Adsorption-desorption kinetics experimental

In this review we put less emphasis on the physics and chemistry of surface processes, for which we refer the reader to recent reviews of adsorption-desorption kinetics which are contained in two books [2,3] with chapters by the present authors where further references to earher work can be found. These articles also discuss relevant experimental techniques employed in the study of surface kinetics and appropriate methods of data analysis. Here we give details of how to set up models under basically two different kinetic conditions, namely (/) when the adsorbate remains in quasi-equihbrium during the relevant processes, in which case nonequilibrium thermodynamics provides the needed framework, and (n) when surface nonequilibrium effects become important and nonequilibrium statistical mechanics becomes the appropriate vehicle. For both approaches we will restrict ourselves to systems for which appropriate lattice gas models can be set up. Further associated theoretical reviews are by Lombardo and Bell [4] with emphasis on Monte Carlo simulations, by Brivio and Grimley [5] on dynamics, and by Persson [6] on the lattice gas model. 

Recently the new SRT approach has been generalized further to take into account the energetic heterogeneity of the actual adsorption systems and the possible role of the interactions between the adsorbed molecules. [10-20] Most recently, the authors have shown, that the SRT approach can be successfully applied to describe the multi-site-occupancy adsorption of molecules which do not dissociate after being adsorbed. [14] Compact simple analytical expressions were developed, and next used successfully to correlate experimental data for adsorption/desorption kinetics in various gas/solid systems. The purpose of this presentation is to show, that the new SRT approach can be also applied to represent the kinetics of dissociative gas adsorption on solids. That kinetics is of a crucial importance in a variety of catalytic reactions occurring on solid surfaces. 

Rather than trying to describe the sparse and occasional results hitherto reported, this work is devoted to the foundation of a general and comprehensive theory of adsorption-desorption kinetics. The emphasis, however, will not be concentrated on the formal mathematical aspects of the theory, but rather on its physical bases in particular, the validity of the methods developed will be checked by verifying how they are able to explain the observed experimental behaviours. Among them one plays a special role — the time-logarithm law known as Elovich equation. 

In our case study, the experimental observations (i.e. concentration versus time data) were used as input to the conceptualization phase of a mathematical model with two sink compartments (the so-called two-sink model). Following the above discussion, this model (schematically represented in Fig. 2.3-1) can be eonsidered as a hybrid-empirical model. Conceptually, this model describes the test chamber kinetics of a VOC for the three types of experiments which have been carried out. The adsorption-desorption kinetics is described by the rate constants k, k, k. Given that the conceptualization... 

Detailed experimental data on the rate constants associated adsorption/desorption kinetics or conformational interconversion of different forms of a protein chromatographed on -alkylsilicas are currently very sparse. The kinetics of de-naturation of several proteins on n-butyl-bonded silica surfaces have been reported. Fig. 18 for example, shows the dependence of peak area on the incubation time of lysozyme on the bonded phase surface, from which rate constants for interconversion on the stationary phase, i.e. were derived [63]. The graphical representations derived from quantitative numerical solutions of the probabihty distributions... 

Although the experimental results are in qualitative agreement [27] with the above simplified proposed mechanism, more detailed analysis is needed to account for the adsorption-desorption kinetics since the phenomenon is occurring far from equilibrium. 

Furthermore, modeling of the esterification reaction was attempted in the presence of silica nanoparticles during the formation of aliphatic polyester nanocomposites. From the experimental data, it was found that on increasing the Si02 content in esterification, the rate of water production decreases [47]. In addition, it was clear that the total quantity of water released does not depend on the nanoparticle concentration. This suggests that the existence of the particles does not influence the esterification reaction itself. Their main effect is to adsorb the produced water before it evaporates, altering in this way the water evaporation curve. The simplest model for this phenomenon is to assume very fast water adsorption/desorption kinetics on the Si02 particles. In this case, the evaporation kinetics must be explicitly taken into account because it is no more very fast compared to the other phenomena that occur. 

Based on the literature indications above-mentioned, and supported by preliminary fits of the experimental data, a nonactivated NH3 adsorption process and Temkin-type NH3 adsorption/desorption kinetics have been assumed, i.e., ... 

Conclusion. The work has demonstrated experimentally that significant improvement in the yield and/or selectivity can be obtained by operating a reactor with competing/consecutive reactions in a periodic mode. The main cause of this improvement appears to be the resonance induced by the adsorption/desorption kinetics during periodic operation. 

It is clear from these experiments that two stable Ga coverages exist, which condense at low temperatures into a (1 x 2)/(4 X 2) reconstruction at bUayer coverage and a (4 x 4) reconstruction at bilayer coverage. On surfaces with noninteger amounts of adsorbed Ga, both reconstructions may coexist (presumably in the form of domains), although we have no experimental evidence for such a coexistence. Furthermore, the recovery behavior of the surface at elevated temperatures provides the opportunity to study the adsorption/desorption kinetics of Ga in real time. 

Abstract. A model of the conformational transitions of the nucleic acid molecule during the water adsorption-desorption cycle is proposed. The nucleic acid-water system is considered as an open system. The model describes the transitions between three main conformations of wet nucleic acid samples A-, B- and unordered forms. The analysis of kinetic equations shows the non-trivial bifurcation behaviour of the system which leads to the multistability. This fact allows one to explain the hysteresis phenomena observed experimentally in the nucleic acid-water system. The problem of self-organization in the nucleic acid-water system is of great importance for revealing physical mechanisms of the functioning of nucleic acids and for many specific practical fields. 

This is the same case with which in Eqs. (2)-(4) we demonstrated the elimination of the time variable, and it may occur in practice when all the reactions of the system are taking place on the same number of identical active centers. Wei and Prater and their co-workers applied this method with success to the treatment of experimental data on the reversible isomerization reactions of n-butenes and xylenes on alumina or on silica-alumina, proceeding according to a triangular network (28, 31). The problems of more complicated catalytic kinetics were treated by Smith and Prater (32) who demonstrated the difficulties arising in an attempt at a complete solution of the kinetics of the cyclohexane-cyclohexene-benzene interconversion on Pt/Al203 catalyst, including adsorption-desorption steps. 

The quantitative solution of the problem, i.e. simultaneous determination of both the sequence of surface chemical steps and the ratios of the rate constants of adsorption-desorption processes to the rate constants of surface reactions from experimental kinetic data, is extraordinarily difficult. The attempt made by Smith and Prater 82) in a study of cyclohexane-cyclohexene-benzene interconversion, using elegant mathematic procedures based on the previous theoretical treatment 28), has met with only partial success. Nevertheless, their work is an example of how a sophisticated approach to the quantitative solution of a coupled heterogeneous catalytic system should be employed if the system is studied as a whole. 

In order to test the reversibility of metal-bacteria interactions, Fowle and Fein (2000) compared the extent of desorption estimated from surface complexation modeling with that obtained from sorption-desorption experiments. Using B. subtilis these workers found that both sorption and desorption of Cd occurred rapidly, and the desorption kinetics were independent of sorption contact time. Steady-state conditions were attained within 2 h for all sorption reactions, and within 1 h for all desorption reactions. The extent of sorption or desorption remained constant for at least 24 h and up to 80 h for Cd. The observed extent of desorption in the experimental systems was in accordance with the amount estimated from a surface complexation model based on independently conducted adsorption experiments. 

The finite kinetics of the adsorption/desorption steps at the interface have been extensively studied by Hudson and Morel [13,15]. A wealth of literature is available on dealing with such interfacial processes [94-96] and its inclusion in the biouptake model should be implemented when experimental evidence of its necessity arises. 

If this mechanism is consistent with the experimental relaxation data, then a plot of xp versus the expression in the brackets of Equation 35 will give a straight line with a slope of kjnt and an intercept at the origin. As shown in Figure 11, the data fit this proposed mechanism quite well. Values for i i0, reactant and product concentrations, and K nt input into Equation 35 are from the equilibrium modeling results calculated at each pH value for which kinetic runs were made. Normally a variety of different mechanisms are tested against the experimental data. Several other more complex mechanisms were tested, including those postulated for metal ion adsorption onto y-A O (7) however, only the above mechanism was consistent with the experimental data. Hence it was concluded that the bimolecular adsorption/desorption reaction was the most plausible mechanism for Pb2+ ion adsorption onto a-FeOOH. 

The fast reactions of ions between aqueous and mineral phases have been studied extensively in a variety of fields including colloidal chemistry, geochemistry, environmental engineering, soil science, and catalysis (1-6). Various experimental approaches and techniques have been utilized to address the questions of interest in any given field as this volume exemplifies. Recently, chemical relaxation techniques have been applied to study the kinetics of interaction of ions with minerals in aqueous suspension (2). These methods allow mechanistic information to be obtained for elementary processes which occur rapidly, e.g., for processes which occur within seconds to as fast as nanoseconds (j0. Many important phenomena can be studied including adsorption/desorption reactions of ions at electri fied interfaces and intercalation/deintercalation of ions with minerals having unique interlayer structure. 

To interpret the kinetics experimental data of an organic pollutant(s) or leachate from complex organic mixtures, it is necessary to determine the adsorption/ desorption process steps in a given experimental system which govern the overall adsorption/desorption rate. For instance, the adsorption process of an organic compound by a porous adsorbent can be categorized as three consecutive steps ... 

Adsorption-desorption coefficients are determined by various experimental techniques related to the status of a contaminant (solute or gas) under static or continuous conditions. Solute adsorption-desorption is determined mainly by batch or column equilibration procedures. A comprehensive description of various experimental techniques for determining the kinetics of soil chemical processes, including adsorption-desorption, may be found in the book by Sparks (1989) and in many papers (e.g., Nielsen and Biggar 1961 Bowman 1979 Boyd and King 1984 Peterson et al. 1988 Podoll et al. 1989 Abdul et al. 1990 Brusseau et al. 1990 Hermosin and Camejo 1992 Farrell and Reinhard 1994 Schrap et al. 1994 Petersen et al. 1995). 

Irrespective of the sources of phenolic compounds in soil, adsorption and desorption from soil colloids will determine their solution-phase concentration. Both processes are described by the same mathematical models, but they are not necessarily completely reversible. Complete reversibility refers to singular adsorption-desorption, an equilibrium in which the adsorbate is fully desorbed, with release as easy as retention. In non-singular adsorption-desorption equilibria, the release of the adsorbate may involve a different mechanism requiring a higher activation energy, resulting in different reaction kinetics and desorption coefficients. This phenomenon is commonly observed with pesticides (41, 42). An acute need exists for experimental data on the adsorption, desorption, and equilibria for phenolic compounds to properly assess their environmental chemistry in soil. 

Of great interest in lots of domains, oxygen has been studied by voltammetry under very different experimental conditions. As its reaction is under kinetic control, the electrode reaction is influenced by various factors, in particular by the nature of the electrode and of the solution. The metal, through its chemical properties and surface states, which influence the adsorption-desorption of electroactive species or... 

The diversity of possibilities of the general picture of a nonuniform surface makes it indefinite and hence scarcely suitable for concrete applications. On the basis of experimental data on heat of adsorption, adsorption equilibria, adsorption and desorption kinetics, the kinetics of reactions and electrode processes, a special model of a nonuniform surface has been formulated (41) that describes the experiments satisfactorily and without excessive mathematical complexity. 

At present we have evidence for the complexity of higher temperature adsorption/desorption phenomena while, in general, the kinetic characteristics observed for many catalytic reactions are perhaps deceptively simple. The estimations of the concentrations of the participating surface intermediates are, in contrast, experimentally very difficult. Mechanistic investigations of many heterogeneous catalytic processes yield insufficient information to allow clear distinctions to be drawn between alternative reaction modelsf 125). 

The adsorption and desorption kinetics of surfactants, such as food emulsifiers, can be measured by the stress relaxation method [4]. In this, a "clean" interface, devoid of surfactants, is first formed by rapidly expanding a new drop to the desired size and, then, this size is maintained and the capillary pressure is monitored. Figure 2 shows experimental relaxation data for a dodecane/ aq. Brij 58 surfactant solution interface, at a concentration below the CMC. An initial rapid relaxation process is followed by a slower relaxation prior to achieving the equilibrium IFT. Initially, the IFT is high, - close to the IFT between the pure solvents. Then, the tension decreases because surfactants diffuse to the interface and adsorb, eventually reaching the equilibrium value. The data provide key information about the diffusion and adsorption kinetics of the surfactants, such as emulsifiers or proteins. 

Analysis of results of experimental measurements of various parameters (isotherms and kinetics of adsorption/desorption, destruction charges, dynamics of percolation, etc.). 

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