Conclusions
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1.
Thermodynamic expressions applicable to the analysis of directly measured adsorption isosteres and also suitable for systems possessing adsorption hysteresis have been obtained on the basis of the representations of the potential theory of adsorption.
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2.
The experimental isosteric data in the range of temperatures and pressures studied are expressed by linear functions not only in plots of log P versus 1/T, but also in plots of log P versus log T, by which the independence of the isosteric heats and entropies from the temperature is confirmed. Constant values of the isosteric quantities (heats and entropies) within the range of temperatures and pressures studied correspond to the isosteres emerging from the adsorption or desorption branches of the isotherm. Two values of the isosteric quantities always correspond to the isosteres emerging from the horizontal portion of the isotherm close to saturation. The first isosteric quantities are always greater than the second. On the horizontal portion of the isotherm close to saturation lie points to which two different values of the isosteric quantities (first and second) correspond, depending on whether these points were obtained during the adsorption or the desorption isosteric process.
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3.
The curves of the dependence of the first and second isosteric heats on the adsorbed amount correspond to calorimetric data obtained on the silica gel-n-paraffin and silica gel-benzene systems. Comparing the results of these two independent methods, we see that the isosteric heats and entropies are an important characteristic of the processes that occur in the adsorption phase. An increase in the first isosteric heats in the horizontal portion of the isotherm and phase transitions in the adsorbed substance are also observed in systems without hysteresis. These phenomena, which are more general in character, are caused by the behavior of the adsorbed substance in the strong field of the adsorbent.
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I am very grateful to M. M. Dubinin and V. V. Serpinskii for their attention to this work and for the critical remarks they have made.
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Godek, I. Thermodynamic treatment of experimental data obtained by direct measurement of adsorption isosteres. Russ Chem Bull 13, 201–209 (1964). https://doi.org/10.1007/BF00853672
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DOI: https://doi.org/10.1007/BF00853672