Dependent Rate Coefficients Research Articles

Apparent amylase diffusion rates in milled cereal grains determined in vitro: potential relevance to digestion in the small intestine of pigs

Starch digestion in grain particles depends on the diffusion of amylase into the particles. Enzyme diffusion rate (EDR) is influenced by grain structure, and may be key in determining the extent of grain digestion in the small intestine. EDR values of sixteen grains (wheat, barley and sorghum) were compared with their ileal digestibility from pigs. Five fractions of each hammer-milled grain were digested in vitro under conditions mimicking monogastric digestion to obtain apparent amylase diffusion coefficients (ADC) from the inverse square dependence of rate-coefficients on particle sizes. The ADC values in sorghum were lower than in wheat (P < 0.05) and barley (P < 0.001) for samples grown under standard conditions. However, grain differences such as sprouting or immaturity, resulted in higher ADC values that were relatable to in vivo digestibility. Grain fibre had large effects on ileal digestibility, with an apparent optimum neutral detergent fibre level of 10–20% that resulted in maximum starch ileal digestibility. It is inferred that the true potential for efficient feed utilisation depends both on fast grain digestion, and on an appropriate rate of passage, so that small intestine residence time is long enough for starch in grains to be digested fully.

Investigation into the Gaussian time-dependence of the rate coefficient in dispersive kinetic models applied to simple gas-phase chemical reactions

Dispersive kinetic models (DKMs) based on the concept of a distribution of activation energies have shown much promise in describing complex reaction transients. In particular, DKMs exhibiting a time (t)-dependent rate coefficient, k, of approximately Gaussian functional form provide the most insight into the kinetics because they do not rely on unit-less, empirical parameters [P.J. Skrdla, J. Phys. Chem. A 113, 9329 (2009)]. The physical basis of the Gaussian behaviour is investigated using a dispersive simple harmonic oscillator (DSHO) description of Marcus-type reactions studied on the ultrafast timescale. The DSHO description is found to support the Gaussian t-dependence of k over relatively short time periods. Moreover, in cases where the transition state survives more than one vibration of the predominant reactive mode, a periodicity in k is observed that is also consistent with empirical observations.

The zeolite–lime pozzolanic reaction: Reaction kinetics and products by in situ synchrotron X-ray powder diffraction

The early pozzolanic reaction in pastes of lime and natural zeolites was studied with time-resolved in situ synchrotron X-ray powder diffraction. For chabazite and Na-, K-, and Ca-exchanged clinoptilolite tuffs, which were mixed with lime and water, the evolution of the quantitative contents of crystalline phases was followed during the early pozzolanic reaction in the first hours and days. The dependence of the rate coefficient on temperature was examined by isothermal curing at 313, 323, and 333 K of lime–chabazite tuff pastes. Kinetic analysis indicates that the reaction rapidly displays a deceleratory character. This is explained by the modified Jander equation as a result of the reaction rate control by diffusion processes through a condensing reaction product boundary layer. The exchangeable cation content of clinoptilolite influences the duration of the initial induction period, the diffusion characteristics of the boundary layer of reaction products, and the structural evolution of the C–S–H reaction product. Alkali-exchanged clinoptilolites show a higher pozzolanic reactivity, a more permeable reaction product layer, and longer silicate chains in the C–S–H phase compared to their Ca-exchanged counterpart. The temperature dependence of the reaction rate coefficients in the lime–chabazite tuff pastes obeys the Arrhenius equation.

Photoinduced Oxidation of Antimony(III) in the Presence of Humic Acid

Interactions of antimony with natural organic matter (NOM) are important for the fate of Sb in aquatic systems. The kinetics of the photosensitized oxidation of Sb(III) to Sb(V) in the presence of Suwannee River Humic Acid (SRHA) was investigated using UV-A and visible light (medium-pressure mercury lamp). At a concentration of 5 mg L(-1) dissolved organic carbon (DOC) the light-induced reaction was 9000 times faster (rate coefficient k(exp) = 7.0 +/- 0.05 x 10(-4) s(-1)) than the dark reaction and followed pseudo-first-order kinetics. Rates increased linearly with the concentration of DOC. Between pH 4 and 8 rates increased by a factor of 5. Further results and kinetic considerations indicate that singlet oxygen, hydroxyl radicals, hydrogen peroxide, and hydroperoxyl radicals/superoxide are not important photooxidants in this system, while other NOM-derived reactive species, in particular excited triplet states and/or phenoxyl radicals, seem to be relevant. The dependence of rate coefficients on Sb(III)/DOC ratio was consistent with a two binding site model including (i) a strong binding site at low concentration inducing fast oxidation, (ii) a weak binding site at high concentration inducing slower oxidation, and (iii) the even slower oxidation of Sb(OH)3. Photoirradiation of natural water samples spiked with Sb(III) showed that the oxidation rates could be well predicted based on DOC.

Kinetics of microwave‐assisted polymerization of ϵ‐caprolactone

AbstractThe kinetics of polymerization of ϵ‐caprolactone (CL) in bulk was studied by irradiating with microwave of 350 W and frequency of 2.45 GHz with different cycle‐heating periods (30–50 s). The molecular weight distributions were determined as a function of reaction time by gel permeation chromatography. Because the temperature of the system continuously varied with reaction time, a model based on continuous distribution kinetics with time/temperature‐dependent rate coefficients was proposed. To quantify the effect of microwave on polymerization, experiments were conducted under thermal heating. The polymerization was also investigated with thermal and microwave heating in the presence of zinc catalyst. The activation energies determined from temperature‐dependent rate coefficients for pure thermal heating, thermally aided catalytic polymerization, and microwave‐aided catalytic polymerization were 24.3, 13.4, and 5.7 kcal/mol, respectively. This indicates that microwaves increase the polymerization rate by lowering the activation energy. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1450–1456, 2004

Polymer-Fraction Dependence of Entry Rate Coefficients in Emulsion Polymerization

Data on the rate of approach to steady state in a series of studies of the seeded emulsion polymerization of styrene yield the dependence of the rate coefficient for entry of free radicals into latex particles as a function of the weight-fraction polymer and hence of monomer concentration in the aqueous phase. The results are in accord with a model for the entry process (i.e., for the initiator efficiency) based on aqueous-phase propagation and termination being the rate-controlling events.

Energy dependence of rate coefficients in nucleation theory

A general expression for the decay rate of a cluster of energy E is derived using equilibrium statistical mechanics. This decay rate is independent of the vapour temperature external to the cluster. Numerical results are presented for a model in which cluster molecules interact via a square well potential. The use of a Gaussian approximation for the equilibrium cluster energy distribution leads to an inaccurate form for the decay rate.

Kinetics and mechanisms of oxidations with nitrous acid. Part 1. Substituted benzyl alcohols and benzyl methyl ether in aqueous sulphuric acid

The kinetics and products of oxidation of several substituted benzyl alcohols and of benzyl methyl ether, by nitrous acid in aqueous sulphuric acid (30–50%) at 25 °C, have been investigated. Conversion into substituted benzaldehyde, when nitrous acid is taken in excess, is quantitative. Stoicheiometry, kinetic form, kinetic and product hydrogen isotope effects, and the dependence of rate coefficients on acidity, support a mechanism in which there is rate-determining abstraction by the nitrosonium ion of a hydride ion, either in one step or by electron transfer followed by hydrogen-atom abstraction, from the α-carbon atom of the alcohol or ether. The fate of the HNO so formed depends upon the presence or absence of oxygen.

The dependence of rate coefficients and product yields upon fluence, intensity, and time in unimolecular reactions induced by monochromatic infrared radiation

The influence of the three parameters (with two degrees of freedom) fluence, intensity, and time on rate coefficients and product yields in collisionless Unimolecular Reactions Induced by Monochromatic Infrared Radiation (URIMIR) is discussed in some detail in terms of the recently proposed logarithmic reactant fluence plots. Model calculations for several archetypes of such plots are presented, based on solutions of the Pauli master equation and solutions of the quantum mechanical equations of motion for spectra involving many states at each level of excitation. Linear diagrams, turnups, and turnovers are found and are discussed systematically. Experimental examples re-evaluated from the literature and new measurements on the laser induced decomposition of CF2HCl are reported which nicely illustrate the various theoretical possibilities. Steady state rate coefficients for six molecules are evaluated and summarized. In some situations the intrinsic nonlinear intensity dependence of the steady state rate coefficients and deviations from simple fluence dependence of the product yields both before and at steady state are shown to be important theoretically and experimentally. The role of the reducibility of the rate coefficient matrix is discussed in connection with turnovers and with the strong influence of initial temperature that is found in the laser induced decomposition of CF2HCl.

Kinetics and mechanism of cyclisation of N-(methylaminocarbonyl)glycine to 3-methylimidazolidine-2,4-dione

The dependence of rate coefficients for cyclisation upon acidity, solvent isotope effects, and the kinetics of oxygen exchange of the product, support the view that in the acid catalysed cyclisation there is rate-determining ring closure of the neutral substrate synchronous with protonation of the carboxy oxygen. This is contrary to what current views of the mechanism of acid-catalysed amide hydrolysis would lead one to expect; reasons for this are discussed. The kinetics of the uncatalysed cyclisation and of the reverse ring opening reaction in alkali are reported, and a mechanistic scheme for the whole pH range is presented. The equilibrium constant of the title reaction in water at 25 °C is shown to lie between 103 and 106; the slowness of the reaction at central values of pH prevents a closer estimate.

Hydrolysis of N-p-nitrobenzylideneaniline: a quantitative study of specific and general catalysis

The paH*–rate profile for the hydrolysis of N-p-nitrobenzylideneaniline was investigated in the paH* range 2·72–10·94, in buffered aqueous methanol solutions (20·0% w/w H2O) at 25° and ionic strength 0·045M. The dependence of rate coefficients on buffer concentration at constant paH*, constant ratio of buffer components at each paH*, and constant ionic strength was also studied in the paH* range 3·02–8·25. The observed rate coefficients show the influence of catalysis, obeying the expression Kobs=Kext+Kcat[HA], in which the straight line parameters show a complex dependence on aH*. Starting from the generally accepted mechanism for these reactions, a steady-state rate equation for specific and general catalysis was developed. An analysis of this rate equation has given not only the entire set of parameters but also the rate-determining steps and the values of some individual rate coefficients. The risk of making erroneous conclusions in the absence of a quantitative study of general catalysis is underlined.