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A Model of Clay Swelling and Tactoid Formation

Published online by Cambridge University Press:  02 April 2024

W. B. Kleijn  and
J. D. Oster
W. B. Kleijn
Affiliation:
U.S. Salinity Laboratory, United States Department of Agriculture, Agricultural Research Service, Riverside, California 92501 Department of Soil and Environmental Sciences, Riverside, California 92521
J. D. Oster
Affiliation:
U.S. Salinity Laboratory, United States Department of Agriculture, Agricultural Research Service, Riverside, California 92501 Department of Soil and Environmental Sciences, Riverside, California 92521
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Abstract

An electrostatic model for the stability of clay tactoids (stacks of parallel clay platelets at ~10 Å separation) in an aqueous solution has been developed. The counter ions located in the interstitial water layers are assumed to be in equilibrium with the bulk solution. Generally, the counter-ion charge density is slightly different in magnitude from the platelet charge density. Approximating the discrete charges by homogeneously charged planes, a one-dimensional potential distribution can be calculated. From this the Gibbs energy of electrostatic interaction (using single platelets as a reference) can be computed. The model predicts that clay minerals with high (vermiculite, mica) and low (pyrophyllite, talc) degrees of cationic substitution form stable tactoids. For smectites, charge density, electrolyte concentration, and counterion species determine the swelling characteristics. At a particular charge density, lower valences of the counter ions and lower electrolyte concentrations lead to increased swelling. If tactoids are formed, the number of platelets is governed by a dynamic equilibrium between electrostatic forces, van der Waals forces, and external forces, such as shear forces due to hydrodynamic flow.

Резюме

Резюме

Резюме—Была развита электростатическая модель стабильности глинистых тактоидов (групп параллельных глинистых пластинок, отделенных 10 Å) в водном растворе. Предполагается, что противоионы, расположенные в промежуточных водных слоях, находятся в равновесии со всем раствором. Обычно, плотность заряда противоионов отличается немножко по величине от плот-ности заряда пластинок. Одноразмерное распределение потенциала может быть вычислено, заменяя дискретные заряды равномерно заряженными площадями. Из этого можно вычислить энергию Гиббса электростатического взаимодействия (используя отдельные пластинки как базу). Модель предсказывает, что глинистые минералы с высокой (вермикулит и слюда) и низкой (пиро-филлит, тальк) степенью катионной подстановки образовывают стабильные тактоиды. Плотность заряда, концентрация электролита и противоионы определяют характеристики набухания смекти-тов. При определенной плотности заряда низшие валенции противоионов и низшие концентрации электролита приводят к увеличению набухания. Если формируются тактоиды, число пластинок определяется динамическим равновесием между силами: электростатическими, Ван дер Ваальса и внешними, такими как срезывающие силы, возникающие при гидродинамическом течении. [E.C.]

Resümee

Resümee

Es wurde ein elektrostatisches Modell für die Stabilität von Ton-Taktoiden (Pakete von parallelen Tonplättchen mit ~10 Å Abstand) in einer wässrigen Lösung entwickelt. Dabei wird angenommen, daß die Gegenionen im Zwischenschichtwasser im Gleichgewicht mit der Gesamtlösung sind. Im allgemeinen ist die Größe der Ladungsdichte der Gegenionen etwas verschieden von der der Lagerungsdichte der Tonschichten. Betrachtet man die einzelnen Ladungen annähernd als homogen geladene Ebenen, so kann eine eindimensionale Potential-Verteilung berechnet werden. Daraus kann die Gibbs'sche Energie der elektrostatischen Wechselwirkung berechnet werden, wobei einzelne Tonschichten als Bezug verwendet werden. Das Modell sagt voraus, daß Tonminerale mit einem hohen (Vermiculit, Glimmer) und einem niedrigen (Pyrophyllit, Talk) Kationenaustausch stabile Ton-Taktoide bilden. Bei Smektiten bestimmen Ladungsdichte, Elektrolytkonzentration und Art des Gegenions die Art der Quellung. Bei einer bestimmten Ladungsdichte führen niedrige Wertigkeiten der Gegenionen und niedrige Elektrolytkonzentrationen zu einer Zunahme der Quellbarkeit. Wenn Ton-Taktoide gebildet werden, wird die Zahl der Tonschichten durch ein dynamisches Gleichgewicht Zwischen elektrostatischen Kräften, Van der Waals'schen Kräften und äußeren Kräften, wie z.B. Scherkräften, aufgrund hydrodynamischen Fließens, bestimmt. [U.W.]

Résumé

Résumé

On a dévelopé un modèle électrostatique pour la stabilité de tactoïdes argile (empilements de plaquettes d'argile parallèles à ~10 Å de séparation) dans une solution aqueuse. Les contre-ions situés dans l'eau interstitiale sont supposés être en équilibre avec la solution en masse. Généralement la densité de charge du contre-ion diffère légèrement en magnitude de la densité de charge de la plaquette. On peut calculer une distribution potentielle à une dimension en approximant les charges discrètes par des plans chargés de manière homogène. A partir de ceci, l’énergie d'interaction électrostatique Gibbs (utilisant des plaquettes simples comme référence) peut être calculée. Le modèle prédit que des minéraux argile ayant de hauts (vermiculite, micas) et bas (pyrophillite, talc) degrés de substitution cationique forment des tactoïdes simples. Pour les smectites, la densité de charge, la concentration d'electrolyte, et l'espèce de contreion déterminent les caractéristiques d'enflement. A une densité de charge particulière, des valences de contre-ions plus basses, et de plus basses concentrations d'electrolyte, mènent à un accroissement d'enflement. Si des tactoïdes sont formés, le nombre de plaquettes est gouverné par un équilibre dynamique entre les forces électrostatiques, les forces van der Waals, et des forces externes telles des forces dues au flot hydrodynamique. [D.J.]

Keywords

Electrostatic forcesMicaPyrophylliteTactoidTalcSwellingVermiculite
Type
Research Article
Information
Clays and Clay Minerals , Volume 30 , Issue 5 , October 1982 , pp. 383 - 390
Copyright
Copyright © 1982, The Clay Minerals Society

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