Analysis of a microencapsulated-urease fluidized-bed reactor under steady and transient state operating conditions

The feasibility of containing microencapsulated urease within a fludized-bed reactor, eliminating problems of membrane rupture, was demonstrated. Hydrolysis of urea under conditions simulating that of an artificial kidney device was measured as a function of reactor residence time, microcapsule diameter, volume of microcapsules, urea concentration in the feed, and enzyme activity. Empirical correlations were developed based on dimensional analysis, which may be used to predict urea conversion within the range of experimental operating conditions. Results under transient state conditions better represent the operation of the reactor in treatment of uremic patients.     

Distinguishing between dynamic yielding and wall slip in a weakly flocculated colloidal dispersion

Silica spheres coated with grafted poly(butyl methacrylate) chains and dispersed in a poor solvent at low to moderate volume fractions display high elastic and loss moduli, together with viscosities that diverge at a dynamic yield stress and decrease by several orders of magnitude with increasing stress. The elastic modulus and dynamic yield stress increase rapidly with volume fraction, but remain in constant ratio. Longer grafted chains produce stronger interparticle attractions, resulting in a smaller yield strain and larger elastic modulus. An apparent Newtonian plateau at extremely low shear rates in creep experiments is argued to result from slip at the wall. Thus, attractions between these grafted layers in a poor solvent generate a pseudo plastic solid, a metastable state that persists indefinitely and demonstrates little affinity for the walls of the rheometer, hence the slip. In contrast, depletion flocculated dispersions with higher volume fractions but weaker attractions produce very similar rheology but with a true Newtonian viscosity and adhesion to the wall.     

Rheology and microstructures formation of immiscible model polymer blends under steady state and transient flows

Viscoelastic properties of model immiscible blend were studied here under steady state condition at different initial conditions and transient flow conditions. The flow‐induced microstructure has been studied on these model blends. For this system, the elastic properties of the blend are mainly governed by the interface. Measurement of the dynamic modulus and of the first normal stress difference, both reflecting this enhanced elasticity, have been used to prove the blend morphology. The dynamic moduli after cessation of shear flow, the mean diameter of the disperse phase as generated by the shear flow, have been calculated using the model of Palierne. A procedure based on a direct fitting of the dynamic moduli with the model is compared with the one that uses a weight relaxation spectrum. On the other hand, the steady state normal stress data have been related to the morphology of the blend by means of Doi and Ohta model. The specific interfacial area is found to be inversely proportional to the ratio of interfacial tension over shear stress for the blend. The flow behavior during transient shear flow was also discussed. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3519–3533, 2005     

Photophysics of Nile red in solution: steady state spectroscopy

Spectroscopic properties of Nile red (NR) in organic solvents, binary solvent mixtures have been studied. Remarkable shifts in the emission band positions have been observed as a function of the polarity of the medium. In solvent mixtures, these shifts can be explained by the process of specific solvation known as dielectric enrichment. The displacement of the fluorescence band was also measured as a function of temperature to obtain the thermochromic shifts (15 cm(-1) K(-1) in methyltetrahydrofuran and 13.8 cm(-1) K(-1) in butanol). Excited state dipole moments were calculated from these shifts.     

Influence of the temporal response of the detection system upon atomic absorption analytical curves for steady state and transient absorption cells and

The general expression for the atomic absorption signal is modified to allow for a possible time-dependence of the source emission line profile and the     

Open-system nonequilibrium steady state: statistical thermodynamics, fluctuations, and chemical oscillations

Gibbsian equilibrium statistical thermodynamics is the theoretical foundation for isothermal, closed chemical, and biochemical reaction systems. This theory, however, is not applicable to most biochemical reactions in living cells, which exhibit a range of interesting phenomena such as free energy transduction, temporal and spatial complexity, and kinetic proofreading. In this article, a nonequilibrium statistical thermodynamic theory based on stochastic kinetics is introduced, mainly through a series of examples: single-molecule enzyme kinetics, nonlinear chemical oscillation, molecular motor, biochemical switch, and specificity amplification. The case studies illustrate an emerging theory for the isothermal nonequilibrium steady state of open systems.     

Carbonate radical induced polymerisation of pyrrole: A steady state and flash photolysis study

A study was undertaken to determine experimentally the uptake of pollutants into of the different parts of the water hyacinth (Eichhornia crassipes) found in “José Antonio Alzate” dam in the State of México, México. There is evidence for efficient and significant root accumulation of Ti, Mn, Fe, and Ba; but in the upper parts concentrations was consistently determined by the degree of watering. However, a significant input could by derived from a common generic source, such as the atmospheric deposition. The experimental study would, therefore, indicate that water hyacinth species can be highly effective in providing a control and treatment buffer for toxic discharges to the dam.     

Transient and steady state CO oxidation kinetics on nanolithographically prepared supported Pd model catalysts: experiments and simulations

Applying molecular-beam methods to a nanolithographically prepared planar PdSiO2 model catalyst, we have performed a detailed study of the kinetics of CO oxidation. The model catalyst was prepared by electron-beam lithography, allowing individual control of particle size and position. The sample was structurally characterized by atomic force microscopy and scanning electron microscopy before and after reaction. In the kinetic experiments, the O-rich and CO-rich regimes were investigated systematically with respect to their transient and steady-state behaviors, both under bistable and monostable reaction conditions. Separate molecular beams were used in order to supply the reactants, allowing individual control over the reactant fluxes. The desorbing CO2 was detected by both angle-resolved and angle-integrated mass spectrometries. The experimental results were analyzed using different types of microkinetic models, including a detailed reaction-diffusion model, which takes into account the structural parameters of the catalyst as well as scattering of the reactants from the support. The model quantitatively reproduces the results as a function of the reactant fluxes and the surface temperature. Various kinetic effects observed are discussed in detail on the basis of the model. Specifically, it is shown that under conditions of limited oxygen mobility, the switching behavior between the kinetic regimes is largely driven by the surface mobility of CO.     

Temperature quenched DODAB dispersions: fluid and solid state coexistence and complex formation with oppositely charged surfactant

Dilute dispersions of the synthetic bilayer forming double-chained cationic lipid dioctadecyldimethylammonium bromide (DODAB) were investigated. In dispersions sonicated above the chain melting temperature Tm (approximately 45 degrees C) it was found by H NMR that about 50% of the surfactant chains remained fluid when the samples were cooled to room temperature, which is 20 degrees C below Tm. In contrast, there was no sign of a fluid fraction in unsonicated samples at room temperature. The addition of the anionic surfactant sodium dodecyl sulfate (SDS) to DODAB dispersions at room temperature resulted in the formation of an essentially stoichiometric DODA-DS complex with frozen chains, as seen by titration calorimetry and H NMR experiments. For sonicated samples, turbidity experiments demonstrated that, after a fast complexation reaction, the system remains colloidally stable unless the SDS-to-DODAB mixing ratio is too close to unity. H NMR experiments also showed that in the unreacted DODAB the fraction of fluid chains remained close to 50%, indicating either that SDS reacts equally fast with fluid and frozen DODAB or that there is a relaxation of the fluid fraction after the complexation. The melting enthalpy and the melting temperature of the alkyl chains rise gradually as the mixing ratio increases. We observed with cryo-TEM that the fraction of large unilamellar vesicles was significantly larger after addition of SDS. This indicates vesicle fusion. Based on both wide- and small-angle X-ray scattering patterns, the structure of the equimolar SDS-DODAB complex at 25 degress C was proposed to be lamellar.     

On the flux-force partitioning and non-equilibrium thermodynamics near a steady state

The flux-force linear relationship is a basic building block in the development of irreversible thermodynamics near equilibrium. Here, we explore the fate of this relationship near a steady state in a finer detail by partitioning the fluxes and the forces into time-independent and time-dependent components. To this end, we use a master equation approach without assuming the condition of detailed balance. The connection of the flux-force components with various state functions and path functions provides a detailed picture of the variations of such quantities in terms of the deviation of probabilities from the steady state. Pilot calculations on an exactly-solvable case furnish insights into the energy-balance mechanism for non-equilibrium systems, revealing additionally how an out-of-equilibrium scenario can be favorable in realizing a minimized free energy state.     

Dimensional analysis of steady state flux for microfiltration and ultrafiltration membranes

Dimensional analysis of the mass, length and time shows that the steady state flux observed for microfiltration or ultrafiltration through inorganic composite membrane can be expressed using two dimensionless numbers. The shear stress number N_S compares the shear stress against the membrane wall to the driving pressure, while the resistance number N_f compares the convective cross-flow transport to the drived transport through a layer, whose resistance is the sum of all the resistances induced by the different processes which limit the mass transport. Experimental data obtained in ultrafiltration of hydrocarbon emulsions and microfiltration of methanogenic bacteria suspensions and secondary treated wastewater were recalculated in terms of these dimensionless groups. Straight lines were plotted whose slope depends solely on the suspension and the membrane and not on the solute concentration. A negative slope and a positive intersection with the N_S axis means that a cake layer or a polarization layer can be completely eliminated at a critical cross-flow velocity; this was the case for an inorganic particles suspension and for the methanogenic suspension. A straight line of negative slope followed by a plateau means that an irreversible fouling is superimposed to the reversible phenomenon; this was observed for a secondary treated wastewater. A positive slope means that fouling predominates; this was observed with hydrocarbon emulsions.     

Mean configurational distribution function for the steady state and stern-volmer representation

A slight discrepancy appears between the treatment of the steady state of diffusion controlled bimolecular quenching of excited species either by the pair model or by the continuum model. A simplifying assumption is pointed out which helps to solve the steady state case but which is responsible for the discrepancy. The error involved is of small importance considering the experimental accuracy.

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Time-resolved and steady state spectroscopy of polydisperse colloidal silver nanoparticle samples

A signal due to coherently excited vibrational motion has been observed in polydisperse silver nanoparticle samples. The particles were synthesized via a wet chemistry seed mediated method, which yields different particle shapes, including spheres, rods, and irregular triangular-shaped particles. The measured vibrational periods were compared to the results from continuum mechanics calculations. This analysis shows that the observed signal arises from the triangular-shaped particles, rather than the rods or spheres. The period of vibration increases as the dimensions of the triangular-shaped particles increase; specifically, we find that the period is given by 2h/c(l), where h is the bisector of the triangle and c(l) is the longitudinal speed of sound in silver.     

Radiolytic degradation of 4-nitrophenol in aqueous solutions: Pulse and steady state radiolysis study

The radiation induced degradation of 4-nitrophenol (4-NP) has been studied by gamma irradiation, while the reactivity and spectral features of the short lived transients formed by reaction with primary transient radicals at different pHs has been investigated by pulse radiolysis technique. In steady state radiolysis a dose of 4.4 k Gy is able to degrade 98% of 1×10⁻⁴ mol dm⁻³ 4-NP. 4-NP has pK_a at 7.1, above which it is present in the anionic form. At pH 5.2, ^•OH and N₃^• radicals were found to react with 4-NP with rate constants of 4.1×10⁹ dm³ mol⁻¹ s⁻¹ and 2.8×10⁸ dm³ mol⁻¹ s⁻¹, respectively. Differences in the absorption spectra of species formed in the reactions of 4-NP with ^•OH and N₃^• radicals suggested that ^•OH radicals add to the aromatic ring of 4-NP along with electron transfer reaction, whereas N₃^• radicals undergo only electron transfer reaction. At pH 9.2, rate constants for the reaction of ^•OH radicals with 4-NP was found to be higher by a factor of 2 compared to that at pH 5.2. This has been assigned to the deprotonation of 4-NP at pH 9.2.     

Wavelet method for steady state immobilized enzyme kinetic model: an operational matrix approach

Journal of Mathematical Chemistry - This paper discusses a mathematical model of the diffusion and reaction in amperometric biosensor response with immobilized enzyme electrodes within a uniform...     

Interaction between certain porphyrins and CdS colloids: a steady state and time resolved fluorescence quenching study

The interaction between porphyrins namely, meso-tetrakis (4-methoxyphenyl)porphyrin (TMeOPP), protoporphyrin IX (PPIX) and Zinc(II) meso-tetraphenylporphyrin (ZnTPP) with colloidal CdS has been studied by using steady state and time resolved fluorescence quenching measurements. The porphyrins adsorbed on the surface of colloidal CdS due to electrostatic interaction. This adsorption leads to changes in the absorption spectra related to the complex formation. The apparent association constant (Kapp) was in the order of 4.34-5.58 x 10(5) M(-1) from the effect of colloidal CdS on the absorption spectra and 0.64-1.6 x 10(5) M(-1) from fluorescence quenching data. Quenching is attributable mainly to static mechanism through ground state complex formation as confirmed by lifetime measurements.     

Small-angle neutron-scattering studies on nonaqueous dispersions Part 5: Magnesium carbonate dispersions in hydrocarbon media

A method is described for the preparation of magnesium carbonate particles in a nonaqueous hydrocarbon environment. The particles were stabilised in the colloidal sense by the adsorption of an alkyl-aryl sulphonic acid. The particle morphology and particle-size distribution were examined by small-angle neutron scattering. It was concluded that the particle structure was of the coreshell type, with a core particle of basic magnesium carbonate, radius ca. 40 Å, and a shell of thickness ca. 13 Å. The results suggested that the latter was predominantly that of the alkyl-aryl part of the stabilising moiety.