Standard adsorption entropy

Measurements of deviation of the standard adsorption entropy at graphitized carbon black have been carried out for thietane and other heterocyclic molecules. The theoretical standard entropy for the thietane molecule shows a greater deviation from the experimental value in comparison to others, which seems to indicate that for thietane a simple model of an adsorbed molecule with 2° of freedom is not accurate <1997ZP1333>. 

The adsorption behavior of atoms and compounds for most of the experiments used in the described correlations were evaluated using differently defined standard adsorption entropies [28,52-57], Adsorption data from more recent experimental results were evaluated applying the model of mobile adsorption [4], In addition, data from previous experiments were reevaluated using this model. 

The dimensionless equilibrium constant of a simple reversible adsorption reaction is related to thermodynamic standard quantities—the standard adsorption enthalpy and the standard adsorption entropy, which are assumed to be temperature independent ... 

Experimental observations of these empirical correlations clearly prove the postulated proportionality. These correlations suggest a similarity between the bond (with lower coordination) of the adsorbed particles to the modified surface and the bond to the surface of the pure macroscopic phase of the compound, which is relevant for the desublimation process. The adsorption behavior of atoms and compounds for most of the experiments used in the described correlations were evaluated using differently dehned standard adsorption entropies [65-70]. Adsorption data from more recent experimental results were evaluated applying the model of mobile adsorption [4]. Hence, data from previous experiments were re-evaluated using the latter model. These correlations based on estimated standard sublimation enthalpies allow predictions of adsorption enthalpies for selected compounds for the case of zero surface coverage. These results are only valid for experimental conditions using the same reactive gases, and thus, similarly modified stationary surfaces. 

It is worth recalling that the entropy of adsorption may be obtained from calorimetric experiments only if the heat exchange is reversible. A formula for evaluating the standard adsorption entropy Aa5 °from a reversible adsorption volumetric-calorimetric data was proposed by Garrone et al. [91] and applied to a selection of quasi-ideal systems, [97] consisting of CO adsorbed on non d/d metal oxides, at the surface of which cus cations acting as Lewis acidic sites were exposed. An isothermal microcalorimeter with a discontinuous (stepwise) introduction of the adsorptive, as the one described here, was fruitfully employed. 

The method is here applied to a very simple but instructive case CO adsorbed via electrostatic polarization, [23] on Na-MFl and K-MFl pre-outgassed at T = 673 K. The adsorption represents in both cases an ideal process, which is characterized (within the experimental error) by a Langmuir-like behavior, as evidenced by the adsorption isotherms (vide supra Fig. 1.7 in Sect. 1.3.1). The calorimetric heat of adsorption was ca. 35 and 28 kJ mol for Na-MFI and K-MFl, respectively (vide supra Sect. 1.4.2.3, Fig. 1.14a, b). The half-coverage equilibrium pressure (obtained by the adsorption isotherms) were pi/2 = 200Torr for Na-MFI and 850 Torr for K-MFI. In both cases the reference state for the gas phase was p° = I Torr, as done in previous work (see Ref. [18, 97]). The obtained standard adsorption entropy was AaS° = -151J mol for Na-MFI and — 140J mol forK—MH. 

FIGURE 3.5 Coverage of species A plotted as a function of pressure at three different values of adsorption enthalpy, A//. The temperature is 300 K and the standard adsorption entropy is... 

FIGURE 6.1 Factors that influence the nature of electrostatic cation adsorption at oxide surfaces, and thus, PL membrane structure, (a) Percentage of surface sites occupied by TMA+ ions compared to Na+ ions. This number is another way of representing AGexcTMA+/Na+. (b) Standard state entropy of cation adsorption. (Modified and reprinted from Sahai, N., J. Colloid Interface Sci., 252, 309, copyright 2002, and Sahai, N., Geochim. Cosmochim. Acta, 64, 3629, copyright 2000. With permission from Elsevier Science.)... 

The thermodynamic characteristic of adsorption from solutions can be determined from the dependence of adsorption on temperature. But the determination of adsorption isotherms from solution at different temperatures is the rather complicate problems. Liquid chromatography may be very useful method for the determination of thermodynamic characteristics of adsorption at small coverage [11] because of the measurement of retention volume (the Henry constant) at different temperatures of the chromatographic columns makes it possible to calculate the heats of adsorption and the differential standard change entropy of adsorption from ... 

Equation (2.182) is commonly used to calculate the standard enthalpy of adsorption [83, 160, 171, 186, 187]. The constant K usually exhibits a weak dependence on temperature. The value of AH° calculated from Eq. (2.182) was found to be in the range of +10 to -20 kJ/mol for various surfactants. As mentioned above AG lies in the range -20 to -60 kJ/mol, hence the standard free energy of adsorption is mainly controlled by the adsorption entropy, see Eq. (2.180), and the value of TAS can amount to 10 to 50kJ/mol. The most significant contribution of entropy was found for the water/oil interface [160]. The increase of AS due to adsorption can be ascribed mainly to the disorder of water structure in the solution bulk [83, 160]. In solution the hydrocarbon chains of the surfactant molecules are surrounded by a structured water shell, while during the adsorption these shells are destructed. This leads to an increase in entropy of the system. The entropy also increases due to the transfer of hydrocarbon chains from the water phase to the gas phase and, especially, to the oil phase where they become more flexible. 

A5a)° should be smaller than the standard-state entropy of the gas phase, (A5 j)°, since the entropy change on adsorption cannot be larger than the entropy of the nonadsorbed state. These two rules are rather obvious ones, which can be termed strong rules. A weak rule, based on the calculation... 

The first attempts to predict macroscopic properties such as Henry constants, initial isosteric heats of adsorption, or changes in the standard differential entropies for noble gases, H2, N2, O2, CO, H2O, NH3, CO2, n-alkanes up to Cg, ethylene, acetylene, and benzene in silicalite were performed by Kiselev and coworkers about 20 years ago.224-227 These workers used potential param-... 

The adsorption of dihydrogen onto the zeohtes Na-ZSM-5 and K-ZSM-5 rendered the fundamental H-H stretching mode as IR active [05A1]. The corresponding IR adsorption bands were fonnd at 4101 and 4112 cm for H2/Na-ZSM-5 and H2/K-ZSM-5, respectively. By means of variable-temperature IR spectroscopy, the standard adsorption enthalpy and entropy were determined. 

According to the constancy of both i/co spectral position and heat of adsorption, species-B adsorption was considered Langmuirian and the Eq. 1.26 could be applied straightforward. The standard molar entropy of adsorption was so evaluated from the calorimetric data 69 kJ mol ) and the Langmuir adsorption isotherm (Pi/2 7 Torr),andturnedouttobe Aa "" = -237Jmol Note that this value... 

Fig. 7.10 Change of the molar adsorbate entropy as a function of the adsorbed amount for localized and mobile adsorption process. S° is the standard molar entropy of the adsorptive in the gas, liquid or solid state...

From the adsorption standard free energies and standard enthalpies, adsorption entropies can be calculated from ... 

The standard entropy of adsorption AS2 of benzene on a certain surface was found to be -25.2 EU at 323.1 K the standard states being the vapor at 1 atm and the film at an area of 22.5 x T per molecule. Discuss, with appropriate calculations, what the state of the adsorbed film might be, particularly as to whether it is mobile or localized. Take the molecular area of benzene to be 22 A. ... 

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