Conditions Of Limited Diffusion Research Articles

Effect of diffusion kinetics on the ice nucleation temperature distribution

The nucleation behavior of water is crucial in many fields, spanning meteorology, glaciology, biology, and astrophysics. We report observations suggesting an effect of diffusion kinetics in water on the heterogeneous immersion/contact mode nucleation temperature distribution of ice. We performed differential scanning calorimetry analyses of repeated freeze/thaw cycles and investigated the effect of several variables on the regularity of the nucleation temperature distributions obtained. We observed that the thawing temperature and residence time above 0 °C affect the width of the measured distributions. We explain the observed phenomena according to the diffusion behavior of an external nucleator. Specifically, conditions of enhanced diffusion of the nucleator translated into broader, more scattered distributions, while conditions of limited diffusion translated into narrower, more regular distributions. Lastly, based on our experimental findings, we propose a theoretical explanation centered on the temperature dependence of diffusion kinetics in water.

Rotating disk electrode analysis of oxygen reduction at platinum particles under limiting diffusion conditions

An analysis is carried out of oxygen reduction under limiting diffusion conditions on a rotating disk electrode partially covered with platinum particles (‘particulate electrode’). First a model is developed for the current response at a rotating particulate electrode, because Levich equation, used for the classical continuous disk electrodes, is not applicable in this case. The model allows for the calculation of the limiting diffusion current by an iterative algorithm, as a function of the density and size of the particles, and the constants for the adsorption/desorption and diffusion of reactants over non-covered areas of the substrate. The model is valid when there is no overlapping of surface diffusion areas around the particles. In a second part, the oxygen reduction on platinum particles electrodeposited on a glassy carbon disk is studied. Platinum particulate electrodes with a variable density and size of particles are prepared by single pulse electrodeposition technique. Limiting diffusion currents for oxygen reduction are analysed on the light of the proposed model. Values for the oxygen surface diffusivity and the equilibrium adsorption/desorption constant on the glassy carbon substrate are obtained from the analysis.

Beneficial Effects of Population Bottlenecks in an RNA Virus Evolving at Increased Error Rate

RNA viruses replicate their genomes with a very high error rate and constitute highly heterogeneous mutant distributions similar to the molecular quasispecies introduced to explain the evolution of prebiotic replicators. The genetic information included in a quasispecies can only be faithfully transmitted below a critical error rate. When the error threshold is crossed, the population structure disorganizes, and it is substituted by a randomly distributed mutant spectrum. For viral quasispecies, the increase in error rate is associated with a decrease in specific infectivity that can lead to the extinction of the population. In contrast, a strong resistance to extinction has been observed in populations subjected to bottleneck events despite the increased accumulation of mutations. In the present study, we show that the mutagenic nucleoside analogue 5-azacytidine (AZC) is a potent mutagen for bacteriophage Qβ. We have evaluated the effect of the increase in the replication error rate in populations of the bacteriophage Qβ evolving either in liquid medium or during development of clonal populations in semisolid agar. Populations evolving in liquid medium in the presence of AZC were extinguished, while during plaque development in the presence of AZC, the virus experienced a significant increase in the replicative ability. Individual viruses isolated from preextinction populations could withstand high error rates during a number of plaque-to-plaque transfers. The response to mutagenesis is interpreted in the light of features of plaque development versus infections by free-moving virus particles and the distance to a mutation–selection equilibrium. The results suggest that clonal bacteriophage populations away from equilibrium derive replicative benefits from increased mutation rates. This is relevant to the application of lethal mutagenesis in vivo, in the case of viruses that encounter changing environments and are transmitted from cell to cell under conditions of limited diffusion that mimic the events taking place during plaque development.

Square-wave thin-film voltammetry: influence of uncompensated resistance and charge transfer kinetics

An electrode reaction occurring in a thin-film having a low electrical conductivity was considered under conditions of square-wave voltammetry (SWV). The ohmic polarization of the system owing to the low conductivity of the film is represented by the complex resistance parameter defined as ρ=R Ω (n 2F 2/RT)Sc Ox * Df , where R Ω is the resistance of the film, S is the electrode surface area, f is the SW frequency and the other symbols have their usual meanings. The influence of the thickness of the film is given by the thickness parameter Λ=L f/D , where L is the thickness of the layer, and D is the diffusion coefficient. For a thin film ( Λ⩽0.949) the dimensionless net-peak current of a reversible electrode reaction depends parabolically on the resistance parameter, exhibiting a well-developed maximum. A detailed study of this phenomenon revealed that it resembles the charge transfer kinetics effect as well as the property known as a “quasireversible maximum” that is typical for an adsorption complicated electrode reaction. The theoretically predicted properties of the SW response are illustrated by experiments with iodine, decamethylferrocene, and azobenzene at the three-phase electrode system. This methodology [Electrochem. Commun. 2 (2000) 112] consists of a single droplet of an organic water immiscible solvent containing an electroactive probe that is attached to the surface of a graphite electrode and immersed in an aqueous electrolyte solution. A good agreement between theory and experiment indicates that the SW voltammetric response of the three-phase electrode system exhibits properties of an electrode reaction occurring under limiting diffusion conditions.

Root Development and Absorption of Ammonium and Nitrate from the Rhizosphere

Plant roots operate in an environment that is extremely heterogeneous, both spatially and temporally. Nonetheless, under conditions of limited diffusion and against intense competition from soil microorganisms, plant roots locate and acquire vital nitrogen resources. Several factors influence the mechanisms by which roots respond to ammonium and nitrate. Nitrogen that is required for cell division and expansion derives primarily from the apex itself absorbing rhizosphere ammonium and nitrate. Root density and extension are greater in nutrient solutions containing ammonium than in those containing nitrate as the sole nitrogen source. Root nitrogen acquisition alters rhizosphere pH and redox potential, which in turn regulate root cell proliferation and mechanical properties. The net result is that roots proliferate in soil zones rich in nitrogen. Moreover, plants develop thinner and longer roots when ammonium is the primary nitrogen source, an appropriate strategy for a relatively immobile nitrogen form.

Use of binary metal deposits on a cylindrical carbon-fiber microelectrode in the voltammetric determination of metal ions

The effect of binary metal deposits on a cylindrical carbon-fiber microelectrode on the determination of metals by direct and stripping voltammetry was studied. The electrolytic deposition of a binary system of copper and thallium, cadmium, lead, or mercury on the electrode in an alkaline solution resulted in the disappearance of the electroreduction peak of dissolved oxygen in the potential range from -0.8 to -1.4 V and in a decrease in the background current. Under the conditions of limited diffusion, the peak currents of Ni(II), Co(II), and Zn(II) in differential pulse voltammograms were 3–7 times higher than those calculated for a reversible electrode process under the conditions of semi-infinite diffusion. Because of this, the determination limit for metal ions in direct voltammetry was lowered to 1 X 10-6 M. With a binary copper-thallium system, the peak current of zinc(II) reduction can be be detected in the presence of 5000-fold molar amounts of copper(II). The deposition of binary copper-lead and copper-thallium systems under the conditions of limited diffusion reduced the effect of negative interaction between the components of these systems and made possible the determination of lead(II) and thallium(I) by stripping voltammetry using additional peaks.

Dynamics of Electrodiffusion Friction Probes. I. Shape-Dependent Potentiostatic Transient

An exact theory is given of the voltage-step transient under limiting diffusion conditions for an electrodiffusion friction probe of arbitrary convex shape. The actual transient courses are given for the strip, circular, elliptic, triangular, and rectangular probes of any orientation with respect to the flow direction. A simple formula for any probe with a single working electrode of convex shape is suggested to facilitate the calibration of electrodiffusion probes based on the voltage-step transient.

Photophysical and photochemical properties of pyrene-doped TiO 2 particle suspensions in water

Both crystalline and amorphous TiO2 impregnated with pyrene have been prepared and the photophysical behavior of the arene in the porous particles is reported. The pyrene can be leached out of the TiO2 particles with hexane. The exact behavior of the pyrene in TiO2 depends on the physical state of the TiO2. In crystalline TiO2 pyrene occupies two polar sites in the particle, one of which is accessible to water and ionic solutes, while the other is isolated from such molecules. The two sites exhibit different photophysical behavior. Pyrene in amorphous TiO2 apparently occupies only one site, which gives ready access to water and ionic quenchers. Photo products normally associated with excited pyrene are not observed in TiO2 particles, a fact that is probably due to the close proximity of the products which are formed under conditions of limited diffusion, thus enhancing back reaction.

The electrocatalytical influence of thallium and lead underpotential adsorbates at silver single-crystal surfaces on the reduction processes of quinone, persulphate and protons

The influence of underpotential Tl- and Bp-adsorbates at (111), (100), and (110) silver single-crystal faces on the electrochemical reduction of quinone (Q), persulphate (S 2O 8 2− and protons (H + was studied in acid perchlorate and sulphate solutions. The reduction of Q and S 2O 8 2− under limiting diffusion conditions was not affected by the metal adsorbates. A specific adsorption of reactants or reaction intermediates involved in the reduction of Q or S 2O 8 2− can be excluded. However, a strong inhibition effect depending on the degree of coverage and the structure of Tl- and Pb-adsorbates was observed in the case of H + results are interpreted in terms of the difference in hydrogen binding energies and of surface reconstruction phenomena.

Influence of lead and thallium underpotential adsorbates at silver single crystal surfaces on different redox reactions

The influence of underpotential Pb- and Tl-adsorbates at rotating disc silver single crystal surfaces (111), (100), and (110) on the kinetics of ‘outer-sphere’ and ‘inner-sphere’ redox reactions was studied in 0.5 M NaClO4 and 0.5 M Na2SO4 solutions (2 < pH < 4) containing Fe3+, Ce4+ or NO 3 − ions. While the reduction of Fe3+ and Ce4+ under limiting diffusion conditions was not affected by the metal adsorbates, a strong inhibition effect correlated to the degree of Pb- or Tl-adsorbate coverages was observed in the case of the reduction process in the presence of NO 3 − ions. The results are interpreted in terms of a strong chemical interaction between the reactant or a reaction intermediate and the silver substrate, taking into account the previously proposed superlattice structures of the metal adsorbates, depending on the orientation of the silver single crystal surfaces and on the underpotential range.

The oxidation of ethylene over evaporated palladium-rhodium alloy films: I. Film structure with special reference to phase-separation

Recent thermodynamic data for Pd-Rh alloys confirm earlier experimental observations that annealing for very prolonged periods below 850 °C causes separation into two FCC phases over a wide composition range. Palladium-rhodium alloy films, prepared by slow simultaneous evaporation of the component metals on to a glass substrate at 400 °C, show good bulk homogeneity (expected lattice constants, symmetrical X-ray diffraction profiles) at the ends of the composition range (0–30 and 80–100% Rh) but tend to phase-separation at intermediate compositions. After use as catalysts for ethylene oxidation, Pd-Rh alloy films with 30–80% Rh show clear evidence of two “phases” contributing to the diffraction profile; Phase I varied in composition and was deficient in Rh compared with the corresponding solid solution and Phase II had a composition of 88 ± 5% Rh in accord with the solubility diagram. Palladium-rich films showed strong (111) preferred orientation, weakening with increasing Rh content so that the crystallites in almost pure Rh films had approximately random orientation. The formation of Pd-Rh alloy films is considered with regard to the interpretation of the catalytic results according to composition: 0–30% Rh; diffusion during film formation and use is limited and inhibits the phase-separation indicated, giving a homogeneous alloy. Evidence was found for hydrogen solubility in alloy films with <10% Rh after carrying out ethylene oxidation with partial consumption of the reactants, indicating a subdivision of this composition range. (Films with up to ~30% Rh could be charged with hydrogen if all the oxygen was consumed, leaving unreacted ethylene.) 40–80% Rh; a mechanism is proposed involving the preferential nucleation of Rh whereby Phase II can form in conditions of limited diffusion as a Rh-rich kernel to the crystallite, surrounded by a Rh-deficient solid solution of variable composition. 80–100% Rh; the process continues but the “phases” are sufficiently close in composition to remain undetected.