroscopy, and the kinetic parameters were estimated using a microkinetic model. At low temperatures, surface cyclohexanone formation is limited by cyclohexane adsorption due to unfavorable desorption of H(2)O, rather than previously proposed slow desorption of the product cyclohexanone. Up to 50 °C, the activation energy for photocatalytic cyclohexanone formation is zero, while carboxylates are formed with an activation energy of 18.4 ± 3.3 kJ mol(-1). Above 50 °C, significant (thermal) oxidation of cyclohexanone contributes to carboxylate formation. The irreversibly adsorbed carboxylates lead to deactivation of the catalyst, and are most likely the predominant cause of the non-Arrhenius behavior at relatively high reaction temperatures, rather than cyclohexane adsorption limitations. The results imply that elevating the reaction temperature of photocatalytic cyclohexane oxidation reduces selectivity, and is not a means to suppress catalyst deactivation.     

Rate coefficients for the OH + HC(O)C(O)H (glyoxal) reaction between 210 and 390 K

Rate coefficients, k1(T), over the temperature range of 210-390 K are reported for the gas-phase reaction OH + HC(O)C(O)H (glyoxal) --> products at pressures between 45 and 300 Torr (He, N2). Rate coefficients were determined under pseudo-first-order conditions in OH using pulsed laser photolysis production of OH radicals coupled with OH detection by laser-induced fluorescence. The rate coefficients obtained were independent of pressure and bath gas. The room-temperature rate coefficient, k1(296 K), was determined to be (9.15 +/- 0.8) x 10-12 cm3 molecule-1 s-1. k1(T) shows a negative temperature dependence with a slight deviation from Arrhenius behavior that is reproduced over the temperature range included in this study by k1(T) = [(6.6 +/- 0.6) x 10-18]T2[exp([820 +/- 30]/T)] cm3 molecule-1 s-1. For atmospheric modeling purposes, a fit to an Arrhenius expression over the temperature range included in this study that is most relevant to the atmosphere, 210-296 K, yields k1(T) = (2.8 +/- 0.7) x 10-12 exp[(340 +/- 50)/T] cm3 molecule-1 s-1 and reproduces the rate coefficient data very well. The quoted uncertainties in k1(T) are at the 95% confidence level (2sigma) and include estimated systematic errors. Comparison of the present results with the single previous determination of k1(296 K) and a discussion of the reaction mechanism and non-Arrhenius temperature dependence are presented.     

Dynamic behavior of the bray-liebhafsky oscillatory reaction controlled by sulfuric acid and temperature

The non-periodic, periodic and chaotic regimes in the Bray-Liebhafsky (BL) oscillatory reaction observed in a continuously fed well stirred tank reactor (CSTR) under isothermal conditions at various inflow concentrations of the sulfuric acid were experimentally studied. In each series (at any fixed temperature), termination of oscillatory behavior via saddle loop infinite period bifurcation (SNIPER) as well as some kind of the Andronov-Hopf bifurcation is presented. In addition, it was found that an increase of temperature, in different series of experiments resulted in the shift of bifurcation point towards higher values of sulfuric acid concentration.     

Do amorphous troglitazones prepared from two diastereomer-pairs have the same molecular mobility and crystallization rate at the surface?

The surface of amorphous compounds crystallizes faster compared to the bulk. This suggests that molecules at the surface have high molecular mobility. Crystallization behavior is affected by various factors including molecular weight and glass transition temperature (T(g)). In this study, we focus on troglitazone which is composed of diastereomers, RR/SS and RS/SR, as model compound, because each diastereomer has the same molecular weight and similar chemical structure. Troglitazone is isolated into each diastereomer, and both amorphous prepared from RR/SS and RS/SR showed similar T(g) (around 60°C). The surface relaxation of each amorphous troglitazone prepared from two diastereomers, RR/SS and RS/SR, was determined to compare surface molecular mobility, using inverse gas chromatography under dry conditions. As a result, amorphous prepared from RS/SR, showed the shorter surface relaxation time at 40°C (temperature below T(g)), which means it has higher molecular mobility than that from RR/SS at the surface although both have the same molecular weight and similar T(g). Microscopy analysis was conducted to observe the crstallization behavior at the surface of amorphous troglitazone in conditions of high temperature and humidity. Micrographs showed that crystallization area at the surface of amorphous prepared from RS/SR, which showed the shorter surface relaxation time, increased faster than that of the amorphous prepared from RR/SS. Although the reason for the difference in the surface relaxation time of each amorphous troglitazone could not be determined, factors such as the difference of configuration might affect the difference.     

Effect of hydration on the dielectric properties of C-S-H gel

The behavior of water dynamics confined in hydrated calcium silicate hydrate (C-S-H) gel has been investigated using broadband dielectric spectroscopy (BDS; 10(-2)-10(6) Hz) in the low-temperature range (110-250 K). Different water contents in C-S-H gel were explored (from 6 to 15 wt%) where water remains amorphous for all the studied temperatures. Three relaxation processes were found by BDS (labeled 1 to 3 from the fastest to the slowest), two of them reported here for the first time. We show that a strong change in the dielectric relaxation of C-S-H gel occurs with increasing hydration, especially at a hydration level in which a monolayer of water around the basic units of cement materials is predicted by different structural models. Below this hydration level both processes 2 and 3 have an Arrhenius temperature dependence. However, at higher hydration level, a non-Arrhenius behavior temperature dependence for process 3 over the whole accessible temperature range and, a crossover from low-temperature Arrhenius to high-temperature non-Arrhenius behavior for process 2 are observed. Characteristics of these processes will be discussed in this work.     

Stable radicals formation in coals undergoing weathering: effect of coal rank

Once coal is excavated it comes into contact with atmospheric oxygen and begins to undergo low temperature oxidation. The mechanism by which the molecular oxygen interacts with the coal macromolecule is suggested to occur in several steps. These steps primarily involve O(2) diffusion to the surface where physical adsorption followed by chemical adsorption takes place. The chemical adsorption forms several types of oxides that can subsequently react to form several products, primarily CO(2). It has also been suggested that some of these oxidation mechanisms might involve radical reactions. As the previous studies were conducted under conditions where significant structural changes occur it is possible that in the low temperature range (T < 100 °C) the oxidation mechanism is different. Several different rank (lignite-subbituminous-bituminous) coals were isothermally heated at 95 °C in an air atmosphere for a period of up to 6 months and samples were collected at two week intervals. The radical concentration of each sample was measured by Continuous Wave Electron Paramagnetic Resonance (CW-EPR). It is apparent that there are distinct differences between the lower rank (lignite) and the higher rank (subbituminous, bituminous) coals. The lower rank coals exhibited only carbon centered radicals with an adjacent oxygen atom and the higher rank coals exhibited only carbon centered radicals. Interestingly, the lower rank coals exhibited no change in radical concentration due to the long term oxidation treatment while the higher rank coals showed a distinct increase in the radical concentration. These findings shed new light on the complex heterogeneous low temperature oxidation reactions occurring at the coal surface.     

Temperature dependence of the Landau-Placzek ratio in glass forming liquids

Here, we studied Rayleigh-Brillouin light scattering in ten different glass-forming liquids (α-picoline, toluene, o-toluidine, ethanol, salol, glycerol, dibutyl phthalate, o-terphenyl, propylene carbonate, and propylene glycol). For each of these liquids it was found that the Landau-Placzek ratio is in a good agreement with the theory at high temperatures and significantly exceeds the theoretical prediction below a certain temperature. Transition between the two temperature regimes occurs near T(A), where T(A) is crossover point from an Arrhenius-like to a non-Arrhenius behavior for the α-relaxation time dependence on temperature. Increase of the Landau-Placzek ratio relative to the theoretical prediction below T(A) seems to be the universal feature of glass-formers. We suggest that formation of locally favored structures in liquids below T(A) causes observed excess of the Landau-Placzek ratio.     

A rate law model for the explanation of complex pH oscillations in the thiourea-iodate-sulfite flow system

In a continuous flow stirred tank reactor (CSTR), the reaction of thiourea-iodate-sulfite (TuIS) exhibits a rich variety of complex oscillations in pH. The transitions from 1(n) type oscillations to 1(3), 1(2) type and simple oscillations were observed on decreasing the flow rate gradually in small steps at 30.2 °C and 20.5 °C, respectively. The transitions from 1(n) type oscillations to 1(0)1(4), 1(0)1(3) type and simple oscillations were observed as well on increasing the temperature in small steps at a given flow rate. Based on the analogous iodate-sulfite-thiosulfate system a simple empirical rate law model is suggested to give a sound agreement between the experimental and simulated results on the complex oscillatory behaviour. A possible explanation of the emergence of the simple empirical rate law model from the mechanism of the individual reactions of the TuIS system is also discussed.     

InP nanowires from surfactant-free thermolysis of single molecule precursors

Indium phosphide nanofibres were grown from a single-molecule precursor, [(PhCH(2))(2)InP(SiMe(3))(2)](2), using hot injection techniques by a solution-liquid-solid (SLS) process, under "surfactant-free" conditions and without the use of protic additives. The fibres are 85-95 nm in diameter and grow from In metal droplets of 100 nm diameter. The length of the nanofibres is a function of the precursor injection temperature (rather than the growth temperature) and can be varied from 6000 nm at 210 °C to 1000 nm at 310 °C. The indium metal tip can be readily removed under mild, non-etching conditions by treatment with thiophenol-P(SiMe(3))(3) mixtures.     

A new optimization strategy for gaseous phase sampling by an internally cooled solid-phase microextraction technique

This study describes a new optimization strategy for internally cooled solid-phase microextraction based on a multivariate approach. The coating temperature was changed in an extraction while manipulating the extraction times to improve the extraction of compounds with different volatilities. Polycyclic aromatic hydrocarbons (PAHs) and phthalic acid esters (PEs) and adipate were used as model compounds in this study. The optimization strategy was in two steps: (1) multivariate optimization of extraction time and initial coating temperature and (2) multivariate optimization of total extraction time and the time required to cool the coating to a lower temperature as determined in step 1. The observed analytical response in relation to the coating temperature was found to be dependent on the analyte volatility and size. The optimized extraction condition for PEs was 23 min extraction while maintaining the coating at 140°C, followed by 7 min of cooling the coating at 10°C. For the PAHs the coating temperature was maintained at 60°C for the first 20 min and at 5°C in the last 20 min of extraction. Comparisons have been made between the proposed optimized conditions with the conventional internally cooled fiber approach and the results thoroughly discussed. The proposed optimization strategy was found to be more effective for all the analytes, especially for the semi-volatiles, compared to the conventional method.     

Reaction of O2 with the hydrogen atom in water up to 350 degrees C

The reaction of the H* atom with O2, giving the hydroperoxyl HO2* radical, has been investigated in pressurized water up to 350 degrees C using pulse radiolysis and deep-UV transient absorption spectroscopy. The reaction rate behavior is highly non-Arrhenius, with near diffusion-limited behavior at room temperature, increasing to a near constant limiting value of approximately 5 x 10(10) M(-1) s(-1) above 250 degrees C. The high-temperature rate constant is in near-perfect agreement with experimental extrapolations and ab initio calculations of the gas-phase high-pressure limiting rate. As part of the study, reaction of the OH* radical with H2 has been reevaluated at 350 degrees C, giving a rate constant of (6.0 +/- 0.5) x 10(8) M(-1) s(-1). The mechanism of the H* atom reaction with the HO2* radical is also investigated and discussed.     

Energetics of association in poly(lactic acid)-based hydrogels with crystalline and nanoparticle-polymer junctions

We report the energetics of association in polymeric gels with two types of junction points: crystalline hydrophobic junctions and polymer-nanoparticle junctions. Time-temperature superposition (TTS) of small-amplitude oscillatory rheological measurements was used to probe crystalline poly(L-lactide) (PLLA)-based gels with and without added laponite nanoparticles. For associative polymer gels, the activation energy derived from the TTS shift factors is generally accepted as the associative strength or energy needed to break a junction point. Our systems were found to obey TTS over a wide temperature range of 15-70 °C. For systems with no added nanoparticles, two distinct behaviors were seen, with a transition occurring at a temperature close to the glass transition temperature of PLLA, T(g). Above T(g), the activation energy was similar to the PLLA crystallization enthalpy, suggesting that the activation energy is related to the energy needed to pull a PLLA chain out of the crystalline domain. Below T(g), the activation energy is expected to be the energy required to increase mobility of the polymer chains and soften the glassy regions of the PLLA core. Similar behavior was seen in the nanocomposite gels with added laponite; however, the added clay appears to reduce the average value of the activation enthalpy. This confirms our SAXS results and suggests that laponite particles are participating in the network structure.     

The fragile-to-strong dynamic crossover transition in confined water: nuclear magnetic resonance results

By means of a nuclear magnetic resonance experiment, we give evidence of the existence of a fragile-to-strong dynamic crossover transition (FST) in confined water at a temperature T(L)=223+/-2 K. We have studied the dynamics of water contained in 1D cylindrical nanoporous matrices (MCM-41-S) in the temperature range 190-280 K, where experiments on bulk water were so far hampered by crystallization. The FST is clearly inferred from the T dependence of the inverse of the self-diffusion coefficient of water (1D) as a crossover point from a non-Arrhenius to an Arrhenius behavior. The combination of the measured self-diffusion coefficient D and the average translational relaxation time tau(T), as measured by neutron scattering, shows the predicted breakdown of Stokes-Einstein relation in deeply supercooled water.     

Stability of carotenoid diets during feed processing and under different storage conditions

The stability of formulated carotenoid diets during feed processing and under different storage conditions were studied. All carotenoid diets were split into two groups with one group containing BHT (acting as an antioxidant) at 250 ppm and the other without BHT. The experiment was divided into two parts. First, all diets were evaluated in total carotenoid (TC) loss during feed processing, in dry mixed feeds after being processed and dried. In the final part, the completed dietary carotenoids were stored in an aluminum foil bag, the top of which was sealed with a bag sealer and kept under different storage conditions at 26-28 °C and 4 °C. The stability of the TC was observed during an 8-week trial period. The results showed that the diet pelleting process did not affect the carotenoid content of the diets, and the best storage temperature for the formulated carotenoid diet was at 4 °C. However, an antioxidant was added to assist in energy saving before feed processing. Thus, the addition of BHT at 250 ppm can be done at normal room temperature in order to reduce oxidation that might cause a loss of TC quantities in diets.     

Communication: On the origin of the non-Arrhenius behavior in water reorientation dynamics

We combine molecular dynamics simulations and analytic modeling to determine the origin of the non-Arrhenius temperature dependence of liquid water's reorientation and hydrogen-bond dynamics between 235 K and 350 K. We present a quantitative model connecting hydrogen-bond exchange dynamics to local structural fluctuations, measured by the asphericity of Voronoi cells associated with each water molecule. For a fixed local structure the regular Arrhenius behavior is recovered, and the global anomalous temperature dependence is demonstrated to essentially result from a continuous shift in the unimodal structure distribution upon cooling. The non-Arrhenius behavior can thus be explained without invoking an equilibrium between distinct structures. In addition, the large width of the homogeneous structural distribution is shown to cause a growing dynamical heterogeneity and a non-exponential relaxation at low temperature.     

Theoretical Investigation for the Abstraction Reaction of H with (CH_3)_3GeH

The reaction of H atom with (CH3)3GeH is considered to play important role in chemical vapor deposition (CVD) processes used in the semiconductor industry 1-2. The reaction mechanism and kinetics nature for this reaction are therefore essential input data for computer-modelling studies directed towards obtaining an understanding of the factors controlling CVD processes. However, despite its importance, the kinetics work about this reaction was very limited. Only two groups studied exper…     

EFFECTS OF PHASIC AND TONIC LIGHT INPUTS ON THE ORCADIAN ORGANIZATION OF THE SQUIRREL MONKEY

Abstract— The organization of the circadian timing system in Saimiri sciureus was probed using the phasic (abrupt transition) and tonic (continuous action) effects of light intensity. The behavior of the simultaneously monitored circadian rhythms of feeding behavior, colonic temperature, and urinary potassium excretion was studied in response to the phasic effects of (a) an abrupt 8-h phase delay in the light–dark (LD) cycle and (b) a series of non-24 h LD cycles ( T = 18 to 30 h). These studies demonstrated that the feeding and temperature rhythms were more tightly coupled to the light-dark cycle than was the rhythm of urinary potassium excretion. The tonic effects of constant levels of illumination confirmed this conclusion. In constant light, internal desynchronization spontaneously occurred in 25% of animals with the potassium rhythm exhibiting a period quite different from that of the feeding and colonic temperature rhythms. Thus, the response of the internal circadian timekeeping system to phasic and tonic light inputs shows that the system in this species comprises at least two potentially independent oscillators with differential light sensitivities.     

Oscillatory Reaction of a Pendant Drop in a Liquid–Air System under Microwave Irradiation

A Belousov–Zhabotinsky reaction in a pendant drop under microwave radiation was observed under nonstirring conditions. The period decreased as the irradiation power and time were increased because the droplet temperature increased. Moreover, the color change of the solution during the oscillatory reaction depended on the irradiation power and time although the droplet temperature returned to room temperature quickly. Irradiation at higher powers or longer times suspended the oscillatory reaction. However, for moderate irradiation powers or times, the oscillatory reaction stopped after irradiation because the concentration distribution of the reagents became homogeneous. After a time, the oscillatory reaction restarted because a concentration gradient developed again. Accordingly, the effect of the microwave irradiation history continued because the oscillatory reaction stopped even after the irradiation was stopped. This is because the rotation of the polar molecule caused by the irradiation produced a flat concentration distribution for the reagents through increased molecular diffusion.     

Micellization and solubilization of a model hydrophobic drug nimesulide in aqueous salt solutions of Tetronic T904

The micellar and phase behavior of an ethylene oxide-propylene oxide branched octablock copolymer Tetronic T 904 (hereafter written as T904) in water and NaCl solutions was examined. The copolymer shows a cloud point (CP) ranging from 74-65°C in the concentration range of 1-10% and forms aggregates (micelles) with a hydrodynamic diameter around 10-12nm in the temperature range 30-40°C. Stable, bluish solutions containing aggregates of variable size (several hundred nm in some cases) were observed even at temperatures much less than the critical micellization temperature (CMT=30°C for a 2% solution in water). The CP and the CMT markedly decrease in the presence of NaCl due to the dehydration of the polyethylene oxide shell. The size of the micelles in water or salt solutions increases at temperatures close to the CP as inferred from viscosity measurments. A model drug compound (nimesulide, NIM) was solubilized in T904 micelles which showed a remarkable increase in solubilization at higher temperature; however, a decrease in solubilization was observed in salt solutions. The thermodynamic parameters for solubilization were obtained, and the location of NIM in the copolymer micelles was investigated by UV-Visible spectroscopy.