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Abstract
Halloysite is a unique 1:1 clay mineral frequently appearing with nanotubular morphology, and having surfaces of different polarity with interesting and important technological applications. HNTs can be consolidated naturally in the earth by pressure and thermal flows. In this study of natural consolidated HNTs, the strength and hardness of these materials were found to be dependent on the presence of impurities (gibbsite, alunite, quartz, and other silica minerals), which accounted for the increased stability of such samples. In the absence of impurities, the strength of consolidated HNTs was significantly lower. The first 3D mapping of the pore structure of natural consolidated HNT is provided. The contributions of the porosity within the nanotubes and between the nanotubes were delineated using a combination of non-invasive ultra-small and small-angle X-ray scattering (USAXS/SAXS) analyses, BET/BJH pore size analyses, and computed tomography studies. A total porosity of 40%, as determined by X-ray attenuation and He porosimetry, was found for the natural consolidated HNTs, of which about one-third was due to the inter-HNT porosity. Nano-X-ray computed tomography (nano-XCT) analyses also indicated that 76% of the inter-HNT pores were smaller than 150 nm in diameter. The intra-HNT pore size determined by combined USAXS/SAXS and BET/BJH was about 10 nm. This pore network information is essential for the utilization of natural consolidated HNTs as a model geomaterial to investigate the effects of surface characteristics on confined fluid flow.
Highlights
The morphological organization of materials, including the sizes and shapes of pores, has been shown to result in anomalous thermodynamic, transport, and reactive properties of nanoconfined fluids [1,2]
Most halloysites are composed of nanoscale tubes, they may appear with platy or spheroidal morphology [3]
This study addresses the following research questions: (i) What are the mechanical characteristics of natural consolidated halloysite nanotube (HNT)? (ii) What is the connectivity between the intra- and interlevel porosity? To address these questions, nanoindentation was used to determine the hardness of natural consolidated HNT samples from different regions of the Dragon Mine, which have differing levels of consolidation
Summary
The morphological organization of materials, including the sizes and shapes of pores, has been shown to result in anomalous thermodynamic, transport, and reactive properties of nanoconfined fluids [1,2] In this context, naturally occurring geomaterials with nanoscale tubular architectures are of interest, given the high likelihood of fluids confined within their materials exhibiting anomalous properties. The studied halloysite nanotube (HNT) had an internal pore (intra-HNT) diameter of 12–15 nm, an outer diameter of 60 nm and above, as well as a length of 0.5–10 μm [4,5] Due to their unique physical, mechanical, and structural properties, HNTs have been used for polymer composites, flame retardation, controlled release, environmental remediation, paints and coatings, agriculture, catalysts, molecular sieves, ceramics, and cosmetics [6,7,8]
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