Region Of Constant Rate Research Articles

Improving Accuracy in Arrhenius Models of Cell Death: Adding a Temperature-Dependent Time Delay.

The Arrhenius formulation for single-step irreversible unimolecular reactions has been used for many decades to describe the thermal damage and cell death processes. Arrhenius predictions are acceptably accurate for structural proteins, for some cell death assays, and for cell death at higher temperatures in most cell lines, above about 55 °C. However, in many cases--and particularly at hyperthermic temperatures, between about 43 and 55 °C--the particular intrinsic cell death or damage process under study exhibits a significant "shoulder" region that constant-rate Arrhenius models are unable to represent with acceptable accuracy. The primary limitation is that Arrhenius calculations always overestimate the cell death fraction, which leads to severely overoptimistic predictions of heating effectiveness in tumor treatment. Several more sophisticated mathematical model approaches have been suggested and show much-improved performance. But simpler models that have adequate accuracy would provide useful and practical alternatives to intricate biochemical analyses. Typical transient intrinsic cell death processes at hyperthermic temperatures consist of a slowly developing shoulder region followed by an essentially constant-rate region. The shoulder regions have been demonstrated to arise chiefly from complex functional protein signaling cascades that generate delays in the onset of the constant-rate region, but may involve heat shock protein activity as well. This paper shows that acceptably accurate and much-improved predictions in the simpler Arrhenius models can be obtained by adding a temperature-dependent time delay. Kinetic coefficients and the appropriate time delay are obtained from the constant-rate regions of the measured survival curves. The resulting predictions are seen to provide acceptably accurate results while not overestimating cell death. The method can be relatively easily incorporated into numerical models. Additionally, evidence is presented to support the application of compensation law behavior to the cell death processes--that is, the strong correlation between the kinetic coefficients, ln{A} and E(a), is confirmed.

Drying of Films Formed by Ordered Poly(ethylene oxide)−Poly(propylene oxide) Block Copolymer Gels

The drying of hydrogel films formed by poly(ethylene oxide)-poly(propylene oxide) (PEO-PPO) block copolymers (Pluronic P105 and Pluronic L64) is investigated at various air relative humidity (RH) conditions in the range 11-94%. These amphiphilic block copolymers self-assemble to form a variety of ordered (lyotropic liquid crystalline) structures as the water content decreases. The amount of water lost increases linearly with the drying time initially (constant rate region, stage I). After this linear region, a falling rate is observed (stage II). The drying rate increases with decreasing RH, thus greatly shortening the drying time. A decrease of the initial film thickness or a decrease in the initial water content shortens the drying time; however, the drying mechanism remains the same. Analysis of the experimental data shows that the hydration level in the Pluronic hydrogel mainly determines the drying rate, rather than the type of ordered structure formed. Two distinct regions (liquid/gel and solid/crystalline) are observed in the drying isotherm for PEO-PPO block copolymers and homopolymer poly(ethylene glycol)s. A model for one-dimensional water diffusion is used to fit the experimental drying results at different RH, initial film thickness, and initial water content conditions. The model accounts for the shrinkage of the film during drying and for a water diffusion coefficient that is a function of the water concentration in the film. For the experimental conditions considered here, the Biot number (Bi) is less than unity and the drying is mainly limited by evaporation at the film surface. The diffusion model is used to obtain information for cases where Bi > 1.

Effect of Oxygen Pressure on Asphalt Oxidation Kinetics

The oxidation of asphalt is a major cause of pavement failure. At a given temperature and pressure, the asphalt oxidizes in two stages: (1) a rapid-rate period followed by (2) a long period with constant oxidation rate. The degree of oxidation that occurs in the constant-oxidation region is asphalt-dependent and varies with oxygen pressure and with temperature. Using pavement-temperature oxidation kinetics obtained for eight asphalts in this study, it has been determined that the activation energies for the constant-rate region are dependent on the oxygen pressure and can be related to the asphaltene composition of the asphalt. An oxidation kinetic model is developed to predict the rate of oxidation in the constant-rate region knowing an initial asphaltene composition variable for the asphalt.

Constant-Rate Drug Release from Novel Anionic Gel Beads with Transient Composite Structure

A new anionic composite bead system with a transient membrane–matrix structure, capable of prolonged constant-rate drug release, has been developed from suspension-polymerized poly(methyl methacrylate-co-methacrylic acid) (PMMA/MAA). These composite beads have a thin PMMA/MAA surface layer and a core consisting of the sodium salt form of the polymer (PMMA/MANa). The high loading (>20%) of a model drug (oxprenolol HCl) that is achievable in this system from a loading solution concentration as low as 0.5% suggests the formation of a drug–polymer complex in the form of an ionic salt in the core. The release of oxprenolol from such composite beads shows an initial burst effect followed by an extended constant-rate region before leveling off. Apparently, the surface PMMA/MAA layer functions as a transient rate-controlling membrane before it is completely ionized. Because the ionization process is slow, the rate-controlling characteristics of the surface layer and the resulting constant rate of drug release are both sustained for an extended period. The unique feature of the present system is not only its high drug loading capability, but also the transient nature of the rate-controlling surface layer, which is completely ionized towards the latter part of the drug release, thus avoiding prolonged tailing of drug release that is normally associated with permanent membrane–matrix systems.

Kinetic behaviour of permeability of polyethylene membrane to chloroform vapour

AbstractThe permeability behaviour of thin polyethylene membrane to chloroform vapour has been studied from the kinetic point of view between 15 and 35 ± 0.5°C. It was found that in the constant rate region, the permeability obeys second‐order reaction kinetics. The activation energy and entropy of activation of the permeability process were found to be 13.81 kcal/deg/mol and 33.74 eu, respectively. The physical nature of the membrane was found to affect the permeability process.

水溶性たんぱく質の電解処理による不溶化

An aqueous solution containing water-soluble protein of Alaska pollack was electrolytically treated in both separated and non-separated type vessels, and the rate of insolubilization of water-soluble protein was measured. In the separated type vessel, the rate of insolubilization showed a characteristic pattern, which had a time lag followed by a region of constant rate. This constant rate of insolubilization was found to be roughly proportional to current but independent of potential. The rate of insolubilization in the non-separated type vessel was much smaller than that in the separated type vessel. The insolubilization of water-soluble protein caused by electrolytic treatment seemed to be due mainly to the isoelectric aggregation in the positive side vessel.

Reactivity of heat-treated coals in hydrogen

Reactivities of eighteen 40 × 100 mesh U.S. coals charred to 1000 °C have been measured in H 2 at 2.7 MPa and 980 °C. The char-hydrogen reaction usually occurs in two stages: a slow induction period followed by a constant-rate region. Reactivities of various chars in the initial stage ( R i ) decrease, in general, with increasing carbon content of the parent coals, whereas reactivities in the constant-rate region ( R c ) are essentially independent of the rank of the parent coals. Reactivities of chars in H 2 differ markedly from those in air and CO 2. Results of surface-area measurements of chars and activation energies for the hydrogasification reaction suggest that during the induction period the reaction is diffusion-controlled whereas in the constant-rate region it is chemically controlled. Upon removal of mineral matter, R i values generally decrease but R c values show a random variation. Removal of mineral matter from coals prior to their carbonization brings about profound changes in surface area and porosity of chars. The effect of char particle size on reactivity is considered.