Effect of adsorption of vapours on the electrical conductivity of some nitroaromatic semiconductors: adsorption and desorption kinetics

On adsorption of various vapours, the electrical conductivity of nitroaromatic semiconductors 9-nitroanthracene, 1,4-dinitronaphthalene and 1,3,5-trinitrobenzene changes appreciably. This phenomenon has been used as a probe to study the adsorption and desorption processes in these nitroaromatic polycrystals. The adsorption and desorption kinetics have been found to follow the modified Roginsky-Zeldovich relation. Two adsorption processes have been identified. One, a two-stage process, the first of which gives a Lennard-Jones potential energy curve and is followed by a rate-determining transition over a potential energy barrier to the second stage of adsorption forming weakly bound complexes between the vapour molecules and the nitroaromatic crystallites. In the second process, in addition to the two stages of the first process, a third stage with a comparatively deep potential energy surface is reached by activation over a second potential barrier and strongly bound complexes are finally formed. On leave from Government Arts and Science College, Durg 491 002, India     

Temperature effect on the adsorption and desorption processes in retinoic acid crystals: An electrical conductivity study

Using electrical conductivity as a probe, the effect of crystal temperature on the process of adsorption and desorption of some chemical vapours on retinoic acid semiconductor has been investigated. It has been observed that a three-stage adsorption process in a sample at low temperature changes over to a two-stage process at higher temperature. Measures of activation energies of adsorption and desorption at different stages have been estimated from kinetic data analysis by using a modified Roginsky-Zeldovich equation.     

Thermal and temperature dependence of electrical conductivity studies on Zn,Cd and Hg hydrazone complexes

The complexes of Zn, Cd and Hg of isatin isonicotinoyl hydrazone were prepared at two differentpHs. Their thermal studies (TG, DTG and DTA) have been made and the DC electrical conductivity of compressed powder samples as a function of temperature was investigated. The activation energies (E) were calculated for the ligand and the complexes which showed that the ligand has a lower value of E than the complexes. The magnitude of E was found to be affected by the nature of the metal and thepH of preparation.

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Zusammenfassung Bei zwei verschiedenenpH-Werten wurden die Zn-, Cd- und Hg-Komplexe von Isatinisonicotinoylhydrazon hergestellt. Diese wurden thermisch untersucht (TG, DTG, DTA) und die Gleichstromleitfähigkeit von gepreßten Pulverproben als eine Funktion der Temperatur untersucht. Sowohl für die Liganden als auch für die Komplexe wurden die Aktivierungsenergien (E) berechnet, wobei sich für die Liganden niedrigere Werte ergaben. Man fand, daß die Größe von E durch die Art des Metalles und denpH-Wert bei der Herstellung beeinflußt wird.

     

On the importance of the purity of surfactant solutions in determining their adsorption kinetics

By means of a theoretical experiment it is demonstrated, that a barrier in the adsorption kinetics of a surfactant can be simulated by the presence of surface-active impurities. Hence, the application of models for the diffusion-kinetic-controlled and/or kinetic controlled adsorption presumes the proof of the surface chemical purity of the surfactant solution under investigation.     

Simultaneous determination of heats,equilibrium and kinetics of adsorption: 1- Ethoxy-2-propanol vapours

The heat, equilibrium, and kinetics of adsorption of 1-ethoxy-2-propanol vapours on granulated activated carbon were determined simultaneously by a reaction calorimeter SETARAM C80 D at T=298.15 K at various relative vapour pressures (0.1< p/p_s<0.8). The adsorption isotherm was correlated by the Freundlich equation. It was observed that the enthalpies of adsorption decrease slightly with increasing of the relative vapour pressure of the adsorptive. The rate of adsorption were calculated from analysis of the heat flux signals and it was found that the mass-transfer coefficient for 1-ethoxy-2-propanol vapours in granulated activated carbon increased with increasing relative vapour pressure of the adsorptive.This revised version was published online in November 2005 with corrections to the Cover Date.     

The kinetics of adsorption of tetracycline on chitosan particles

Experiments to monitor and characterize the kinetics of adsorption of tetracycline on chitosan particles are reported in this work. The same pseudo-order kinetics that has been widely used for describing the adsorption in systems related to wastewater purification and drug loading was used to treat the present data. As some unexpected results came out from the experiments, it was necessary a detailed deduction for this sort of kinetics to be carried out, so that approximations related to short and long times were obtained. Firstly it was shown that an apparently linear t/q(t) versus t relationship did not imply a pseudo-second-order sorption kinetics, differently of what has been repeatedly reported in the literature. It was found that this misinterpretation could be avoided by using non-linear regression. Finally, the adsorption of tetracycline on chitosan particles was analyzed, using the insights obtained from theoretical analysis, and the parameters generated were used to analyze to adsorption kinetics and to propose an adsorption mechanism.     

Numerical approximation of a two-dimensional model of non-isothermal reactive liquid chromatography involving slow rates of adsorption–desorption kinetics

Abstract

A two-dimensional general rate model of non-isothermal reactive column chromatography is formulated considering homogenous and heterogeneous reaction rates, slow rates of adsorption–desorption kinetics, and enthalpies of adsorption and reaction. The model is expressed by a system of six nonlinear partial differential equations (PDEs) coupled with algebraic expressions for the adsorption and reaction rates. The nonlinearity of adsorption isotherm and reaction term hinders the derivation of analytical solutions. For that reason, a flux-limiting high-resolution finite volume scheme is suggested to numerically approximate the model equations. The effects of several kinetic and thermodynamic parameters are rigorously analyzed on the reactant conversion and components separation.     

Thermal and electrical conductivity properties of some CrIII, MnII and UO 2 II amino alcohol complexes

Cr^III, Mn^II and UO₂ ^II amino alcohol complexes were prepared and analyzed. The IR spectra proved that the ligation occurs via oxygen and nitrogen atoms. The nujol mull electronic absorption spectra joined with the room temperature magnetic moment values proved that the complexes are of O_h geometry. The thermal behaviour of the complexes was studied by DTA and TG techniques. The mechanism of the decomposition was suggested. The thermodynamic parameters of the decomposition were evaluated and discussed. The entropy of activation changes, S*, showed that the transition states are more ordered than the reacting complexes. The D.C. electrical conductivity measurements of Cr- and Mn-amino alcohol complexes are of semiconductor behaviour. Empirical equations were deduced for the conduction of the investigated complexes.This revised version was published online in November 2005 with corrections to the Cover Date.     

类金刚石碳薄膜对碳膜导电性能的影响

以聚酰亚胺薄膜为原料,经炭化形成碳膜;进而在碳膜表面制备了类金刚石碳(DLC)薄膜,研究了制备条件对碳膜导电性能的影响.采用扫描电镜分析了薄膜的表面形貌和微观结构;采用X射线衍射仪分析了薄膜的晶体结构.结果表明,DLC薄膜的电阻率随着沉积时间的延长先减小后增加;当沉积时间达到3 h时,相应DLC薄膜的电阻率达到最小值5.66×10-5Ω.m.     

Heterogeneous photocatalytic kinetics: beyond the adsorption/desorption equilibrium concept

Summary Rigorous kinetic analysis is presented for a range of photocatalytic mechanisms based on the steady-state approximation, while typically photochemical kinetics is discussed within the framework of Langmuir-Hinshelwood rate equations, assuming a slow rate-determining step and adsorption/desorption equilibrium. Performed comparison with experimental data reveals additional features inconsistent with adsorption/desorption equilibrium concept.     

Effect of porosity on the effective electrical conductivity of different ceramic membranes used as separators in eletrochemical reactors

Different microporous ceramic membranes have been investigated to be used as separators in electrochemical reactors. The effect of porosity on the effective electrical conductivity of the ceramic membranes has been studied. The porosity of the membranes has been modified by changing the manufacturing pressure and by the addition of starch to the alumina–kaolin matrix. In the absence of starch the pore size distribution becomes more uniform with the increase of the manufacturing pressure, and lower porosities and average pore sizes are obtained. On the other hand, the porosity and the average pore size increase with the addition of starch to the alumina–kaolin matrix, but pore size distribution is less uniform and becomes bimodal with two different characteristic pore diameters.

The effective electrical conductivity of the membranes, κ_eff, increases with the decrease of manufacturing pressure and with the increase of starch content. The following correlation between the effective electrical conductivity and the porosity has been obtained: f_c = κ_eff/κ = 0.35 ^1.04, where κ is the electrolyte electrical conductivity.     

11种吸附剂上二噁英的热吸附/脱附性能评价

筛选在低温下高效捕集并在一定的高温下可以快速完全脱附二噁英的吸附/脱附材料是二噁英在线热捕集的关键.该研究以1,2,3,4-四氯代二苯并-对-二噁英(1,2,3,4-TCDD)和1,2,3,8,9-五氯代二苯并呋喃(1,2,3,8,9-PCDF)为二噁英模型物,以电子捕获检测器(ECD)作为检测器,利用填充柱气相色谱系...     

不同吸附剂上动态吸附-脱附挥发性有机气体性能研究

采用气相色谱法和热重分析(TG)研究了活性炭以及5A、NaY、13X、ZSM-5(Si0_2/Al_2O_3=27、300)、Hβ和MCM-41分子筛对正己烷、甲苯和乙酸乙酯的动态吸附-脱附性能,系统考察了挥发性有机气体(VOCs)浓度与种类及体积空速对吸附容量的影响。结果表明,增加体积空速和VOCs浓度,一定程度上能够提升吸附容量;活性炭吸附剂对三种VOCs具有较高的单位质量吸附量,而13X与NaY对三种VOCs具有更大的单位体积吸附量。     

Experimental investigation on phosphate adsorption,mechanism and desorption properties of Mn-Zn-Ti oxide trimetal alloy nanocomposite

Recently composite metal oxides have gained significant attention to be used as adsorbent because of their synergetic effects. Particularly Manganese containing composite oxides are useful for removal of inorganic oxyacids such as phosphate or arsenate. In present study fabrication of Mn-Zn-Ti Oxide adsorbent for phosphate removal carried out via co precipitation method. Surface properties deduced by TEM, FESEM, EDAX and XRD, revealed nanosized composite material has a porous nature constitute of alloy type mixing of the metals. Size of the nanocomposite found to be as small as 6?nm. Adsorption capacity for phosphate estimated at different pH, time and adsorbent dose by batch mode. In addition desorption properties and thermodynamic study also carried out. Several isotherms and kinetic models applied to observe adsorption properties of the Mn-Zn-Ti Oxide nanocomposite. Adsorption capacity found to be 151?mg/g at pH 6, time 90?min, adsorbent dose 0.20?g/L and phosphate concentration of 200?mg/L. Adsorption data fitted to second order kinetics and Freundlich isotherm. Formation of complex between nanocomposite and phosphate predicted from FTIR and supported by pH kinetic and isotherm studies. Desorption and reusability found to be well maintained over five cycles.     

Time-dependent electrical properties of liquid crystal cells: unravelling the origin of ion generation

An understanding of possible sources of ion generation in liquid crystal materials is of utmost importance to ensure uncompromised performance of modern liquid crystal devices. In this paper, time-dependent electrical properties of filled liquid crystal cells are used to reveal important information about often overlooked source of ion generation in liquid crystals. This source of ions originates from the ionic contamination of substrates of the liquid crystal cell. Ionic contaminants can be inherently present in the alignment layers or can be induced by external factors such as mechanical rubbing or irradiation with light. The model of this ion generation process is presented and tested using existing experimental data.     

Analytical models for describing cation adsorption/desorption kinetics as considering the electrostatic field from surface charges of particles

Surface charges of particles together with the adsorbed counter ions in diffuse layer can set up a strong electrostatic field around the particles in aqueous solution. The existent kinetic models for describing cation exchange on solid/liquid interface were either empirical or semi-empirical, and in which the electrostatic field is not considered. In this paper, as considering the important effect of electrostatic field around particles on cations adsorption/desorption, for the first time the dynamic distribution equations of cations in diffuse layer for adsorption and desorption processes in both flow method and batch technique have been established. Those equations clearly show how the cation concentration changes with time in different position of diffuse layer during the cation exchange process, and the corresponding new kinetic models have been obtained upon them. The new models indicate that, in both flow method and batch technique, for the adsorption process, experimental results should appear zero order kinetic process caused by the strong force adsorption in the initial stage of adsorption, and then transform to the first order kinetic process of the weak force adsorption; and for the desorption process, however, only first order kinetic process may exist. The new models are essentially different from the classic apparent or empirical kinetic models since all the parameters have their defined physical meanings in the new models, thus the rate parameters in the new models have the potential to theoretically predict. Theoretical analyses also indicated that, the adsorption/desorption rate in flow method experiment will be much higher than that in batch technique experiment.     

A study of surface adsorption on silver powders

DSC and TG-DTA techniques were used to investigate micro-sized silver powder particles and the adsorption of ethyl cellulose on these particles in a solution of ethyl acetate. The apparent specific heat of the silver particles was determined, and the kinetics of temperature-programmed desorption (TPD) of these adsorbed silver particles was investigated. Results show that the apparent specific heat and desorption kinetic parameters obtained by thermal analysis techniques could be used to characterize certain physico-chemical properties of such a particulate system.     

Spreading of proteins and its effect on adsorption and desorption kinetics

The kinetics of adsorption of lysozyme and alpha-lactalbumin from aqueous solution on silica and hydrophobized silica has been studied. The initial rate of adsorption of lysozyme at the hydrophilic surface is comparable with the limiting flux. For lysozyme at the hydrophobic surface and alpha-lactalbumin on both surfaces, the rate of adsorption is lower than the limiting flux, but the adsorption proceeds cooperatively, as manifested by an increase in the adsorption rate after the first protein molecules are adsorbed. At the hydrophilic surface, adsorption saturation (reflected in a steady-state value of the adsorbed amount) of both proteins strongly depends on the rate of adsorption, but for the hydrophobic surface no such dependency is observed. It points to structural relaxation ("spreading") of the adsorbed protein molecules, which occurs at the hydrophobic surface faster than at the hydrophilic one. For lysozyme, desorption has been studied as well. It is found that the desorbable fraction decreases after longer residence time of the protein at the interface.     

Adsorption and desorption of macromolecules on solid surfaces studied by on-line size exclusion chromatography 2. Preferential adsorption and exchange processes

Preferential and exchange adsorption of polymers differing in molar mass and/or chemical nature under dynamic conditions were investigated using on-line size-exclusion chromatography (SEC). The sample investigated dissolved in an appropriate solvent was injected into a small adsorption–desorption column packed with nonporous silica. A nonadsorbed or desorbed fraction of the polymer was directed into an SEC column for determination of both the amount and the molecular characteristics. This approach is in many aspects superior to other techniques for studies of polymer adsorption onto solid surfaces due to its low sample and time consumption. At a low degree of surface coverage, adsorption and desorption of macromolecules were rapid and were affected by the rate of supply of macromolecules to the adsorbent surface. The exchange between macromolecules at the stage of surface saturation was found to depend on the mean molar masses of preadsorbed and displacing polymer species and possibly also on the chain flexibility of the macromolecules. It was shown that the preferential adsorption driven by the chain-length difference upon saturation of the adsorbent surface was more noticeable if the preadsorbed macromolecules were smaller. Received: 7 April 1999 Accepted in revised form: 21 July 1999     

Quasi-equilibrated temperature programmed desorption and adsorption: A new method for determination of the isosteric adsorption heat

Temperature programmed desorption and adsorption (TPDA) studied under quasi-equilibrium conditions using typical flow TPD setup equipped with a chromatographic TCD detector has been found a good method of obtaining the equilibrium data characterizing adsorption of n-hexane on high silica HZSM-5 and HY zeolites. The equilibrium control of sorption has been confirmed by linearity of the isosters constructed from the TPDA profiles. For HZSM-5 it was corroborated by very good agreement obtained in fitting the experimental data with the model based on the dual site Langmuir (DSL) adsorption function. The values of the adsorption enthalpy and entropy, either determined from the adsorption isosters or as the fitted model parameters, were close to the literature data. Increase of the isosteric adsorption heat of hexane with coverage was observed for both zeolites (72-90 kJ mol⁻¹ for HZSM-5 and 46-61 kJ mol⁻¹ for HY).