Correlations between Growth Kinetics and Microstructure for Scales Formed by High-Temperature Oxidation of Pure Nickel. II. Growth Kinetics

The oxidation kinetics of high-purity nickel were studied between 500 and 1200°C, in pure oxygen at atmospheric pressure, for aûerage oxide-scale thicknesses of 1, 5, 10, and 30 μm. In the oûerall temperature range studied, a decrease in the parabolic rate constant kp with increasing scale thickness was observed. Depending on temperature and oxide-scale thickness, growth kinetics can be interpreted as a mixture of parabolic- and cubic-growth kinetics. Possible correlations between growth kinetics and microstructures of the oxide scales were inûestigated. From this set of experimental data, oxidation-kinetics models were tested. In particular, the effect of grain-boundary diffusion on NiO-growth kinetics was discussed. The correlations between growth kinetics and oxide microstructures appear to be more complex than usually reported

Dilation kinetics of glassy, aromatic polyimides induced by carbon dioxide sorption

Over the past years, the equilibrium sorption of gases in polymers has been intensively studied. Mostly, glassy polymers were investigated because of their excellent selective mass transport properties. This work does not focus on the equilibrium sorption but on the kinetics to reach the equilibrium. We developed a new experimental method measuring the sorption-induced dilation kinetics of a polymer film. Carbon dioxide and glassy, aromatic polyimides were chosen as model systems. Low-pressure experiments demonstrate that the measured dilation kinetics represent the sorption kinetics. A significant delay between the sorption and dilation kinetics is based on the fact that dilation kinetics occurs simultaneously with the concentration increase in the center of the polymer film. High-pressure experiments reveal significant differences in dilation kinetics compared to low-pressure experiments. Generally, three regimes can be distinguished in the dilation kinetics: a first, fast volume increase followed by two much slower regimes of volume increase. The magnitude of fast and slow dilation kinetics strongly depends on the swelling history of the polymer sample. The results of the experiments are analyzed in the light of a model relating the fast dilation kinetics to a reversible Fickian dilation and the slower dilation kinetics to an irreversible, relaxational dilation

Fundamental electrode kinetics

Report presents the fundamentals of electrode kinetics and the methods used in evaluating the characteristic parameters of rapid-charge transfer processes at electrode-electrolyte interfaces. The concept of electrode kinetics is outlined, followed by the principles underlying the experimental techniques for the investigation of electrode kinetics

A Method to Study Complex Enzyme Kinetics Involving Numerical Analysis of Enzymatic Schemes. The Mannitol Permease of Escherichia coli as an Example

An analysis of complex kinetic mechanisms is proposed that consists of two steps, (i) building of an kinetic scheme from experimental data other than steady-state kinetics and (ii) numerical simulation and analysis of the kinetics of the proposed scheme in relation to the experimental kinetics. Procedures are introduced to deal with large numbers of enzymatic states and rate constants, and numerical tools are defined to support the analysis of the scheme. The approach is explored by taking the mannitol permease of Escherichia coli as an example. This enzyme catalyzes both the transport of mannitol across the cytoplasmic membrane and the phosphorylation of mannitol. The challenge is to deduce the transport properties of this dimeric enzyme from the phosphorylation kinetics. It is concluded that (i) the steady-state kinetic behavior is largely consistent with the proposed catalytic cycle of the monomeric subunit, (ii) the kinetics provide no direct support but also do not disprove a coupled translocation of the binding sites on the two monomeric subunits. The approach reveals the need for further experimentation where the implementation of experimental results in the scheme conflict with the experimental kinetics and where specific experimental characteristics do not show up in the simulations of the proposed kinetic scheme.

Interfacial Reaction Kinetics

We study irreversible A-B reaction kinetics at a fixed interface separating two immiscible bulk phases, A and B. We consider general dynamical exponent $z$, where $x_t\sim t^{1/z}$ is the rms diffusion distance after time $t$. At short times the number of reactions per unit area, $R_t$, is {\em 2nd order} in the far-field reactant densities $n_A^{\infty},n_B^{\infty}$. For spatial dimensions $d$ above a critical value $d_c=z-1$, simple mean field (MF) kinetics pertain, $R_t\sim Q_b t n_A^{\infty} n_B^{\infty}$ where $Q_b$ is the local reactivity. For low dimensions $d<d_c$, this MF regime is followed by 2nd order diffusion controlled (DC) kinetics, $R_t \approx x_t^{d+1} n_A^{\infty} n_B^{\infty}$, provided $Q_b > Q_b^* \sim (n_B^{\infty})^{[z-(d+1)]/d}$. Logarithmic corrections arise in marginal cases. At long times, a cross-over to {\em 1st order} DC kinetics occurs: $R_t \approx x_t n_A^{\infty}$. A density depletion hole grows on the more dilute A side. In the symmetric case ($n_A^{\infty}=n_B^{\infty}$), when $d<d_c$ the long time decay of the interfacial reactant density, $n_A^s$, is determined by fluctuations in the initial reactant distribution, giving $n_A^s \sim t^{-d/(2z)}$. Correspondingly, A-rich and B-rich regions develop at the interface analogously to the segregation effects established by other authors for the bulk reaction $A+B\to\emptyset$. For $d>d_c$ fluctuations are unimportant: local mean field theory applies at the interface (joint density distribution approximating the product of A and B densities) and $n_A^s \sim t^{(1-z)/(2z)}$. We apply our results to simple molecules (Fickian diffusion, $z=2$) and to several models of short-time polymer diffusion ($z>2$).Comment: 39 pages, 7 figures, uses fund2.sty, submitted to Eur. Phys. J. B, 1 figure added, for short version see cond-mat/980409

Collective behaviours: from biochemical kinetics to electronic circuits

In this work we aim to highlight a close analogy between cooperative behaviors in chemical kinetics and cybernetics; this is realized by using a common language for their description, that is mean-field statistical mechanics. First, we perform a one-to-one mapping between paradigmatic behaviors in chemical kinetics (i.e., non-cooperative, cooperative, ultra-sensitive, anti-cooperative) and in mean-field statistical mechanics (i.e., paramagnetic, high and low temperature ferromagnetic, anti-ferromagnetic). Interestingly, the statistical mechanics approach allows a unified, broad theory for all scenarios and, in particular, Michaelis-Menten, Hill and Adair equations are consistently recovered. This framework is then tested against experimental biological data with an overall excellent agreement. One step forward, we consistently read the whole mapping from a cybernetic perspective, highlighting deep structural analogies between the above-mentioned kinetics and fundamental bricks in electronics (i.e. operational amplifiers, flashes, flip-flops), so to build a clear bridge linking biochemical kinetics and cybernetics.Comment: 15 pages, 6 figures; to appear on Scientific Reports: Nature Publishing Grou