Halloysite clay tubes have 50 nm diameter and chemically different inner and outer walls (inner surface of aluminum oxide and outer surface of silica). Due to this different chemistry, the selective etching of alumina from inside the tube was realized, while preserving their external diameter (lumen diameter changed from 15 to 25 nm). This increases 2-3 times the tube lumen capacity for loading and further sustained release of active chemical agents such as metals, corrosion inhibitors, and drugs. In particular, halloysite loading efficiency for the benzotriazole increased 4 times by selective etching of 60% alumina within the tubes' lumens. Specific surface area of the tubes increased over 6 times, from 40 to 250 m(2)/g, upon acid treatment.

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During drying, an aqueous suspension of strongly charged halloysite clay nanotubes concentrates at the edge of the droplet ("coffee-ring" effect) which provides alignment of the tubes along the liquid-substrate contact line. First, the surface charge of the nanotubes was enhanced by polyanion adsorption inside of the lumen to compensate for the internal positive charges. This increased the magnitude of the ξ-potential of the tubes from -36 to -81 mV and stabilized the colloids. Then, colloidal halloysite was dropped onto the substrate, dried at 65 °C and after a concentration of ∼0.05 mg mL(-1) was reached, the alignment of nanotubes occurred starting from the droplet edges. The process was described with Onsager's theory, in which longer nanorods, which have higher surface charge, give better ordering after a critical concentration is reached. This study indicates a new application of halloysite clay nanotubes in polymeric composites with anisotropic properties, microchannel orientation, and production of coatings with aligned nanotubes.

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Natural halloysite nanotubes (HNTs) were modified with a silane coupling agent, N-β-aminoethyl-γ-aminopropyl trimethoxysilane (KH-792), to form a new adsorbent for Cr(VI) removal. The as-prepared product was characterized by FTIR spectroscopy, TGA, TEM, and specific surface analysis. The results showed that KH-792 was successfully grafted onto the halloysite surface. Modified HNTs exhibited a rapid adsorption rate for Cr(VI) and approached 95% of the maximum adsorption capacity within 5 min. The effects of initial Cr(VI) concentration, temperature, pH, and ionic strength on the adsorption capacity were investigated in batch experiments. The results showed that low temperature was favorable to improve adsorption efficiency, and the adsorption capacity decreased significantly with the increase of pH and ionic strength. The optimum pH was found to be 3À5. The main adsorption mechanism was considered to be electrostatic interaction between protonated amino groups on the adsorbent surface and negatively charged Cr(VI). The results above confirmed that modified HNTs had the potential to be utilized as a low-cost and relatively effective adsorbent for Cr(VI) removal.

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