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1.
过渡状态理论是化学动力学上继碰撞理论的又一理论,是20世纪30年代由埃林等人在量子力学和统计力学发展的基础上提出的。理论的形成过程中曾引入一些假设和模型,意思是化学反应不是只通过简单碰撞就形成产物,而是经过一个以一定构型存在的过渡态。理论中势能面概念的提出引导人们从分子间作用的微观层次来考察化学反应的机理。这个理论的提出解决了该阶段上碰撞理论解决不了的问题。 相似文献
2.
In this study, the adsorption of some organic acids from aqueous solutions on modified and unmodified montmorillonite clays was carried out as a function of temperature and initial concentration of the adsorbate, through adsorption isotherms and their thermodynamic parameters (ΔS, ΔH, and ΔG). The results showed that the adsorption capacity of the adsorbents increased with the concentration, at constant temperature, and decreased with the increase of temperature, at constant concentration. The negative values of ΔG and ΔH showed that the adsorption was a spontaneous and exothermic process. In general the experimental data were well represented by the Langmuir and Freundlich isotherms models. 相似文献
3.
The electric conductance of ion across titanium arsenate membrane has been recorded. Aqueous solutions of KCl, NaCl and LiCl were used. The conductance values of titanium arsenate membrane have been found to increase with increase in concentrations as well as temperature (10-50 °C) in these cases. The conductance values of electrolytes follow the sequence for the cations: K+ > Na+ > Li+. Negative ΔS values are considered to indicate formation of bond between the permeating species and the membrane material. The physico-chemical characterization of the hybrid material was established by XRD, TGA and simultaneous SEM studies. 相似文献
4.
Ewa Ostrowska‐Ligeza Witold Bekas Dorota Kowalska Marta Lobacz Malgorzata Wroniak Boleslaw Kowalski 《European Journal of Lipid Science and Technology》2010,112(2):268-274
Four samples of olive oil were oxidized under polythermal (dynamic) conditions in the cell of a normal‐pressure differential scanning calorimeter (DSC) and in the Metrohm Rancimat apparatus. The DSC experiments were carried out in an oxygen flow atmosphere using different linearly programmed heating rates in the range of 4–20 °C/min. Through DSC exotherms, the extrapolated onset temperatures were determined and used for the assessment of the thermal‐oxidative stabilities of the samples. Using the Ozawa‐Flynn‐Wall method and the Arrhenius equation, the activation energies (Ea), pre‐exponential factors (Z) and reaction rate constants (k) for oil oxidation under DSC conditions were calculated. The Rancimat measurements of oxidation induction times were carried out under isothermal conditions in an air atmosphere at temperatures from 100 to 140 °C with intervals of 10 °C. Using the Arrhenius‐type correlation between the inverse of the induction times and the absolute temperature of the measurements, Ea, Z, and k for oil oxidation under Rancimat conditions were calculated. The primary kinetic parameters derived from both methods were qualitatively consistent and they help to evaluate the oxidative stabilities of oils at increased temperatures. 相似文献
5.
Temperature effects on the determination of oxidative stability with the metrohm rancimat 总被引:1,自引:0,他引:1
Gerard L. Hasenhuettl Peter J. Wan 《Journal of the American Oil Chemists' Society》1992,69(6):525-527
Reproducibility of Oil Stability Index (OSI) values determined on the Metrohm Rancimat was measured with a single run and
between experimental runs. Within a single experiment, a range of 0.13 h and a standard deviation of 0.066 h were determined.
For multiple experiments, standard deviations of 0.24 and 0.26 were obtained for soybean and low-erucic rapeseed oil, respectively.
The effect of temperature was determined for safflower, soybean, lowerucic rapeseed, corn, peanut and olive oils. A linear
relationship was established between log (OSI) and temperature. The linear equation obtained for soybean oil was utilized
to calculate variability of the OSI due to temperature differences in the heating block. 相似文献
6.
Fabiano B. Gonzaga Celio Pasquini Christianne E. C. Rodrigues Antônio J. A. Meirelles 《European Journal of Lipid Science and Technology》2007,109(1):61-65
This work presents a comparison between a new method for the determination of the oxidative stability of edible oils at frying temperatures, based on near‐infrared emission spectroscopy (NIRES), and the Rancimat method at 110 °C. In the NIRES‐based method, the induction time (IT) is determined by means of the variation of the emission band at 2900 nm during heating at 160 °C. The comparison between the IT values obtained with the two methods for 12 samples of edible oils shows some correlation for samples of the same type once there is an agreement on the sequence of highest to lowest IT values between the methods, but a poor correlation considering all samples (correlation coefficient of 0.78). This lack of correlation demonstrates that the results obtained with the Rancimat method cannot be used as an indication of the oxidative stability, or the resistance to degradation, of edible oils at frying temperatures. The difference in the heating temperatures used in the two methods leads to 20–36 times higher IT values for the Rancimat method in relation to the NIRES‐based method, but with similar repeatabilities (2.0 and 2.8%, respectively). 相似文献
7.
三元硝酸熔盐高温热稳定性实验研究与机理分析 总被引:5,自引:3,他引:2
通过熔盐质量损失率和试样中NO2-含量变化状况,以及熔盐材料的热重(TG)曲线,研究了三元硝酸熔盐(53%KNO3-40%NaNO2-7%NaNO3)在空气和氮气气氛中高温条件下的热稳定性性能。结果表明,三元硝酸熔盐空气中的上限使用温度为773K,高温时存在劣化现象;而在氮气气氛中三元熔盐的热分解温度为723K。同时,从热力学和动力学角度分析得到,三元硝酸熔盐在773K以下空气中发生的反应为亚硝酸钠的分解和氧化;而在氮气气氛中三元熔盐723K以下时主要发生的是亚硝酸钠的分解反应。在氧气含量一定的情况下,氧气的扩散和亚硝酸盐的分解反应符合一级反应动力学模型。 相似文献
8.
A parametric semi-empirical thermodynamic theory for the dissolution of glassy polymers in liquids is formulated based on the experimental results on Gibbs energy, enthalpy and entropy of mixing in isothermal conditions for a large number of systems. The values of these thermodynamic functions include the contribution from the glassy structure of the polymer, which depends on the volume fraction of metastable voids, the cohesion energy of polymer, and the entropy depression parameter. The glassy-state contribution to the enthalpy and entropy of mixing is large and negative, which provides sufficient difference from the same functions for the elastic polymers. However, this also leads to the enthalpy/entropy compensation, and the values of Gibbs energy of mixing only weakly depend on the glassy nature of the polymer. The theory is shown to provide adequate explanation for all specific features of the concentration dependences of different thermodynamic functions found in experiment at constant temperature. The parameters of the model are tabulated for a large variety of solutions of polymer glasses. 相似文献
9.
Tang Lee Han H. D. Rozman Wan Rosli Wan Daud 《Polymer-Plastics Technology and Engineering》2013,52(6):551-557
One of the important characteristics of the thermosetting resins such as urea-formaldehyde resin is the cure property. This is an invaluable characteristic for determining the processing capabilities of the resin. Differential scanning calorimetry (DSC) measurements are ideally suited to achieve this end. This paper presents the results of the effect of process variables on the enthalpy of cure of a low formaldehyde emission UF resin prepared by a four-stage process as described in the text. Response surface methodology (RSM) was used to design and the analyze the experiments. Urea (U) was added to the reaction mixture according to a pre-determined program of varying the number of additions and the duration between the additions during the second stage of the process. A mathematical relationship between the process variables and enthalpy of cure was established. The urea-formaldehyde (UF) resin was prepared under optimum conditions. Experiments were performed with the resin so prepared to determine the energy of activation of the curing reaction catalyzed by ammonium chloride. 相似文献
10.
《分离科学与技术》2012,47(20):2843-2853
Abstract Kinetics and thermodynamic parameters of Cr6+ separation by fly ash from aqueous solution has been evaluated. It was found that the separation process is partly diffusion controlled and endothermic in nature. The mass transfer coefficient increases with an increase in temperature. The activation energy and change of enthalpy were also evaluated and found to be 17.5 kJ/mol and 14.47 kcal/mol, respectively. The free energy of the process was small but positive, and it decreases with an increase of temperature. The change of entropy was found to be small and remains unchanged over the 30 to 50°C temperature range. This process follows the Langmuir isotherm model where Q o and b were also determined at different temperatures. 相似文献
11.
利用热重分析法比较研究了新型硬质聚氨酯泡沫[超支化聚氨酯多元醇型(HBPU型)]和硬质聚氨酯泡沫(RPUF)在氮气中的热分解行为,探讨了HBPU型RPUF在不同升温速率下的热分解动力学,运用Kissinger最大失重率法和Flynn-Wall-Ozawa等失重百分率法计算获得了其热分解过程的活化能。研究结果表明,HBPU型RPUF的初始分解温度(T5%)为205℃,半寿温度(T50%)为361℃,略低于传统的RPUF。Kissinger法得到的HBPU型RPUF的热分解表观活化能为159.8 kJ/mol;Flynn-Wall-Ozawa法得到的热分解过程分为三个阶段:第一阶段的平均活化能为82.8 kJ/mol,第二阶段的平均活化能为140.7 kJ/mol,第三阶段的平均活化能为111.3 kJ/mol,HBPU型RPUF具有较好的热稳定性。 相似文献
12.
Thermodynamic stability is an important property of proteins that is linked to many of the trade-offs that characterize a protein molecule and therefore its function. Designing a protein with a desired stability is a complicated task given the intrinsic trade-off between enthalpy and entropy which applies for both the folded and unfolded states. Traditionally, protein stability is manipulated by point mutations which regulate the folded state enthalpy. In some cases, the entropy of the unfolded state has also been manipulated by means that drastically restrict its conformational dynamics such as engineering disulfide bonds. In this mini-review, we survey various approaches to modify protein stability by manipulating the entropy of either the unfolded or the folded states. We show that point mutations that involve elimination of long-range contacts may have a greater destabilization effect than mutations that eliminate shorter-range contacts. Protein conjugation can also affect the entropy of the unfolded state and thus the overall stability. In addition, we show that entropy can contribute to shape the folded state and yield greater protein stabilization. Hence, we argue that the entropy component can be practically manipulated both in the folded and unfolded state to modify protein stability. 相似文献
13.
利用含阻聚剂和不含阻聚剂的DSC曲线,分别确定其热分解反应的动力学参数,经此来研究阻聚剂的存在对于碳氢燃料的性能的影响。 相似文献
14.
C. M. Fantoni A. P. Cuccio D. Barrera-Arellano 《Journal of the American Oil Chemists' Society》1996,73(2):251-253
Encapsulated fish oils are extensively commercialized in Brazil. These products could have an effect in the reduction of heart
diseases because of their high content of polyunsaturated fatty acids. However, information about their composition and quality
are still lacking. Fatty acid composition, oxidative stability (Rancimat, 80°C, 2.5 g sample and 8.3 L/h air), peroxide value
(PV), and polar compound content were determined in sixteen trademarked encapsulated fish and cod-liver oils, purchased from
Brazilian markets. The highly polyunsaturated fatty acid (eicosapentaenoic acid+docosahexaenoic acid) level appear to be typical
of marine oils (16.2 and 32.1%). The PV ranged from 2.1 to 20.3 meq/kg, which is considered high, whereas the Rancimat induction
periods varied from 1.95 to 8.45 h. The samples analyzed contained from 0.1 to 8.3% polar components. In some cases, both
composition and quality were inadequate for this kind of product. One of the samples did not contain cod-liver oil, it appears
that it contained soybean oil.
Presented at the 6th Latin American Congress and Exhibit on Fats and Oil Processing, LA-AOCS, Campinas, SP, Brazil, Sept.
1995. 相似文献
15.
The Helmholtz free energy or an equation of state relating temperature, pressure, volume and composition plays a key role
in the calculation of phase equilibria and thermodynamic properties. Such information is usually available for vapor, partially
available for liquids, and rarely available for solids. Depending on the information available, different methods are used
for properties calculation. In this study, various methods were systematically presented and their relations with available
information were comprehensively discussed. 相似文献
16.
Two methods for the evaluation of resistance to the thermooxidation of different classes of polyurethane films (PURs) throughout thermogravimetric analyses were used. Normal and waterborne polyurethanes were characterized using either isothermal or dynamic heating. In the isothermal method, the times required to reach degradations of 0.025, 0.05, and 0.10 were determined at several temperatures in the interval of 190–250 °C. In the dynamic method, heating rates of 0.5, 1, 2, 5, and 10 °C/min were used in the range of 30–500 °C, and degradations of 0.025, 0.05, 0.10, 0.20, and 0.40 were considered. From the Arrhenius plots, activation energies were evaluated ranging between 130 and 230 kJ/mol for normal PURs and between 80 and 170 kJ/mol for waterborne PURs depending on the method, temperature interval, and degree of degradation. Advantages and disadvantages of the two methods were compared. The isothermal method requires a preliminary test, and it was more useful for degradation studies up to 0.05. The dynamic method offered evidence for the presence of different degradation processes, and it was more suitable for the evaluation of kinetics parameters at higher degrees of degradation. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1216–1225, 2001 相似文献
17.
Kinematic viscosity (η) is an important property of diesel fuels, including biodiesels, which are marketed mostly as the blends in many countries around the world. In this study, the free energy of viscous flow (ΔGvis) for a non-associated liquid mixture is assumed to be the summed of ΔGvis of individual components. Hence, the Eyring’s equation, η = Ae(−ΔGvis/RT), is transformed to ln ηblend = a + bn1 + c/T + dn1/T (where, a, b, c and d, T and n1 are thermodynamically related constants, absolute temperature and mole fraction of biodiesel, respectively). The transformed equation is used to predict kinematic viscosity of biodiesel blends (ηblend) of different degree of blending at any temperatures from pour point to 100 °C. The predicted kinematic viscosities are in good agreement with those reported in literatures at all temperatures. The highest deviation is ±5.4% and the average absolute deviation (AAD) is less than 2.86%. The transformed equation can also be used to predict kinematic viscosities of pure fatty acid methyl esters in diesel fuel. Methyl ricinoleate is an exception. The AAD is 4.50% and the deviation is as high as 12.80%. The high deviation suggests that molecular interactions between methyl ricinoleate and diesel fuel is high and cannot be ignored. 相似文献
18.
《Ceramics International》2016,42(6):7054-7059
The results of heat capacity measurements for the lanthanum orthoniobate substituted with 10, 20 and 30 mol% of antimony (LaNb0.9Sb0.1O4, LaNb0.8Sb0.2O4 and LaNb0.7Sb0.3O4) are presented and discussed. Temperature dependence of low temperature heat capacity was analyzed within the Debye and Einstein models. The Debye temperature decreased, whereas the Einstein temperature increased with antimony content. The decrease of the Debye temperature with increasing antimony content was correlated with decreasing scheelite–fergusonite transition temperature. The increase of the Einstein temperature of LaSbxNb1−xO4 with increasing antimony content may indicate increasing frequency of optical vibrations of Nb(Sb)–O4−2 polyhedra relative to La3+ cations. Using the heat capacity data, standard entropies of the phases were calculated and combined with previously measured enthalpies of formation to obtain Gibbs energies of formation. Standard thermodynamic properties were tabulated. 相似文献
19.
PMDA-ODA型聚酰亚胺纤维的热稳定性研究 总被引:1,自引:0,他引:1
通过干法纺丝制备了聚酰亚胺(PI)纤维,采用热失重(TGA)测试分析了其热稳定性能.TGA测试表明,均苯四甲酸酐-4,4.二氨基二苯醚(PMDA-ODA)型PI纤维500℃之前不发生分解,其热分解稳定性要优于P84纤维.并利用Kissinger和Flynn-Wall-Ozawa方法计算并比较了PI和P84纤维在空气中热... 相似文献
20.
用热空气老化试验、差示扫描量热法和热重分析法研究添加少量氯磺化聚乙烯(CSM)的EPDM硫化胶的热氧稳定性。结果表明:添加CSM的EPDM硫化胶第一分解阶段的活化能低于未添加CSM硫化胶的,而第二分解阶段的活化能却高于未添加CSM硫化胶的。这说明,添加CSM能提高EPDM硫化胶耐长期热氧老化性能,但对硫化胶早期的热分解起加速作用 相似文献