Kinetic Curves (kinetic + curve)

Distribution by Scientific Domains

Selected Abstracts

Experimental and numerical study of sorption/diffusion of esters into polypropylene packaging films

Laurent Safa
Abstract The sorption and diffusion of four esters into polypropylene packaging films immersed in four different solutions have been studied by the FTIR method at 45C. Kinetic curves have been generated for these systems to compute diffusion coefficients. The calculated results are discussed in terms of size and shape of diffusing molecules into a polymer film. Isovalues and profiles of liquid concentrations have been calculated by solving Fick's diffusion equation using a numerical method based on a two-dimensional finite element method. Copyright 2002 John Wiley & Sons, Ltd. [source]

Volume recovery, surface properties and membrane integrity of Lactobacillus delbrueckii subsp. bulgaricus dehydrated in the presence of trehalose or sucrose

E.E. Tymczyszyn
Abstract Aims:, Although the practical importance of adding sugars before drying is well known, the mechanism of protection of bacteria by sugars is not clear. The response of the dehydrated micro-organisms to rehydration is analysed in terms of structural and functional changes, and correlated with their potentiality to grow in rich media. These aspects are related with the membrane integrity and the metabolic state of the rehydrated bacteria, measured by means of surface properties and permeability. To attain this objective, Lactobacillus delbrueckii subsp. bulgaricus was dehydrated in the presence and in the absence of sucrose and trehalose. The bacterial response upon rehydration was investigated by determining: (i) the lag time of the bacterial growing in rich media, (ii) the restoration of the surface properties and the cellular volume and (iii) the membrane integrity. Methods and Results:,Lactobacillus delbrueckii subsp. bulgaricus was grown in MRS at 37C overnight [De Man et al. (1960)J Appl Bacteriol 23, 130] and then dehydrated for 10, 20 and 30 min at 70C in a vacuum centrifuge. The lag time of micro-organisms was determined by optical density changes after rehydration. The surface properties were determined by measuring the zeta potential of the bacteria suspended in aqueous solution. The cellular volume recovery was measured, after stabilization in saline solution, by light scattering and by the haematocrit method [Alemohammad and Knowles (1974)J Gen Microbiol 82, 125]. Finally, the membrane integrity has been determined by using specific fluorescent probes [SYTO 9 and propidium iodide, (PI)] that bind differentially depending on the integrity of the bacterial membrane. The lag time of Lact. delbrueckii subsp bulgaricus, dehydrated by heat in the presence of sucrose or trehalose and after that rehydrated, was significantly shortened, when compared with that obtained for bacteria dried in the absence of sugars. In these conditions, trehalose and sucrose maintained the zeta potential and the cell volume close to the control (nondried) cells. However, the membrane integrity, measured with fluorescent probes, was maintained only when cells were dehydrated for 10 min in the presence of sugars. For larger times of dehydration, the membrane integrity was not preserved, even in the presence of sugars. Conclusions:, When the micro-organisms are dehydrated in the absence of protectants, the membrane damage occurs with a decrease in the absolute value of the zeta potential and a decrease in the cellular volume recovered after rehydration. In contrast, when the zeta potential and the cellular volume are restored after rehydration to that corresponding to nondried cells, the micro-organisms are able to recover and grow with a reduced lag time. This can only be achieved when the dehydration is carried out in the presence of sugars. At short dehydration times, the response is associated with the preservation of the membrane integrity. However, for longer times of dehydration the zeta potential and volume recovery occurs in the presence of sugars in spite of a severe damage at membrane level. In this condition, cells are also recovered. In conclusion, to predict the ability of growing after dehydration, other bacterial structural parameters besides membrane integrity, such as zeta potential and cellular volume, should be taken into account. Significance and Impact of the Study:, The correlation of the lag time with the surface and permeability properties is of practical importance because the correlation of these two parameters with cell viability, allow to determine the potential bacterial capacity to grow in a rich medium after the preservation procedure, without necessity of performing a kinetic curve of growth, which is certainly time-consuming. [source]

Comparison of the roles of two shrinkage-controlled low-profile additives in water aging of polyster resin,glass fiber composites

G. Camino
A model previously formulated or water sorption in polyester resin-glass fiber composites has been applied to the kinetic analysis of experimental data for two composites containing a hydrolysable (polyvinyl acetate-PVAc) and a non-hydrolyzable (polystyrene-PS) shrinkage-controlling low-profile additive (LPA) respectively. It was found that the equilibrium water uptake in the composites and their unreinforced matrix is not drastically affected by the type of LPA. The kinetics of water sorption in the composites, however, were substantially different. The PVAc composite displayed a two-stage sorption process, the first stage being attributed to diffusion combined with hydrolysis, and the second to matrix swelling and plasticizing. A maximum in the kinetic curve was observed, and was due to changes in water solubility inside the matrix during sorption. In contrast, the PS composite displayed the typical kinetics of materials with slowly increasing hydrophilicity. [source]

Degree of conversion and permeability of dental adhesives

Milena Cadenaro
The aim of this study was to analyse the extent of polymerization of different adhesive films in relation to their permeability. One adhesive of each class was investigated: OptiBond FL; One-Step; Clearfil Protect Bond; and Xeno III. Adhesive films were prepared and cured with XL-2500 (3M ESPE) for 20, 40 or 60 s. Polymerization kinetic curves of the adhesives tested were obtained with differential scanning calorimetry (DSC) and data were correlated with microhardness. The permeability of the adhesives under the same experimental conditions was evaluated on human extracted teeth connected to a permeability device and analysed statistically. The results showed that the extent of polymerization obtained from DSC exotherms was directly correlated with microhardness. An increased level of polymerization after prolonged light-curing was confirmed for all adhesives. Simplified adhesives exhibited a lower extent of polymerization and showed incomplete polymerization, even after 60 s. An inverse correlation was found between the degree of cure and the permeability. This study supports the hypothesis that the permeability of simplified adhesives is correlated with incomplete polymerization of resin monomers and the extent of light exposure. These adhesives may be rendered less permeable by using longer curing times than those recommended by the respective manufacturer. [source]

Kinetic evidences for facilitation of peptide channelling by the proteasome activator PA28

FEBS JOURNAL, Issue 20 2000
Ralf Stohwasser
The activation kinetics of constitutive and IFN,-stimulated 20S proteasomes obtained with homomeric (recPA28,, recPA28,) and heteromeric (recPA28,,) forms of recombinant 11S regulator PA28 was analysed by means of kinetic modelling. The activation curves obtained with increasing concentrations of the individual PA28 subunits (RecP28,/RecP28,/RecP28,+ RecP28,) exhibit biphasic characteristics which can be attributed to a low-level activation by PA28 monomers and full proteasome activation by assembled activator complexes. The dissociation constants do not reveal significant differences between the constitutive and the immunoproteasome. Intriguingly, the affinity of the proteasome towards the recPA28,, complex is about two orders of magnitude higher than towards the homomeric PA28, and PA28, complexes. Striking similarities can been revealed in the way how PA28 mediates the kinetics of latent proteasomes with respect to three different fluorogenic peptides probing the chymotrypsin-like, trypsin-like and peptidylglutamyl-peptide hydrolyzing like activity: (a) positive cooperativity disappears as indicated by a lack of sigmoid initial parts of the kinetic curves, (b) substrate affinity is increased, whereby (c), the maximal activity remains virtually constant. As these kinetic features are independent of the peptide substrates, we conclude that PA28 exerts its activating influence on the proteasome by enhancing the uptake (and release) of shorter peptides. [source]

Identification of the effective distribution function for determination of the distributed activation energy models using the maximum likelihood method: Isothermal thermogravimetric data

Bojan Jankovi
The new procedure for identification of the effective distribution function for determination of the distributed activation energy models, which is based on use the maximum likelihood method (MLM), was established. The five different continuous probability functions (exponential, logistic, normal, gamma, and Weibull probability functions (the extended set of distributions)) were used for searching the best reactivity model for two heterogeneous processes: (a) the isothermal reduction process of nickel oxide under hydrogen atmosphere and (b) the isothermal degradation process of bisphenol-A polycarbonate (Lexan) under nitrogen atmosphere. The MLM showed that for both processes, the most suitable reactivity model represents the Weibull distribution model. It was concluded that the values of Arrhenius parameters (ln A and Ea), evaluated from the Weibull distribution model, represent the effective kinetic values for both considered processes. This procedure enables identification the suitable distribution model for considered process only from the experimental data (based on the shapes of obtained integral kinetic curves), and this fact represents the advantage of established analysis. The established mathematical procedure, which is based on the MLM, can be applied as the preliminary analysis for evaluating the distribution of activation energies for complex heterogeneous processes. 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 27,44, 2009 [source]

Modeling the mass transfers during the elaboration of chitosan-activated carbon composites for medical applications

AICHE JOURNAL, Issue 6 2010
A. Venault
Abstract Hydrogels composites composed of chitosan and activated carbon were prepared for medical applications using the vapor-induced phase separation process. Since the gelation process involves mass exchanges between the polymer solution and the air, the kinetics of mass transfer were investigated through experimental and modeling approaches. Among the formulation and process parameters, gravimetric measurements exhibited that mass transfers were mostly controlled by the initial ammonia partial pressure. A nonisotherm mass-transfer model was developed to predict the nonsolvent and solvent exchange rates, therefore, the water and ammonia concentration profiles within the sample during the process. The numerical results were successively validated with gravimetrical kinetic curves obtained in a chamber where the process parameters were controlled. The model aimed also at predicting the pH moving front along the film thickness. The gelation time could also be predicted for different operating conditions (formulation and process parameters). 2009 American Institute of Chemical Engineers AIChE J, 2009 [source]

Safety and quality of plastic food contact materials.

Optimization of extraction time, based on arithmetic rules derived from mathematical description of diffusion., extraction yield
Abstract Migration of packaging constituents into food may raise concerns about food safety. This paper describes the conclusions of a EU research project (AIR 941025), aiming to facilitate the introduction of migration control into good manufacturing practice and into enforcement policies. The first part describes a re-evaluation of analytical approaches to extract and identify potential migrants released by plastic materials, viz. comparison of analytical methods, choice of extraction solvents and of fat simulants. Here we focus on the extraction time needed to achieve a given extraction yield. By correlating these parameters with simple and practical equations, it is possible to design alternative tests for control of compliance of packaging plastics. Using a reference experiment (where there is good agreement between experimental and calculated kinetic curves), it is possible to calculate the percentage of extraction which can be achieved in a given time, or the time necessary to reach a target extraction level for other polymer/solvent combinations. A global control scheme is proposed, which indicates whether and when calculation and testing should be applied. Guidelines are proposed, and can be adapted to both industrial control and to enforcement laboratories. Copyright 2003 John Wiley & Sons, Ltd. [source]