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Clay-Polymer Interactions: Summary and Perspectives

Published online by Cambridge University Press:  02 April 2024

B. K. G. Theng
B. K. G. Theng*
Affiliation:
Soil Bureau, Department of Scientific and Industrial Research, Lower Hutt, New Zealand
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Abstract

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The adsorption of uncharged polymers by clays is largely “entropy-driven.” Polymer conformation changes from a random coil in solution to an extended form at the surface in which adsorbed polymer segments or trains alternate with loops and tails extending away from the surface. Although the net interaction energy, ε, per segment-surface contact is small (~1 kT unit), the total energy of adsorption is large because the fraction of train segments, p, is commonly between 0.3 and 0.5. The adsorption isotherms are typically of the high-affinity type, and there is an apparent lack of desorption on dilution. Positively charged polymers (polycations) are adsorbed largely through electrostatic interactions between the cationic groups of the polymer and the negatively charged sites at the clay surface. Here ε ≫ 1 kT unit and p > 0.7, leading to an almost complete collapse of the polymer chain onto the surface. Indeed, beyond a given level of adsorption charge reversal can occur in that the clay-polycation system effectively behaves as an anion exchanger. Little adsorption occurs with negatively charged polymers (polyanions) due to initial charge repulsion between the polymer and the clay surface. Acid pH, a high ionic strength, and the presence of polyvalent cations in the system enhance and promote polyanion adsorption. Uncharged polymers and polycations can enter the interlayer space of expanding 2:1 type layer silicates but polyanions generally fail to intercalate.

The interactions of clays with biopolymers, such as proteins, nucleic acids, and polysaccharides, can be rationalized in similar terms. When intercalation occurs, the interlayer biopolymer is further stabilized against microbial (enzymatic) degradation giving rise to practical applications of clay-polymer complexes as flocculants and soil conditioners. Polyanions are effective as flocculants because of their large “grappling distance,” whereas uncharged polymers are better suited as soil conditioners because they can spread over adjacent clay/soil particle surfaces like a coat of paint.

Резюме

Резюме

Движущей силой адсорбции незаряженных полимеров глинами является, главным образом, энтропия. Конформация полимера изменяется от случайной спирали в растворе до вытянутой формы на поверхности, в которой сегменты адсорбированных полимеров либо их последовательные ряды чередуются с изогнутостями и концевыми частями, простирающимися от поверхности. Хотя чистая энергия взаимодействия, е, на контакт сегмент, поверхность небольшая (~1 кТ), общая энергия адсорбции большая вследствие того, что фракция серийных сегментов, р, обычно находится в пределах между 0,3 и 0,5. Адсорбционные изотермы обычно большого родства и имеется также кажущееся отсутствие десорбции при разбавленнии. Положительно заряженные полимеры (поликатионы) адсорбируются, главным образом, посредством электростатических взаимодействий между катионными группами полимера и отрицательно заряженными местами на поверхности глины. При этом е *** 1 кТ и р >0,7, что приводит к почти полному разрушению полимерных цепочек на поверхности. Действительно, вне данного уровна адсорбции может выступить перемена знака заряда с тем, что система глина—поликатион эффективно ведет себя как обменник анионов. Вследствие начального отталкивания заряда между полимером и поверхностью глины появляется небольшая адсорбция с отрицательно заряженными полимерами (полианионами). Кислотность рН, большая ионная сила и присутствие многовалентных катионов в системе благоприятствуют и ускоряют адсорбцию полианионов. Незаряженные полимеры и поликатионы могут входить в межслойные пространства расширяющихся слоевых силикатов типа 2:1, только полианиоиы, в основном, отказываются переслаиваться.

Взаимодействия глин с биополимерами, такими как белки, нуклеиновые кислоты и полисака-риды могут быть рационализированы подобным образом. Когда пояавляется прослойка, промежуточный биополимер стабилизируется в дальнейшем от микробного (ензиматического) распада приводя к практическому применению глинополимерных комплексов в качестве флоккулянтов и кондиционеров почвы. Полианионы эффективны как флоккулянты из-за их большого „схватывающего расстояния”, в то время как незаряженные полимеры лучше удовлетворяют требованиям в качестве кондиционеров почвы, так как они могут растекаться по прилегающих частицах глины/ почвы подобно растеканию краски. [Е.С.]

Resümee

Resümee

Die Adsorption von ungeladenen Polymeren durch Tone hängt hauptsächlich von der Entropie ab. Die Polymerkonformation ändert sich von einer Spirale in der Lösung zu einer gestreckten Form an der Oberfläche, an der adsorbierte Polymer-Segmente oder Züge mit Polymer-Schleifen und -Enden abwechseln, die von der Oberfläche wegstehen. Obwohl die Vernetzungsenergie, ε, pro Segmentoberflächenkontakt gering ist (~1 kT), ist die gesamte Adsorptionsenergie groß, da die Spaltung der Polymersegmente, p, meistens zwischen 0,3 und 0,5 ist. Die Adsorptionsisotherme sind typisch vom Hochaffinitätstyp, wobei auch bei der Verdünnung eine Desorption offensichtlich fehlt. Positiv geladene Polymere (Polyka-tionen) werden hauptsächlich durch elektrostatische Wechselwirkungen zwischen den kationischen Gruppen des Polymers und den negativ geladenen Stellen auf der Tonoberfläche adsorbiert. Dabei ist ε ≫ 1 kT und p > 0,7, was zu einem nahezu vollständigen Zusammenbrechen der Polymerketten auf der Oberfläche führt. Tatsächlich kann über einem bestimmten Grad der Adsorption eine Ladungsumkehr auftreten, wobei das Ton-Polykation-System wie ein Anionenaustauscher wirkt. Eine geringe Adsorption tritt bei negativ geladenen Polymeren (Polyanionen) auf, die auf dem anfänglichen Ladungsabstoß zwischen dem Polymer und der Tonoberfläche beruht. Ein saurer pH, eine groß Ionenstärke und die Anwesentheit von polyvalenten Kationen im System erhöhen und fördern die Adsorption von Polyanionen. Ungeladene Polymere und Polykationen können in den Zwischenraum von quellfähigen 2:1 Schichtsilikaten gehen, Polyanionen können dagegen im allgemeinen nicht eingebaut werden.

Die Wechselwirkungen von Tonen mit Biopolymeren, wie Proteinen, Nukleinsäuren und Polysaccharide, können auf ähnlichem Weg verwirklicht werden. Wenn ein Einbau stattfindet, ist das Zwischenschichtpolymer gegen mikrobiologischen (enzymischen) Abbau geschützt. Dies führt dazu, daß man Ton-Polymerkomplexe als Flockungsmittel und Bodenstabilisierungsmittel verwenden kann. Polykationen wirken wegen ihres großen “grappling distance” als Flockungsmittel, während ungeladene Polymere besser als Bodenstabilisierungsmittel geeignet sind, da sie sich wie eine Farbhaut über aneinander grenzende Ton/ Boden-Teilchen breiten können. [U.W.]

Résumé

Résumé

L'adsorption de corps polymères sans charge par des argiles est pour la plupart “conduite par entropie.” La conformation de corps polymères change d'une bobine au hasard en solution en une forme étendue à la surface dans laquelle des segments ou des trains de corps polymères adsorbés alternent avec des boucles et des queues s’éloignant de la surface. Malgré que l’énergie nette d'interaction, ε, par contact segment-surface, est petite (~1 kT) l’énergie totale d'adsorption est grande parce que la fraction des segments de train, p, est souvent entre 0,3 et 0,5. Les isothermes d'adsorption sont typiquement de la sorte à haute affinité et il y a un manque apparent de désorption lors de la dilution. Les corps polymères chargés positivement (polycations) sont pour la plupart adsorbés par interactions électrostatiques entre les groupes cationiques du corps polymère et les sites chargés négativement à la surface de l'argile. Dans ce cas, ε ≫ 1 kT et p > 0,7, menant à un effondrement presque complet de la chaîne polymère sur la surface. En effet, au delà d'un niveau donné d'adsorption, un renversement de charge peut se produire en le fait que le système argile-polycation se conduit effectivement comme un échangeur d'anions. Peu d'adsorption se produit avec les corps polymères chargés négativement (polyanions) à cause d'une répulsion de charge initiale entre le corps polymère et la surface argileuse. Un pH acide, une force ionique élevée et la présence de cations polyvalents dans le système accroit et promeut l'adsorption de polyanions. Des corps polymères et des polycations peuvent entrer dans l'espace interfeuillet des silicates en expansion à couches du type 2:1, mais les polyanions ne s'intercalent généralement pas.

Les interactions des argiles avec des corps biopolymères tels que des protéines, des acides nucléiques et des polysaccharides peuvent être rationalisées en des termes semblables. Lorsque l'intercalation se produit, le corps polymère interfeuillet est stabilisé d'avantage face à la dégradation microbiale (enzy-matique), menant à des applications pratiques de complexes argile-polymère en tant que flocculants et de conditionneurs de sols. Les polyanions sont de bons flocculants à cause de leur grande “distance étreig-nante,” tandis que les corps polymères sans charge sont de meilleurs conditionneurs de sols parcequ'ils peuvent s’étendre par dessus des particules argile/sol adjacentes comme une couche de peinture. [D.J.]

Keywords

AdsorptionAnion exchangeFlocculantsMontmorillonitePolyanionPolymerSoil conditioner
Type
Research Article
Information
Clays and Clay Minerals , Volume 30 , Issue 1 , February 1982 , pp. 1 - 10
Copyright
Copyright © 1982, The Clay Minerals Society

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