Structure and crystal chemistry of clay minerals

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CHAPTER 2: LITERATURE OVERVIEW

The following pages present a concise overview of clay science in general, and fundamental research into clays and clay minerals, with more comprehensive elaboration on the crystal chemistry of mixed-layer structures and rectorite, and mica and smectite group species as component layers in the structure of rectorite. The state of applied research on nanoclays (rectorite), biopolymers (chitosan) and nanocomposites is given, with emphasis on clay films and bionanocomposite films (NCF), specifically rectorite/chitosan composites. A literature review on all major analytical and experimental techniques relevant to this study is also presented.

Clays, clay minerals and mixed-layer structures

The study of clays and clay minerals dates back several centuries with a series of scientific and technological advances published in an impressive volume of knowledge available today. Among others, some of the fundamentals of clay science are set out in well-known books authored by (Bergaya and Lagaly, 2013; Bergaya et al., 2006; Brindley and Brown, 1984; Grim, 1962; Moore and Raynolds, 1997; Newman, 1987; Velde, 1985). It is a common belief that clays and clay minerals are the materials of the 21st century because they are environmentally friendly, naturally abundant and relatively inexpensive. Both fundamental and applied research are driven by a multi-disciplinary and multi-scale approach linking materials science and engineering with colloid science at macro-, micro- and nano-scale.

Definitions and terminology

The word “clays” was first assigned to fine-grained material found in geological formations by Agricola (1494–1555) almost five centuries ago in 1546 (Guggenheim and Martin, 1995), but was only identified as a mineral species at the beginning of the 19th century  in  the  production  of  ceramic  materials.  The  working  bodies  dealing   with nomenclature and definitions of clay and clay minerals are the Joint Nomenclature Committee (JNCs) of the Association Internationale pour l’Etude des Argiles (AIPEA) and the Clay Mineral Society (CMS). Because clay research, fundamental or applied, is such a dynamic field of science, there is an ongoing debate about and numerous revisions of clay nomenclature and definitions. These constantly evolve due to the expanding multidisciplinary approach to the study and the use of these unique materials. However, the distinction between clays and clay minerals is commonly accepted across all fields and the two terms should not be used interchangeably (Guggenheim and Martin, 1995). In essence, clay minerals are defined as naturally occurring, or synthetic, phyllosilicates or non-phyllosilicates, primarily fine-grained porous materials that impart plasticity when wet and harden upon drying and firing (Brindley, 1966; Brindley et al., 1968; Guggenheim et al., 2006; Guggenheim and Martin, 1995; Martin et al., 1991).
For clarity, the following terminology and definitions are defined here and will be applied as such throughout this document:

• • Lattice: to be used in a strict crystallographic sense for a uniform distribution of points in space. There are 14 standard types of lattice, known as Bravais lattices. Phyllosilicates are based mainly on monoclinic and triclinic
  • Plane, sheet, layer, interlayer and structure unit: to be used in the following manner: a plane of atoms, a tetrahedral or octahedral sheet, a 1:1 or 2:1 layer, interlayer In this order they refer to increasingly thicker arrangements, i.e. a sheet is a combination of planes and a layer is composed of sheets. Layers may be separated by interlayer space occupied by cations, hydrated cations, hydroxide octahedral groups or organic molecules (Brindley et al., 1968). An arrangement of a layer plus an interlayer is referred to as a structure unit. It may contain one or more chemical formula units (Z) (Bailey, 1984).
  • Unit cell, lattice constants and Miller indices (usage of the terms is as agreed by the IUCr (International Union of Crystallography)): a unit cell is the parallelepiped built on the vectors a, b, c of a crystallographic basis of the direct lattice. A unit cell is defined by lattice constants a, b and c, lattice vectors and interaxial angles α, β, and γ (always given in italic font). There are 7 different types of unit cell defined as crystal systems and 14 standard (Bravais) unit cells that describe all possible lattice networks. Miller indices (hkl) for crystallographic planes – these are the reciprocals of the fractional intercepts that the plane makes with the x, y, and z axes of the three nonparallel edges of the cubic unit cell.
  • Particles, aggregates and assembly: clay particle is a generalised term also referred to as a crystallite, crystal or It is defined as an assembly of layers; an assembly of particles is referred to as an aggregate (Figure 2-1). An arrangement of particles and aggregates results in different morphologies, i.e. plates, laths or fibres, and the formation of interlayer, interparticle and interaggregate spaces or pores (Bergaya and Lagaly, 2013).
  • Delamination and exfoliation: these occur in aqueous dispersions when intercalated water molecules cause the separation of two successive layers. When interaction between the separating layers is sufficient enough to maintain some crystallographic integrity in the system, the separation is called delamination. With increasing separation no further interaction between the delaminated units is possible and eventually they cease to exist as a structure unit but become independently mobile in the liquid media, i.e. layers are completely exfoliated (Bergaya and Lagaly, 2013).

Structure and crystal chemistry of clay minerals

Clay minerals are hydrous layered silicates and belong to the larger group of phyllosilicates. The elementary structural unit is the Si-O tetrahedral linkage extending in continuous two-dimensional tetrahedral sheets (T) of composition T2O5 in a hexagonally symmetric network along the a-b crystallographic directions. All phyllosilicates contain silicate or aluminosilicate layers in which sheets of tetrahedrally coordinated cations (T – usually Si4+, Al3+ or Fe3+) link through shared oxygens to adjacent sheets of octahedrally coordinated cations (O – most commonly Al3+, Fe3+, Fe2+ or Mg2+). Each tetrahedron links to adjacent tetrahedra in a lateral direction by three corner oxygens (the basal oxygens), while the fourth corner oxygen (the apical oxygen) is positioned in a direction normal to the tetrahedral basal plane and is shared with a neighbouring octahedral sheet. The Si-O bond distance is about 1.62 Å and the O-O distance about 2.64 Å. Up to half of the Si4+ atoms may be replaced by the larger Al3+ atom, causing an increase in sheet dimensions as the Al-O distance is about 1.77 Å (Moore and Raynolds, 1997). Each octahedron consists of a cation coordinated by six oxygens or hydroxyl groups and links to adjacent octahedra by sharing edges. The resulting sheets of edge-shared octahedra have a hexagonal or pseudo-hexagonal symmetry. There are two configurations of octahedra depending on the crystallographic position of the OH- group, i.e. a cis-oriented octahedron with OH- along the edge and a trans-oriented octahedron with OH- along the diagonal. When one tetrahedral sheet (T) is linked to one octahedral sheet (O), a 1:1 (TO) layer is formed as in kaolinites. Respectively there are six octahedral sites and four tetrahedral sites in the unit cell of a 1:1 (TO) layer structure.
When one octahedral sheet (O) is linked to two tetrahedral sheets (T), i.e. one on each side, a 2:1 layer structure (TOT) is produced as in smectites and micas (Figures 2-3 and 2-4). This configuration results in six octahedral and eight (four on each side) tetrahedral sites in the 2:1 (TOT) layer unit cell. The two tetrahedral sheets are in inverted orientation to each other, i.e. they are both oriented with their apical oxygens (tips) towards the central octahedral sheet (Figures 2-3 and 2-4). Furthermore, two-thirds of the hydroxyl groups are replaced by tetrahedral apical oxygen atoms. The remaining third hydroxyl ion fits into the hole (gallery) in the hexagonal ring made by the apical oxygens of the tetrahedral sheet. Thus, the surfaces on both sides of the 2:1 layer consist of basal oxygens. The thickness of a 2:1 layer varies, depending on interlayer occupancy, from 0.91 – 0.95 nm in talc and pyrophyllite (empty interlayer space) to 1.40 – 1.45 nm in chlorite. The higher value for chlorites is ascribed to the additional octahedral interlayer sheet of cations, i.e.  showing a TOTOint  configuration (Brindley and Brown, 1984).

ABSTRACT 
ACKNOWLEDGEMENTS 
DECLARATION
THESIS OUTLINE 
LIST OF FIGURES 
LIST OF TABLES 
LIST OF ABBREVIATIONS 
CHAPTER 1 INTRODUCTION
1.1 Background
1.2 Research objectives
1.3 Research methodology
1.3.1 Purification, modification and characterisation of rectorite clay
1.3.2 Rectorite clay films (RecF): preparation, characterisation and functional properties
1.3.3 Rectorite/chitosan nanocomposite films (Rec/CS NCF): preparation, characterisation and functional properties
CHAPTER 2 LITERATURE OVERVIEW 
2.1 Clays, clay minerals and mixed-layer structures
2.1.1 Definitions and terminology
2.1.2 Structure and crystal chemistry of clay minerals
2.1.3 Mixed-layer clay minerals
2.1.4 Nomenclature and classification
2.1.5 Properties of clay minerals
2.2 Smectite group
2.3 Mica group
2.5 Chitosan
2.6 Rectorite, nanoclays, biopolymers, bionanocomposites: state of research – fundamental and applied
2.6.1 Rectorite research
2.6.2 Clay films
2.6.3 Bionanocomposites: polysaccharides, rectorite-chitosan nanocomposite films
2.7 Analytical and characterisation techniques
2.7.1 Brunauer–Emmett–Teller (BET)
2.7.2 Infrared and Fourier transform infrared spectroscopy (FTIR)
2.7.3 Characterisation of functional properties
2.7.3.1 Mechanical properties
2.7.3.2 Optical property
2.7.4 Microscopy
2.7.4.1 Scanning electron microscopy (SEM)
2.7.4.2 Transmission electron microscopy (TEM)
2.7.5 Nuclear magnetic resonance (NMR) spectroscopy
2.7.6 Particle size determination and -potential
2.7.6.1 Particle size distribution (PSD) in submicron to millimetre size range
2.7.6.2 Particle size distribution (PSD) in nanometre to submicron size range
2.7.6.3 Zeta potential
2.7.7 Thermal analysis
2.7.8 X-ray diffraction (XRD)
2.7.9 X-ray fluorescence (XRF) spectroscopy
CHAPTER 3 EXPERIMENTAL: MATERIALS, METHODOLOGY, CHARACTERISATION TECHNIQUES AND INSTRUMENTATION
3.1 Materials
3.2 Primary processing of rectorite clay material
3.3 Purification of rectorite
3.4 Modifications of purified rectorite
3.5 Preparation of rectorite clay films (RecF)
3.6 Preparation of NH4+-rectorite-chitosan nanocomposite films (Rec/CSNCF)
3.7 Sample preparation and treatments for XRD analysis
3.8 Rectorite structure refinement, modelling and quantitative phase analysis
3.9 Characterisation techniques
3.10 Characterisation of functional properties
3.11 Statistical analysis
CHAPTER 4 RESULTS 
4.1 Characterisation of neat and cation-exchanged rectorite
4.2 Characterisation of organically modified rectorite
4.3 Rectorite clay films (RecF)
4.4 NH4-rectorite-chitosan bionanocomposite films (NH4-Rec/CS NCF)
CHAPTER 5 KEY FINDINGS AND DISCUSSION 
5.1 Characterisation of rectorite
5.2 Rectorite clay films
5.3 NH4-rectorite-chitosan bionanocomposite films (NH4-Rec/CS NCF)
CHAPTER 6 CONCLUSIONS 
6.1 Characterisation of rectorite
6.2 Rectorite clay films
6.3 NH4-rectorite-chitosan bionanocomposite films (NH4-Rec/CS NCF)
Appendices 
Appendix I: Chitosan Certificate of Analysis
Appendix II. Refinement data for modelled rt_hkl phase used for QPA
Appendix III. Rectorite-only films dataset
Appendix IV. Publications emanating from this research
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