Table Of ContentDevelopments in Soil Science 9
FORMATION AND PROPERTIES OF
CLAY- POLYMER COMPLEXES
B.K.G. THENG
Soil Bureau, Department of Scientific and Industrial Research,
Lower Hutt, New Zealand
ELSEVIER SCIENTIFIC PUBLISHING COMPANY
Amsterdam Oxford New York 1979
ELSEVIER SCIENTIFIC PUBLISHING COMPANY
335 Jan van Galenstraat
P.O. Box 211, 1000 AE Amsterdam, The Netherlands
Distributors for the United Stutes and Canada:
ELSEVIER/NORTH-HOLLAND INC.
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New York, N.Y. 10017
Library of Congress Cataloging in Publication Data
Theng, B. K. G.
Formation and properties of clay-polymer com-
plexes.
(Developments in soil science ; v. 9)
Bibliography: p.
Includes index.
1. Clay minerals. 2. organic geochemistry.
3. Soilchemistry. I. Title. 11. Series.
~~389.62.5T4 a 631.4 '17 78-13704
ISBN: 0-444-41706-0 (VO~9.)
ISBN: 0-444-40882-7 (series)
0E lsevier Scientific Publishing Company, 1979.
All rights reserved. No part of this publication may be reproduced, stored in a retrieval
system or transmitted in any form or by any means, electronic, mechanical, photocopy-
ing, recording or otherwise, without the prior written permission of the publisher,
Elsevier Scientific Publishing Company, P.O. Box. 330, Amsterdam, The Netherlands.
Printed in The Netherlands
To the memory of
MORICE FIELDES
In appreciation of his
integrity and vision
ix
PREFACE
This book has been in planning since 1968 when I first began to search
and abstract the literature on clay-organic systems. The intention then was
to produce a critical and comprehensive account of the many and varied
aspects of clay-organic chemistry, embracing those reactions in which
organic polymers were involved. I still had this idea in mind when the oppor-
tunity for writing such a review arose in 1971 with the announcement (by
the London publishing house of Adam Hilger Ltd) of a competition for the
Adam Hilger Prize 1972. It would not have been possible, however, for me
to enter the competition without the active support and enthusiastic encour-
agement from the late Dr. M. Fieldes who was then Director of the Soil
Bureau. It seems fitting, therefore, that I should dedicate this book to his
memory.
On sifting through the published material, it soon became evident that the
reactions of clays with organic polymers should not be discussed concur-
rently with those involving small, well-defined organic species. To do so
would not only lead to excessive length but would also result in a certain
loss of perspective since, although both systems share a great many features,
complex formation between clays and polymers is controlled by variables
which are essentially different from those encountered in the adsorption of
organic micromolecules.
Once the decision was taken to treat each system separately, it seemed
only logical to begin with summarising the extensive data on the behaviour
of small organic compounds at clay mineral surfaces. This work came out in
the form of a monograph (Theng, 1974) in which I stated that the clay-
polymer interaction would form the subject of a separate book. What I have
done here, therefore, is simply to complete the task I set out to do and keep
the promise I made to myself. In writing the present review, I have been
much encouraged by the kind comments which my peers have expressed
with regard to my previous effort. Also, the fact that a synthesis of the data
on clay-polymer systems has not been previously attempted, has provided
me with the necessary inspiration at moments when I would have preferred
to be in the laboratory.
The systematic study of clay-organic reactions, as a whole, is a young
science since the essential crystallinity of the main groups of clay minerals
and the macromolecular concept have not become generally accepted until
relatively recently. However, complex formation between clays and the
organic constituents of soils, has been recognised and described for over a
century (Schloesing, 1874). Indeed, the clay-organic interaction has been
implicated in the very origin of life on earth; certainly, this process is
". . . as vital to the continuance of life as, and less understood than, photo-
synthesis . . ." (Jacks, 1973).
X
Despite its late beginning much progress has been made over the last two
decades in understanding the behaviour of polymeric materials at the solid/
solution interface, research being stimulated by the importance and potential
of the solid-polymer interaction in agriculture, biology, and industry. It can
even be said that theory has outrun practice in the case of uncharged, linear
homopolymers. The picture is less clear as regards polyelectrolytes which, of
course, are more relevant than their non-ionic counterparts to soil and bio-
logical systems. For this reason, much of the data on the clay-polymer
interaction, in general, are not amenable to a quantitative interpretation.
Nevertheless, it seems timely to survey the advances that have so far been
made in this subject and to outline the current state of knowledge. In this
task I have been much assisted by the availability of a number of excellent
reports on some of the topics under discussion. I have attempted, as much as
possible, to present a balanced and coherent account of each topic, pointing
out areas of discordance and uncertainty, and suggesting ways in which such
discrepancies may be resolved. In doing so, I hope to have given something
more than just an annotated catalogue of references. How well or otherwise
I have succeeded in this direction is for the reader to judge.
The growing literature on clay-polymer systems has made it necessary to
be selective as to what aspects and how much, of a given topic, are to be
included if the size of the book is to be kept within manageable bounds
without sacrificing essential depth and detail. The practical applications of
the clay-polymer interaction, for example, are not discussed at great length
because much of the information on this topic is published in the form of
patents which are difficult to assess and collate. The rapid growth of the sub-
ject has also meant that some parts of the book, at least, will become out of
date in a very short time. One also had to draw a line as regards the selection
of published material which, apart from the few last minute additions, does
not go beyond the end of 1976.
For the sake of clarity and convenience, I have divided the contents of the
book into three parts. Part I gives an outline of clay minerals structures, the
properties of aqueous clay suspensions, and the behaviour of polymers at
clay and mineral surfaces. Part I1 describes the reactions of clays with syn-
thetic polymers and Part I11 deals with complex formation between clays
and various classes of naturally occurring polymers. I have tried to write each
chapter in Parts I1 and I11 in the form of a review paper ending with a rather
comprehensive list of references to the original literature. This was done for
the benefit of those readers who may wish only to get quickly acquainted
with the state of the art in a topic of their own specialty.
Although this book is directed primarily to soil scientists and agronomists,
it may also appeal and be of interest to a variety of chemists who, at some
time in their respective professional careers, have to deal with “environmen-
tal” problems in which clays, polymers, and their reaction products feature.
Besides serving as a work of reference, the book may be found useful for
xi
teaching at the graduate and higher levels of agricultural and soil science
courses.
It goes without saying that the reliance of a book of this type on previous
publications extends to illustrated and tabulated data. In this connection, I
wish to acknowledge the many authors, councils of scientific societies, and
publishers who have kindly permitted me to reproduce figures and tables.
Thanks are also due to the draughting staff of the Science Information
Division of D.S.I.R. for drawing up the numerous diagrams. I am indebted
to the library and typing staff of the Soil Bureau for their unstinting assis-
tance, respectively, in procuring journal articles from other libraries and in
converting a long stream of untidily written drafts into neat typescripts. I
am very grateful to Dr. R.B. Miller, Director of the Soil Bureau, for allowing
me to devote myself entirely to bringmg this book to completion over an
extended period of time. Lastly, I would like to thank my family for their
forbearance and understanding.
Lower Hutt B.K.G. THENG
December 1977
REFERENCES
Jacks, G.V., 1973. The biological nature of soil productivity. Soils and Fertilizers, 26:
14 7 -1 50.
Schloesing, Th., 1874. Determination de l’argile dans la terre arable. Comptes Rendus
Hebdomadaires des Seances de 1’Academie des Sciences, 78: 1276-1279.
Theng, B.K.G., 1974. The Chemistry of Clay-Organic Reactions. Adam Hilger Ltd,
London, 343 pp.
3
Chapter 1
THE CLAY MINERALS
1 .l.S TRUCTURAL ASPECTS
Clays are hydrous silicates or alumino silicates and may broadly be de-
fined as those minerals which dominantly make up the colloidal fraction of
soils, sediments, rocks, and waters. The crystalline nature of the majority
of clay minerals is now generally accepted. It should be recalled, however,
that this is a relatively recent concept, dating from the late nineteen-thirties.
By this time the crystallinity and structures of the main groups of layer sili-
cates related to clay minerals had been established, largely through the
pioneering studies of such workers as Ross (1927), Hendricks (1929), Hen-
dricks and Fry (1930), and Pauling (1930a, b).
In the soils literature, the term clay or clay fraction denotes a textural
class of minerals consisting of particles with an equivalent spherical diam-
eter of less than 2 pm, not all of which may be crystalline. Indeed, the finely
divided, highly disordered minerals in soil are probably the most active in
terms of their behaviour towards plant nutrients and organic constituents.
For a detailed discussion of the structures and properties of layer silicate
minerals together with their identification by X-ray diffraction, infrared
spectroscopy, and differential thermal analysis, the reader should consult
the series of monographs written or edited by Brown (1961), Marshall
(1964), Carroll (1970), Mackenzie (1970), Farmer (1974), Gieseking
(1975), and Thorez (1975). A number of books dealing with some particular
aspect of clay chemistry are also mailable, notably that by Van Olphen
(1963) on the colloid chemistry, by Weaver and Pollard (1973) on the miner-
al chemistry, and by Theng (1974) on the clay-organic interaction.
The majority of the clay minerals belong to the class of layer silicates or
phyllosilicates because their structural framework is basically composed of
layers comprising silica and alumina sheets, joined together in varying pro-
portions and stacked on top of each other in a certain way. A silica sheet
has two planes of oxygen/hydroxyl ions, one of which consists of the bases
and the other of the tips or apices of linked Si(0, OH)4 tetrahedra. Similarly,
an alumina sheet has an upper and a lower plane both consisting of hydroxyl
ions between which is a plane of A13+ ions, octahedrally coordinated to the
hydroxyl groups (Fig. 1.1).
Condensation in a 1 : 1 ratio of a silica sheet with an alumina sheet gives
rise to a 1 : 1, dimorphic or two-sheet mineral as in kaolinite (cf. Fig. 1.2).
Here, the tips of the silica tetrahedra project into an hydroxyl plane of the
octahedral sheet, replacing two-thirds of the hydroxyl ions. In a 2 : 1, tri-
morphic or three-sheet mineral, two silica sheets condense with an alumina
4
w
0 oxyga
@ Hydroxyl
o Aluminium
a Silicon
Fig. 1.1. Schematic representation of a silica tetrahedral sheet and an alumina octahedral
sheet. The mode of condensation between the two sheets is indicated by the rectangles.
sheet. The resultant layer structure is such that the alumina sheet is sand-
wiched between two sheets of inward-pointing, linked silica tetrahedra, as
exemplified by pyrophyllite (cf. Fig. 1.4). The possibility also exists that a
trimorphic layer alternates regularly with an hydroxide sheet to form a
2 : 1 : 1, tetramorphic or four-sheet mineral, such as in chlorite. In the so-
called pseudo-layer silicates (“chain-lattice silicates”), the trimorphic layers
are arranged in chains or bands which are linked together through oxygen
ions. Here, the silica sheets are continuous but the apices of the tetrahedra
in adjacent chains point in opposite directions. The structures so formed
have a corrugated surface with channels running parallel to the chain as in
palygorskite (cf. Fig. 1.9). The clay fraction of soils, derived from volcanic
ash, is mainly composed of highly disordered or non-crystalline hydrous
alumino-silicates collectively termed allophane. In many instances, a gel-
like material consisting of bundles of fine tubes, called imogolite, occurs
in conjunction with allophane (Quantin, 1972).
Since the type of cation occupying tetrahedral and octahedral sites is lim-
ited more by ionic size and coordination than by valency, there is consider-
able scope for isomorphous replacement or substitution in these structures.
The site and extent of this substitution are at the basis for distinguishing
between groups, series, species, and varieties of the phyllosilicates. As indi-
cated above, where A13+ ions occupy octahedral positions, two out of three
sites are filled, giving rise to dioctahedral minerals. In the trioctahedral min-
erals, 3 Mg2+r eplace 2 A13+t hereby filling all octahedral positions. The more
common situation is the partial substitution of Si4+a nd A13+ in tetrahedral
and octahedral sites, respectively, for cations of similar size and coordina-
tion but of different (usually lower) valency. Such a process gives rise to
structures with a permanent net negative charge. Apart from the small com-
pensating effect of internal substitution, this positive charge deficiency is
5
balanced by sorption of extraneous cations which may or may not be ex-
changeable.
The charge (x) per formula unit or half layer-unit-cell which ranges from
-0 to -2 electron charges, is therefore an important parameter in the classi-
fication of the phyllosilicate and related minerals. Another criterion enter-
ing the scheme is the manner in which successive layers are stacked within
a crystal. Thus, many of the groups listed in Table 1.1, have a number of
polymorphs or structural varieties depending on the relative order or disor-
der of layer stacking. As might be expected, a crystal may also consist of
different types of layers which are either randomly or regularly interstrati-
fied. Many pedogenic clay materials have this kind of structural arrange-
ment.
In the structures just described, the symmetry is basically hexagonal. In
practice, however, there is some departure from this ideal. Thus, the arrange-
ment of the surface oxygens of the silica sheet, for example, is more ditrigo-
nal than hexagonal (e.g. Radoslovich, 1975). This is ascribed to the opposed
rotation of alternate silica tetrahedra which, in addition, may be slightly
tilted with respect to the plane of surface oxygens. The extent of rotation
and tilting is influenced by such factors as the geometric fit between the
larger tetrahedral and the smaller octahedral sheets, the amount and site of
isomorphous substitution, and the nature of the charge-balancing cation.
The classification scheme for the phyllosilicates proposed by the Nomen-
clature Committee of the Clay Minerals Society (Bailey et al., 1971) is now
widely accepted. However, we shall adopt the earlier scheme of Mackenzie
and Mitchell (1966) here because it seems more appropriate to our purposes.
Thus, this scheme, set out in Table 1.1, includes the pseudo-layer silicates
(Pedro, 1970) and also allots a place for illite or hydromica, a group of min-
erals which have been used as adsorbents of organic polymers. Of the other
minerals listed, those belonging to the kaolinite and smectite (or mont-
morillonite) groups are of particular relevance. Indeed, the bulk of the data
on the clay-polymer interaction relates to montmorillonite.
Allophane and imogolite which are not included in the phyllosilicate class,
have also featured as adsorbents mainly of proteins (Milestone, 1971) and
humic substances (Wada and Inoue, 1967; Inoue and Wada, 1971). Refer-
ences to chlorite, mica, vermiculite and the palygorskitesepiolite group of
minerals are rare (Orlov et al., 1973; Kodama and Schnitzer, 1974). Only
brief mention will therefore be made of these minerals here.
1.1. l.T he kaolinite group
The layer structure of kaolinite is shown in Fig. 1.2. The indicated layer
thickness of -0.72 nm also represents the basal or d(001) spacing of kaoli-
nite. When viewed under the electron microscope kaolinite appears as more
or less well-defined, hexagonal platelets, ranging in thickness from 0.05 to
TABLE 1.1
Classification scheme for the crystalline clay minerals and related phyllosilicates (from
Mackenzie and Mitchell, 1966)
-
Class Type Formula unit Group Series
charge (x)
Phyllosilicates 1:l -0 Kaolinite Dioctahedral
or layer
silicates
Serpentine Trioctahedral
2:l -0 Pyrophyllite- Dioctahedral
talc Trioctahedral
-
0.2 5-0.6 Smectite or Dioctahedral
Montmorillonite
Trioctahedral
-0.64.9 Vermiculite Dioctahedral
Trioctahedral
-
0.9 Illite Dioctahedral
Trioctahedral
-1.0 Mica Dioctahedral
Trioctahedral
-2.0 Brittle mica Dioctahedral
Trioctahedral
2 : I : 1 Variable Chlorite Dioc tahedral
Di-tri-octahedral
Trioctahedral
Pseudo-layer (2:l) -0.1 Palygorskite- Di-tri-octahedral
silicates sepiolite Trioctahedral
a Only the more common are listed. The nature of isomorphous substitution indicated
refers to the predominant cation types involved.
This is the name, recommended by the Nomenclature Committee of Association Inter-
national pour I’Etude des Argiles (AIPEA), Pedro (1970),r eplacing the term “chain-
lattice silicates”.
The name “hormite” for this subdivision has been rejected by the AIPEA Nomenclature
Committee.
d See text.
