Lamellar materials : hybridization, an insertion problem

The historical overview described in the previous lecture shows that hybrid materials with lamellar or tunnel structures have often been among the precursors of high-performance nanocomposites. Indeed, hybrid materials in this family were conceived and shaped at a very early stage, both in ancient applications such as Mayan pigments (Mayan blue) and Chinese "eggshell" ceramics, porcelains characterized by their extreme finesse, and through numerous industrial developments beginning in the second half of the ^20th century or academic work carried out from the late 1950s by the scientific community studying clays. The physico-chemical properties of these compounds and their low cost have led to strong industrial development in many fields, including those associated with special fluids (drilling, refining, paints), catalysis, the automotive industry (reinforcement, lightening), packaging (barrier effect, lightening), construction (binders and cements), sporting goods, textiles (flame retardants), agronomy (vectors and reservoirs) and cosmetics.

For all these historical, scientific and economic reasons, we thought it wise to describe and discuss in greater depth, right from the first lectures, the concepts and work underlying the major advances in lamellar hybrid materials.

Lamellar hybrid materials consist of mineral layers between which organic or biological components are inserted, integrated or grafted. The properties of these hybrid compounds depend on the cohesion of the mineral layers and their ability to be modified by hybridization, exfoliation, reconstruction and so on. Hybridization strategies need to be tailored to the structure and chemical composition of the mineral layers and their electrostatic charge, as well as to the nature of the organic species inserted and their mode of association.

Lamellar organic-inorganic hybrid materials can be synthesized by post-hybridization or direct synthesis. The various processes and mechanisms for integrating organic or biological components into the interleaf space (by ion-exchange intercalation, acid-base interaction, electron transfer, grafting) or by integration via a direct synthesis mode (metal salt co-precipitation, sol-gel, hydrothermal synthesis...) have been described. The description covers lamellar compounds with positive layers (double lamellar hydroxides [HDL], lacunar hydroxides), negative layers (tetravalent metal phosphonates, smectic clays and vermiculites) and neutral layers (kaolinite-type clays, divalent metal phosphonates, chalcogenides, metal oxides and oxychlorides, metal phosphates, single divalent metal hydroxides).

The second part of the lecture was devoted to describing relevant examples in which hybridization of lamellar compounds with organic or biological molecules or macromolecules creates or improves a physicochemical property.

The improvement of polymer mechanical properties (with gains of over 50% in tensile strength and Young's modulus) via the dispersion of exfoliated nanoclays by insertion of reactive monomers or organic polymers is a landmark result that has enabled the industrial development of hybrid nanocomposites in the automotive, packaging and sports equipment sectors.

Other examples include the inclusion of organic dyes for optics, the insertion of polymers such as polyethylene oxide complexing alkali cations to synthesize ion-conducting hybrids, or polycationic polymers such as chitosan to develop potentiometric sensors for anion detection, electronically conductive lamellar hybrid materials based on clay-carbon or vanadium oxide-polyaniline nanocomposites, which can achieve interesting electrical conductivities of the order of ^10-1 s. ^cm-1. This research paves the way for lightweight hybrid materials that can combine good mechanical properties with the transport or elimination of electrical charges. The multifunctionality accessible in lamellar hybrid materials is then discussed on the basis of three main examples:

1) Palladium-colloid-loaded double lamellar hydroxide (HDL) materials feature acidic, basic and hydrogenating functions which, via catalytic cascade reactions, enable higher yields and increased selectivity for the synthesis of molecules of pharmaceutical interest such as 2-methyl-3-phenyl propanal (MKC-442 analogue active against HIV-1) by reaction of benzaldehyde and propanal with hydrogen.

2) Numerous lamellar hybrid materials are also being developed at the interface with biology. The intercalation of DNA into HDLs enables biohybrids to be obtained for use as non-viral vectors, while hybrid talc analogues easily insert proteins (hemoglobin, etc.), enzymes (glucose oxidase) and nucleic acids (DNA) from which biosensors and biocatalysts can be developed. Without diminishing the performance of the biological part, these biocomposites generally offer better stability than those observed when biocomponents are simply dispersed in a homogeneous medium.

3) The last examples in this lecture concern the very recent development of hybrid materials based on magnetic lamellar simple hydroxides, made up of sheets of divalent metal hydroxides (Ni(II), Co(II)) to which luminescent or photochromic molecules are anchored. In these materials, the magnetic coupling existing within the sheets can be modulated by variations in the optical response of the grafted molecules located in the inter-foil space. These are the first examples of multifunctional magnetic lamellar hybrid materials.