No abstract

We have recently introduced a new method of creating homogeneous ultrathin films on solid supports, which is based on the electrostatic attraction between opposite charges. Consecutively alternating adsorption of anionic and cationic polyelectrolytes from their aqueous solution leads to the formation of multilayer assemblies. The multilayer buildup is easily monitored by small angle X-ray scattering (SAXS) and a linear increase of the film thickness with the number of adsorbed layers is observed. In the present study we concentrate on the polyelectrolytes poly(vinyl sulfate) (PVS) and poly(allylamine) (PAH), discussing especially the influence of the adsorption conditions of PVS on the growth and the properties of the resulting films. A stable film growth was found to depend on two prerequisites for the PVS deposition: an adsorption time of at least 15 min and a maximum concentration of added sodium chloride of 0.2 mol/L. In the regime of stable growth the multilayer film is able to smooth the surface roughness of the underlying glass substrate from 18 A to a value of 4.5 A. The addition of small amounts of electrolyte to the PVS solution leads to an increase of the thickness of one layer pair from 13 A (at 0.0 mol of NaCl/L) to 34 A (at 0.9 mol of NaCl/L). This behavior is similar to the well-known "rod-to-coü" transition of polyelectrolytes in solution and due to screening of electrostatic charges on the polymer chain. The films are stable up to temperatures of at least 160 °C but seem to lose trapped water above 60 °C.

Halloysite aluminosilicate nanotubes with a 15 nm lumen, 50 nm external diameter, and length of 800 +/- 300 nm have been developed as an entrapment system for loading, storage, and controlled release of anticorrosion agents and biocides. Fundamental research to enable the control of release rates from hours to months is being undertaken. By variation of internal fluidic properties, the formation of nanoshells over the nanotubes and by creation of smart caps at the tube ends it is possible to develop further means of controlling the rate of release. Anticorrosive halloysite coatings are in development and a self-healing approach has been developed for repair mechanisms through response activation to external impacts. In this Perspective, applications of halloysite as nanometer-scale containers are discussed, including the use of halloysite tubes as drug releasing agents, as biomimetic reaction vessels, and as additives in biocide and protective coatings. Halloysite nanotubes are available in thousands of tons, and remain sophisticated and novel natural nanomaterials which can be used for the loading of agents for metal and plastic anticorrosion and biocide protection.

Halloysite is an alumosilicate tubular clay with a diameter of 50 nm, an inner lumen of 15 nm and a length of 600-900 nm. It is a natural biocompatible nanomaterial available in thousands of tons at low price, which makes it a good candidate for nanoarchitectural composites. The inner lumen of halloysite may be adjusted by etching to 20-30% of the tube volume and loading with functional agents (antioxidants, anticorrosion agents, flame-retardant agents, drugs, or proteins) allowing for formulations with sustained release tuned by the tube end-stoppers for hours and days. Clogging the tube ends in polymeric composites allows further extension of the release time. Thus, antioxidant-loaded halloysite doped into rubber enhances anti-aging properties for at least 12 months. The addition of 3-5 wt% of halloysite increases the strength of polymeric materials, and the possibility of the tube's orientation promises a gradient of properties. Halloysite nanotubes are a promising mesoporous media for catalytic nanoparticles that may be seeded on the tube surface or synthesized exclusively in the lumens, providing enhanced catalytic properties, especially at high temperatures. In vitro and in vivo studies on biological cells and worms indicate the safety of halloysite, and tests for efficient adsorption of mycotoxins in animals' stomachs are also carried out.