Nanoclays are nanoparticles of layered mineral silicates. Depending on chemical composition and nanoparticle morphology, nanoclays are organized into several classes such as montmorillonite, bentonite, kaolinite, hectorite, and halloysite. Organically-modified nanoclays (organoclays) are an attractive class of hybrid organic-inorganic nanomaterials with potential uses in polymer nanocomposites, as rheological modifiers, gas absorbents and drug delivery carriers.
Plate-like montmorillonite is the most common nanoclay used in materials applications. Montmorillonite consists of ~ 1 nm thick aluminosilicate layers surface-substituted with metal cations and stacked in ~ 10 µm-sized multilayer stacks. Depending on surface modification of the clay layers, montmorillonite can be dispersed in a polymer matrix to form polymer-clay nanocomposite. Within the nanocomposite individual nm-thick clay layers become fully separated to form plate-like nanoparticles with very high (nm × µm) aspect ratio (Figure 1A). Sigma-Aldrich, in collaboration with the Nanocor Corporation, offers a range of montmorillonite nanoclay products with different organic modifications optimized to be compatible with various polymer systems.
A. TEM micrograph of 2% nylon-clay nanocomposite showing nm-thick platelets of montmorillonite clay fully dispersed in the polymer matrix. Inset shows schematic of montmorillonite aluminosilicate layer structure.
B. TEM micrograph of halloysite clay nanotubes.
Halloysite is a naturally occurring aluminosilicate nanotube (Figure 1B). The two-layer halloysite tubes are chemically similar to kaoline and have average dimensions of 15 × 1000 nm comparable to carbon nanotubes. Halloysite tubes are hollow and can be used for controlled delivery and release of drugs as well as nanocomposite and rheology modification applications.