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Water Vapor Isotherms and Heat of Immersion of Na- and Ca-Montmorillonite Systems. III. Thermodynamics

Published online by Cambridge University Press:  01 July 2024

R. Keren  and
I. Shainberg
R. Keren
Affiliation:
Institute of Soils and Water, ARO, The Volcani Center, Bet Dagan, Israel
I. Shainberg
Affiliation:
Institute of Soils and Water, ARO, The Volcani Center, Bet Dagan, Israel
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Abstract

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The integral thermodynamic quantities of adsorbed water on Na- and Ca-montmorillonite have been calculated from water adsorption isotherms on Na- and Ca-montmorillonite at 298° and 313°K and from one adsorption isotherm and calorimetric measurements at 298°K. The integral entropy values decrease and then increase as the amount of adsorbed water approaches zero. In both systems, the curves approach the entropy value of free liquid water at the high content water. The thermodynamics of adsorbate on a non-inert adsorbent is discussed in some detail. The two-isotherm method gives the energy change of the water phase only, whereas the colorimetric method gives the energy change of the whole system (clay, exchangeable cations, and the adsorbed water). Only when the energy changes in the solid phase are negligible (=inert surface) should the two methods give similar results. An hypothesis was developed to explain the entropy-change data of water adsorbed on clay surfaces, in which the clay surface behaves as a non-inert adsorbent.

Резюме

Резюме

Высчитывались полные термодинамические количества воды, адсорбированной Na- и Са-монтмориллонитом, по изотермам адсорбции воды Na- и Са-монтмориллонитом при 298° и 313°К и по одной адсорбционной изотерме и калориметрическим измерениям при 298°К. Полные величины энтропии уменьшаются и затем возрастают, когда количество адсорбированной воды приближается к нулю. В обеих системах, кривые приближаются к величине энтропии свободной жидкой воды при высоком содержании воды. Обсуждаются некоторые детали термодинамики адсорбата на неинертном адсорбенте. Двух-изотермный метод дает изменение энергии только водной фазы, тогда как кадотетрический метод дает изменив энергии всей системы (глины, обменных катионов, и адсорбированной воды). Только когда изменения энергии в твердой фазе незначительны (= инертной поверхности) оба метода должны показать сходные результаты. Была предложена гипотеза для объяснения изменений энтропии воды, адсорбированной на глинистых поверхностях, в которых глинистая поверхность ведет себя как неинертные адсорбенты. [N. R.]

Resümee

Resümee

Die integralen thermodynamischen Mengen von an Na- und Ca-Montmorillonit adsorbiertem Wasser wurden aus den Wasseradsorptionsisothermen auf Na- und Ca-Montmorillonit bei 298° und 313°K berechnet, sowie aus einem Adsorptionsisotherm und aus kalorimetrischen Messungen bei 298°K. Die Werte der intergralen Entropie nehmen ab und steigen dann wieder an, wenn die Menge des adsorbierten Wassers gegen Null geht. In beiden Systemen erreichen die Kurven den Entropiewert von freiem flüssigem Wasser bei hohem Wassergehalt. Die Thermodynamik des adsorbierten Stoffes auf einen nicht-inerten Adsorber wird detailliert diskutiert. Die Zweiisothermenmethode gibt nur die Energieänderung der Wasserphase, während die kalorimetrische Methode die Energieänderung des ganzen Systems (Ton, austauschbare Kationen, adsorbiertes Wasser) liefert. Nur wenn die Energieänderungen in der festen Phase zu vernachlässigen sind (= inerte Oberfläche), sollten die beiden Methoden zu ähnlichen Resultaten führen. Es wurde eine Hypothese entwickelt, um die Werte der Entropieänderung von Wasser zu erklären, das an Tonoberflächen adsorbiert ist, bei denen die Tonoberfläche als nicht-inerter Adsorber wirkt. [U.W.]

Résumé

Résumé

Les quantités intégrales thermodynamiques d'eau adsorbée sur la montmorillonite-Na et -Ca ont été calculées à partir d'isothermes d'adsorption d'eau sur la montmorillonite-Na et -Ca à 298° et 313°K, et à partir d'un isotherme d'adsorption et de mesures calorimétriques à 298°K. Les valeurs intégrales d'entropie décroissent et ensuite augmentent à mesure que la quantité d'eau adsorbée approche zero. Dans les deux systèmes les courbes approchent la valeur d'entropie de l'eau liquide à haute teneur en eau. Les thermodynamiques d'un adsorbat sur un adsorbant non-inerte sont discutés en détail. La méthode à deux isothermes ne donne que le changement d’énergie de la phase eau, tandis que la méthode calorimétrique donne le changement d’énergie du système entier (argile, cations échangeables, et eau adsorbée). Les deux méthodes ne devraient donner des résultats semblables que lorsque les changements d’énergie dans la phase solide sont négligeables (surface inerte). Une hypothèse a été developée pour expliquer les données de changements d'entropie d'eau adsorbée sur les surfaces argileuses dans lesquelles la surface argileuse se comporte comme adsorbant non-inerte. [D.J.]

Keywords

AdsorptionHeat of immersionMontmorilloniteThermodynamicsWater
Type
Research Article
Information
Clays and Clay Minerals , Volume 28 , Issue 3 , June 1980 , pp. 204 - 210
Copyright
Copyright © Clay Minerals Society 1980

Footnotes

1

Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. No. 233-E, 1978 series.

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