Heterogeneous adsorption isotherm Isosteric heat

According to second and third observations, it is difficult to appreciate the maximum value of the surface free energy and surface enthalpy of a solid, especially in the case of microporous materials which are widely efficient adsorption properties of the surface (sample V). Therefore, for this material, more works may be needed on the adsorption isotherm, spreading pressure, isosteric heat of adsorption, and even heterogeneities of solid surfaces. They are concerned with the finite concentration technique with increasing amount adsorbed, which will be dealt to some extent in the next section. 

An illustration of the surface heterogeneity of silica, deduced from isotherm measurements, is shown in Figure 10 which indicates the variation, with the coverage degree (0), of the isosteric heat of adsorption Qst of heptane and toluene, calculated from adsorption isotherms determined at 37° and 47°C, on a talc by Jagiello et al.[19]. 

Equations (1) and (2) are applicable when the alumina is energetically homogeneous (isosteric heat of adsorption, q, is independent of water loading, n). The Toth equation can be used when the alumina is energetically heterogeneous (q decreases with increasing n) and the isotherm shape is Type I [7] ... 

It would be highly desirable to have other quantities simulated in this model, as they would allow a more thorough testing of the model through comparisons with the available experimental data (e.g., quantities such as adsorption isotherms at several temperatures, or the coverage dependence of the isosteric heat of adsorption). These simulated data are not presently available because the heterogeneous deformable bundle model is very recent. 

Despite the above-mentioned problem, numerous works have been published to micromodel adsorbent heterogeneity by assuming that the adsorbent comprises a patchwise distribution of homogeneous sites of different isosteric heats of adsorption [23], According to this concept, the overall surface excess isotherm of a pure gas i on a heterogeneous adsorbent ( f ) is given by... 

The sorption data were obtained using the adsorption isotherm representing the equilibrium uptake versus the free solute concentration at a constant temperature. The Freundlich equation has been used because P-CD polymer has a heterogeneous surface able to act both with the cavity of P-CD and with the polymer network. The relationship between the sorbate equilibrium concentration and the temperature at a constant equilibrium uptake is shown by the adsorption isostere. The values of isosteric heat decrease in the order propranolol > 4-methoxyindole = indole pindolol. 

These observations suggest that the heterogeneous effect in the S02-modenite system represents an extreme case, so much so that chemisorp-tive bonds may be stipulated (probably between the S02 and the cations). These effects would, of course, involve energy emission and show up in the calorimetric measurements. However, the specificity of the adsorption would tend to show a relatively temperature-insensitive isotherm in the low-pressure region, thus rendering the isosteric techniques of obtaining heats of adsorption/chemisorption ineffective. 

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