Re-Inventing Kaolinite

The Potential of Kaolinite

Clays, whether used in paper coating, cosmetics, or the arts, are largely known as soft squidgy materials with slippery sticky features. Kaolinite is the world's most abundant clay mineral, and has the opportunity to be transformed from a commodity by taking advantage of its complex properties. Developments in areas such as nanotechnology have improved our understanding of these properties, transforming this material from a classic colloid whose nature was deduced from rheology and adsorption/desorption experiments, to one with wide-ranging potential as a building-block in future technologies.

In order to exploit this newly-appreciated potential, the complexities of the particle shape distributions, surface chemistries, and defect structures of kaolinite must be understood, as they can open the way to radically different development programs.

Refining & Redefining

Commercial use of kaolinite has been dominated initially by ceramics and latterly by paper coating and filling.¹ Paper coating developments have driven the search for deposits that are of adequate brightness together with rheological properties that allow it to be coated on to paper at sufficient levels. Current kaolinite processing plants are run as refineries where crude clays are selected by way of particle size and shape, run through diverse mineral separation processes to purify and then distinct particle size fractions are "distilled off" using a variety of centrifuges.

Refining kaolin requires the clay to be dispersed so as to allow de-gritting, mineral separation and particle size selection. Dispersion is best carried out above pH 7, when the edges take on a negative charge that together with the positive charges on the faces results in particle repulsion that provides low-viscosity suspensions. The surface charge is further enhanced through the use of dispersants such as poly-phosphates, sodium silicate and, dominantly in latter years, sodium polyacrylates. Such dispersants cover up the inherent surface chemistry of each kaolin platelet and render them as "inert" particles with a measurable aspect ratio and particle size distribution.

Proprietary techniques to rapidly measure particle shape have allowed the more precise mining of deposits with diverse aspect ratios. What were once thought of as useless "viscous" kaolins of no use in paper coating applications, have turned out to be very high aspect ratio thin crystal kaolins. This presents opportunities to develop new uses in research areas such as barrier coatings, new composites,biomimetic structures and flame retardant properties.

Biomimetics

Ceramicists, when “throwing” on a wheel initially work the clay so as to realign the plates of kaolinite (whose size ranges from 20 to 200 nm in thickness), consequently developing better plasticity and strength of the formed body. Materials scientists should take note of this technique as well as the insights around the formation of aligned platelets similar to mineral liquid crystal structures, so as to develop the best possible structure for strength development.

Previous work with kaolinite has not taken structure into account; it has been difficult to look at the structure in suspension. A deeper understanding of the nature of the dispersion and dispersant types used together with the control of ionic strength give the opportunity to control mineral liquid structure leading to useful ordering of assemblies of clay platelets, creating structures mimicking those found in nature. The well-formed hexagonal platelet structure of kaolinite will lend itself as a building block for biomimetic structures akin to abalone seashells provided the interface chemistry and ability to align particles is taken into account.

Two-Faced “Janus” Kaolin

Creative use of the atomic force microscope by the Jan Miller² group at Utah University has confirmed the presence of two distinct “faces” to the hexagonal kaolin plate; each face is associated with the aluminum hydroxide and silicate layers. They were fortunate enough to have had access to a kaolinite produced by the English China Clay research group, led by Brian Jepson³ in the early 1970's, where they refined it without using dispersants. Jepson’s group dispersed the clay using pH adjustments above pH 7 to de-grit and select for particle size fraction, then filtering and drying at a lower pH after the fine particles flocculated (clumping together).

The Miller group, under the atomic force microscope, sees each planar surface to have different isoelectric points and therefore distinct surface properties. The aluminum hydroxide surface is profoundly hydrophilic while the silicate surface, which should be hydrophobic in a way similar to talc, has a modest hydrophilicity driven by a low-level isomorphic substitution of silicon by aluminum, which stabilizes some hydroxyl groups on the silicate.

Josef Breu⁴ and his group at Bayreuth University have taken advantage of the distinctly different faces by selective surface treatments where the aluminum hydroxide surface is made even more hydrophilic through treatments with a polymethacrylate treatment and the silicate surface hydrophobic by treatments with an appropriate silane. Such two-faced particles are called “Janus particles" and open up the use of kaolinite as a surfactant, use in Pickering emulsions, and to stabilize mixtures on incompatible polymers to form polymer alloys.

Defect Structures

While we describe kaolinite as a layer aluminosilicate with a pure, well-defined crystal structure, many of its properties are the consequence of defect structures and centers. Defect structures drive the majority of materials properties, but the complexities of clay minerals have made it difficult to understand their implications. Defects range from layer shift mismatches, screw dislocations as seen via gold decoration, and Frenkel and Schottky defects (isomorphous substitutions interstitial atoms and lattice vacancies).

As with many valuable materials applications low level doping drives fundamental and important properties. The kaolin lattice can only accommodate very low levels of isomorphous substitutions as higher levels result in different layer silicate structures. The most frequent substitutions are Aluminum for Silicon, Magnesium for Aluminum; Iron (II or III) for Aluminum or Silicon. Where a valence difference occurs, such as Magnesium II for Aluminum III, the deficit in positive charge either results in a Bronsted acid site on the surface or the trapping of a "hole" center (positive charge) on the oxygen bridging Aluminum and Silicon. Jean-Pierre Muller⁵ and his group have done the clearest work in mapping out the nature of this center and others seen by Electron Spin Resonance building on the earlier pioneering work of Jones⁶ et al.

These centers are formed via background radiation over millions of years from very low levels of radionucleotides. A century ago the Bronsted acidity was used as an oil cracking catalyst but was since replaced by higher acidity catalysts such as zeolites. There exists a clear opportunity to take advantage of this surface acidity in applications development and, at the very least, to help explain apparently anomalous results when the performance of a pure kaolinite had been assumed.

Kaolin Nanotubes and nano-plates

The aluminosilicate layers are hydrogen bonded together and help produce vermicular stacks of crystals. These individual crystals can vary in thickness from typically 20 nm to 200 nm depending on the geological conditions of crystallization. Thinner crystals tend to exhibit very high aspect ratio and viscous behavior, the thicker crystals are around 5 to 0:1 aspect ratio and considered to be fluid clays. The use of molecules with high dipole moment, such as urea dimethyl sulfoxide etc., can be inserted between the hydrogen-bonded layers so as to intercalate the kaolin. Christian Detellier⁷ at the University of Ottawa has being able to displace these intercalated molecules with other species, such as triethanolamine, enabling the formation of nano composite material. In some cases it is possible to fully separate the kaolin layers forming extremely thin sheets often with the result of them rolling up to form nano tubes.

Summary

The more detailed understanding of kaolinite resulting from the enhanced tools developed by investments in nanotechnology have opened up the opportunity to reinvent Kaolinite.

The main properties of use are as follows

1. Rapid particle shape measurement
2. Characterization of defect centers
3. Intercalation leading to nano composites applications
4. Identification of the different face surfaces
5. Production of low-cost Janus particles
6. The assembly of biomimetic structures

Endnotes

<sup>1. W Bundy, Applied Clay Science (1991) 5, 397 ↩
2. V Gupta et al; J Colloid Interface Sci., (2010) 344, 362 ↩
3. A Ferris et al, (1975) J Colloid Interface Sci., 51, 245 ↩
4. S Weiss et al., Polymer (2013), 54, 1388 ↩
5. B Clozel et al., Clays & Clay Minerals, (1994), 42(6) 657 ↩
6. B Angel et al, Clay Minerals (1974) 10, 247 ↩
7. CS Lataief et al, Langmuir, (2009) 25, 10975 ↩</sup>