Learning Nature's Lessons: Biomimetic Kaolinite
Phil Jones
Biomimicry, an “approach to innovation that seeks sustainable solutions to human challenges by emulating nature's time-tested patterns and strategies”, has greatly informed humankind’s scientific progress. From the avian influences of our earliest attempts at flight to more recent advances in fields as diverse as medicine and electronics, we’ve taken inspiration from natural processes and structures tried and tested over millennia.
In particular, we can look to nature’s drive towards efficient use of resources and energy as an inspiration for how we develop new, sustainable technologies to meet the needs of a growing, modernizing world. By leveraging kaolinite’s abundance, along with its recently-discovered nano-scale properties, we can better develop innovative solutions to a range of materials science questions.
Looking to the Sea: Kaolinite for biomimetic applications
Abalone shells have become a major point of interest for materials scientists, as their “tiled” calcium carbonate (“nacre”) provides a remarkably resilient, yet lightweight, structure. The lessons drawn from the study of these shells have drawn attention for diverse applications such as reinforced glass and body armor.
The nacre as found in abalone seashells is comprised of platelets of aragonite (a form of calcium carbonate) bonded together with approximately 5% of a protein with elastic properties.
As it is difficult to make platelets of the calcium carbonate form of aragonite many people have used either plays such as Montmorillonite or synthetic aluminas. Kaolinite, a layer structured alumino silicate, occurs as fine-particle-sized pseudo-hexagonal platelets typically 1 µm in diameter and 1 µm thick. Because of the high elemental abundance kaolin is the world's most abundant clay mineral and deposits occur in many parts of the world.
Different kaolin deposits yield a wide variety of crystal morphology and careful selection of the deposit is necessary for best performance. The kaolinite structure is very sensitive to isomorphic substitutions and will not accommodate very high levels of different cations. As a result many deposits are unusually white and as a result the major use of kaolinite has been for use in paper coating and paper filling applications.
François Barthelat1 at McGill University has probably done the most extensive work of mapping out basic mechanisms for matching nacre. He has identified the faces of the calcium carbonate aragonite are not necessarily flat (similar to some of the kaolinite's in Georgia) and that the polymer interface needs to have an ability to have elastic properties to relieve the strain. His recent work has identified in some detail what may be needed for an assembly of platelets, which calls for a hierarchical assembly of different sizes and a range of overlap structure.
A number of other researchers have been involved in looking at ways to make a biomimetic nacre. Some of the early work was carried out Santa Barbara with people like Herb Waite2, Daniel Morse3, and Angela Belcher4 where they identified some unique polymers that the called elastin. At Northwestern University, Philip Messersmith5 showed that L-dopa simulated the bonding chemistry that sea-based mussels used to attach to rocks. More interestingly this molecule has a bond strength between that of hydrogen bonding and covalent bonding and his work with an atomic force microscope shows that this bond, when broken, can easily reform.
More recently, researchers at Santa Barbara 6 and J J Wilker at Purdue University 7 , have identified that the use of transition metal ions such as iron can make the L-dopa/catechol chemistry even stronger by complexing a number of the molecules together. Philip Messersmith has commented to me privately that it is possible to use borane chemistry as a protectant molecule that can be switched on and off with pH changes. That is to say, one can have a solution or suspension protected by the borane at a neutral pH, apply the formulation, and then with the appropriate change of pH remove the protective nature of the borate enabling the L-dopa to bond. The typical mineral used by Philip Messersmith has been montmorillonite which, while work, or at such low solids of the dewatering rate is impractically slow.
In addition, the layer by layer assembly technique used is exceedingly slow and impractical. Private discussions with Philip Messersmith led to the idea the L-dopa molecule could be attached to a polysaccharide for ease of usage. We will need to work with the starch company ideally to follow this up.
Nanometre-scale Layerd Assemblies
At the University of Michigan, Ann Arbor, Nicholas Kotov8 has also used montmorillonite together with polyvinyl alcohol in a layer by layer assembly. The assembly was then treated with glutaraldehyde so as to cross-link the polyvinyl alcohol and make a surprisingly strong structure. Nicholas Kotov has a very strong background in colloid chemistry, and in his recent article in Science he makes a strong case about other forces being at work once we're at a scale of tens of nanometres. That is to say, the usual theories of colloid chemistry no longer apply and we need to rethink the bonding interactions. When thinking of the hyper-platy kaolinites that can be in the range of 20 to 60 nm thick, we should remember that the double layer, or the immobilized water layer, can be of the range 10 to 40 nm on each side of the crystal.
At RWTH Aachen University, Andreas Walther9, working with researchers such as Olli Ikkala10 at the VTT Technical Research Centre of Finland, have also used montmorillonite pre-treated with polyvinyl alcohol to be applied with a blade applicator to form a robust coating. They first treated the montmorillonite with the polyvinyl alcohols at a low level to get a monolayer coverage, then drying it before re-dispersion and application with a blade applicator. Glutaraldehyde and borates were also used to toughen the structure. This work was published in over four papers, and some of the recent papers lay claim to remarkably good oxygen barrier properties with water stability as well. Their papers describe the clay as core-shell materials and is a novel feature in modifying the way the clay interacts with binders. The key lesson is to pay attention to the surface chemistry of the clay. Most previous work has not taken into account the nature of the clays, the nature and level of dispersants present, or the ability to bond polymers to the surface.
These coatings also show remarkably good thermal barrier properties and fire resistance properties, with numerous potential applications. These coatings with pretreated clay applied to conventional cellulosic base stocks can enable a broader usage of applications and especially in areas where it is possible to avoid the usual bromine based fire retardants.
It should be possible to build this concept and incorporate some of the work done with Josef Breu’s work at Bayreuth so as to make even stronger coupling with the clay. It will be necessary to think through the associated polymers to maintain enough elasticity to dissipate strain
The Wallenberg Wood Science Centre, under the leadership of Lars Berglund, has published a large number of papers building on these biomimetic concepts. While the majority of the work has involved the use of montmorillonite, they have used, in addition to the polyvinyl alcohol, materials such as nano fibrillar cellulose. The oriented clay nanopaper from bio-based component reports superior fire-protection properties11. In earlier work with Andreas Walther at Aachen, reported above, they pretreated the clay with a monolayer of PVOH which provided higher strength. In addition they use Xyloglucan, with attention the interface interactions, as a way of extending the range of mechanical and barrier properties12. Management of the clay polymer interface will be the key way to get past the limitations of simple formulation. The effects of the sphere of hydration and associated cations will be an area to pay closer attention so as to control the way that the kaolin plates interact and bond through a range of binder chemistries. By doing this the Berglund group was able to report multifunctional nano clay hybrids with high toughness with thermal and barrier performances13.
In an attempt to overcome the major solids limitations of bentonite/montmorillonite Berglund found that talc, being coarser, is a step in the right direction.
Looking for Goldilocks
While previous work with Monmorillonite to replicate the natural structures found in abalone has lead us in promising directions, the substance proves too thin for practical applications due to slow dewatering and long drying times. The very low solids also means that the amount of material laid down will be low which leads to very low coat-weights. Similarly, while Talc represents a step in the right direction, it is ultimately too thick to provide the barrier and strength properties needed. By turning to another phyllosilicate, Kaolinite, we find the "just right" particle thicknesses for this particular biomimetic application.
As very few people realize that thin crystal kaolins are available, there is opportunity further research. Kaolinite's ubiquity and unique nanoscale features render it the right choice for building upon the lessons learned from Mother Nature.
Endnotes
1. S M M Valashani et al, RSC Advances (2015) 5, 4780 ↩
2. J H Waite et Al, Biochemistry, (2004) 4,(24) 7653 ↩
3. B L Smith et al, Nature (1999), 399, 761 ↩
4. S Whaley et al., Nature (2000), 405, 665 ↩
5. P Podsiadlo, et al., Advanced Materials, (2007) 19, 949 ↩
6. N Holten-Andersen et al, PNAS, (2011) 108(7) 2651 ↩
7. M J Sever et al, Angew Chem. Commun., (2004) 43, 447 ↩
8. P Podsiadlo et al. Science, (2007) 318, 30 ↩
9. P Das, et al., Nature Commun, (2015) DOI:10.1038/ncomms6967 ↩
10. A Walther, et al., Nano Letts (2010) 10, 2742 ↩
11. F Carosio et al, Apllied Materials & Interfaces, (2015), 7(10), 5847 ↩
12. J J Kochumalayil et al., Biomacromolecules (2013), 14, 84 ↩
13. H Sehaqui et al., ACS Applied Materials & Interfaces, (2013) 5(15) 7613 ↩