Thermodynamic parameters adsorption

Though thermodynamics cannot provide direct insight into the metal/so-lution interface on a microscale, it generates important information about the energies of interaction of metals with the solvent and the solute. Adsorption parameters are of great value to understanding mutual interactions of the system components at the interface. 

As we have seen, an adsorption isotherm is one way of describing the thermodynamics of gas adsorption. However, it is by no means the only way. Calorimetric measurements can be made for the process of adsorption, and thermodynamic parameters may be evaluated from the results. To discuss all of these in detail would require another chapter. Rather than develop all the theoretical and experimental aspects of this subject, therefore, it seems preferable to continue focusing on adsorption isotherms, extracting as much thermodynamic insight from this topic as possible. Within this context, results from adsorption calorimetry may be cited for comparison without a full development of this latter topic. 

Tfor the determination of thermodynamic adsorption parameters, we used a drop calorimeter to measure specific heats and a conduction calorimeter to measure enthalpies of adsorption. 

Few thermodynamic adsorption parameters are listed in the International Critical Tables, and rightly so. The literature commonly reveals considerable disagreement between investigators for these parameters, even for surfaces which should be structurally simple. 

The fifth mechanism, proposed by Stranahan and Deming (1982), utihses a four-parameter thermodynamic model which assumes Langmuir adsorption of the charged counter-ion at the stationary phase-mobile phase interface. The model does not require the formation of ion-pairs between counter-ions and sample ions and assumes that the primary effects of the counter-ion are on the interfacial tension between the adsorbed phase and the bulk liquid phase, together with direct ionic interactions with the charged sample. 

A volcano plot correlates a kinetic parameter, such as the activation energy, with a thermodynamic parameter, such as the adsorption energy. The maximum in the volcano plot corresponds to the Sabatier principle maximum, where the rate of activation of reactant molecules and the desorption of product molecules balance. 

Once the kinetic parameters of elementary steps, as well as thermodynamic quantities such as heats of adsorption (Chapter 6), are available one can construct a micro-kinetic model to describe the overall reaction. Otherwise, one has to rely on fitting a rate expression that is based on an assumed reaction mechanism. Examples of both cases are discussed this chapter. 

Parameter estimates should not differ by orders of magnitude from those evaluated using well established methods of thermodynamics or known from the literature several rules concerning adsorption phenomena have been worked out by Boudart et al. (1967) the optimal parameter estimates should not differ very much from the initial guesses if the latter were determined in well designed separate dedicated experiments. 

The thermodynamic properties of U-Th series nuclides in solution are important parameters to take into account when explaining the U-Th-Ra mobility in surface environments. They are, however, not the only ones controlling radionuclide fractionations in surface waters and weathering profiles. These fractionations and the resulting radioactive disequilibria are also influenced by the adsorption of radionuclides onto mineral surfaces and their reactions with organic matter, micro-organisms and colloids. 

As with experimental work on polymer adsorption, experiments in the area of dispersion stability in the presence of polymers require detailed characterisation of the systems under study and the various controlling parameters (discussed above) to be varied in a systematic way. One should seek the answer to several questions. Is the system (thermodynamically) stable If not, what is the nature of the equilibrium state and what are the kinetics of flocculation If it is stable, under what critical conditions ( s, T, x> p etc.) can flocculation be induced ... 

Very often thermodynamic parameters of adsorption, calculated from different isotherms and reported in the literature, can scarcely be compared because the standard-state conditions used in the calculation are not specified. 

On the contrary, a more advanced methodology makes use of nonlinear chromatography experiments If the adsorption isotherms are measured under variable temperatures, the corresponding thermodynamic parameters for each site can be obtained in view of the van t Hoff dependency (site-selective thermodynamics measurements) [51,54]. Thus, the adsorption equilibrium constants of the distinct sites bi a = ns, s) are related to the enthalpy (A// ) and entropy (A5j) according to the following equation [54] ... 

The site-selectively derived thermodynamic parameters obtained by adaptation of Equation 1.17 (Table 1.8) clearly revealed that the heat of adsorptions are exothermic on both enantioselective and nonenantioselective sites, and the difference in the adsorption enthalpies on enantioselective and nonenantioselective sites is about 10 and 15 kJ mol for/ - and 5-enantiomers, respectively. The differential enthalpy change upon adsorption of R- and 5-enantiomers at the enantioselective site AAEIg... 

In contrast to this, there is little information available (11) on the thermodynamics of adsorption of alkyl betaines and no data on the thermodynamic parameters of adsorption or micellization for sulfobetaines. 

Standard Thermodynamic Parameters of Adsorption. Table IV lists the standard thermodynamic parameters of adsorption. Values have been... 

Ba(NO3)2, decomposition of BaOa to BaO and oxygen and the reversible spillover of NO2 between Pt sites and BaO sites. Essentially the model assumes that the adsorption of NO proceeds through the nitrate route and does not consider the nitrite route. Olsson et al. [76] estimated part of the rate parameters in their model from theoretical considerations, part were taken from the literature or calculated from thermodynamic constraints and part were estimated by fitting a set of experimental data. 

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