Characterisation of properties of various halloysites relevant to their use as nanotubes and microfibre fillers Pooria Pasbakhsh a, ⁎, G. Jock Churchman b , John L. Keeling c a Department of Mechanical Engineering, School of Engineering, Monash University Sunway Campus, Bandar Sunway, 46150 Selangor, Malaysia b The University of Adelaide, School of Agriculture, Food and Wine, Adelaide, Australia c Geological Survey of South Australia, Department for Manufacturing, Innovation, Trade, Resources and Energy, Adelaide, Australia abstract article info Article history: Received 12 March 2012 Received in revised form 6 June 2012 Accepted 27 June 2012 Available online 28 August 2012 Keywords: Halloysite nanotubes Elemental analysis X-ray diffraction analysis Transmission electron microscopy Zeta potential measurements N 2 adsorption analysis There is increasing research interest on new industrial applications for the clay mineral halloysite where greater use is made of its natural tubular morphology, nano-scale diameter and contrasting chemistry on ex- ternal and internal surfaces. Halloysite nanotubes, commonly referred to as HNTs, have potential applications as microfibre fillers, carriers for the supply and controlled or sustained release of active agents for drug deliv- ery and anticorrosion coatings, in nanoreactors or nanotemplates, and for the uptake of contaminants or pol- lutants. In this study, various properties were measured on 6 halloysites from different geographical and geological environments from Australia, New Zealand and the USA. From the results, inferences were drawn on their comparative suitability for new uses. The characterisation included identification of impuri- ties by X-ray diffraction (XRD), morphology, surface area and pore volume by electron microscopy and nitro- gen absorption, the determination of exchangeable cations, and measurement of zeta potential over a wide range of pH. Halloysite content in individual samples ranged from 84 to 98%. Impurities included minor quartz, cristobalite, kaolinite, gibbsite, alunite, iron oxides and anatase. Variation in halloysite morphology and the levels of impurities had the most effect on surface area and internal pore volume. Samples with low levels of impurities and regular, thin-walled tubes reported the highest pore volumes associated with the cylindrical cavity or lumen in halloysite tubes. Surface areas varied from 22 to 81 m 2 .g -1 and the propor- tion of pore space associated with the HNT lumen ranged from 11 to 39%. When the properties of the 6 dif- ferent halloysites were assessed relative to the requirements for halloysite as nanotubes for either additives or carriers, one showed exceptional characteristics for both types of application but it occurs only rarely. An- other halloysite that is moderately suitable for use as an additive but not a carrier occurs in a large deposit. The other samples each showed some limitations of suitability for use as an additive and/or as a carrier. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Halloysite is a 1:1 layer silicate clay mineral where each layer is composed of a tetrahedral (Si–O) and an octahedral (Al–OH) sheet identical to those in kaolinite, but halloysite has a generally higher, but variable, water content in the interlayer spaces (Churchman et al., 1995; Joussein et al., 2005). The presence of water in the interlayer at the time of formation acts to reduce the electrostatic interaction between adjacent layers and facilitates curvature of the layers to ac- commodate the mismatch in the dimensions of the octahedral and tetrahedral sheets (Bailey, 1990; Singh and Mackinnon, 1996). Conse- quently, halloysite often crystallises with a tubular morphology with the Al–OH sheet forming the inside and the Si–O sheet the outside. In kaolinite, without interlayer water, the mismatch between the oc- tahedral and tetrahedral sheets is reduced by rotation of silica tetrahedral and distortion of the tetrahedral sheet to give a platy morphology. The interlayer water in halloysite is readily and irreversibly lost on heating or drying but the tubular morphology is retained. The con- trasting chemical composition of halloysite on the inside of the cen- tral void or lumen compared with the outer surfaces would be expected to affect the charges carried by the tubes at different pH. It is known that at low pH, the lumen space of halloysite carries a pos- itive charge on the inside of the tube, while the outer surface carries a negative charge (Levis and Deasy, 2002). While halloysite can also form as spherical, platy and partly rolled particles (Churchman et al., 1995; Joussein et al., 2005), it is the tubu- lar form that has attracted most interest for new technology applica- tions. Individual tubes vary from submicron to several microns in length and typically are less than 100 nm in diameter with the lumen forming a cylindrical void with a diameter between 5 and 30 nm. Halloysite and kaolinite have been used in industry for centu- ries and are known to have a low environmental impact and to result in few health issues. Halloysite nanotubes (HNTs) are a potential Applied Clay Science 74 (2013) 47–57 ⁎ Corresponding author. Tel.: +60 551 46211; fax: +60 551 46020. E-mail address: pooria.pasbakhsh@monash.edu (P. Pasbakhsh). 0169-1317/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2012.06.014 Contents lists available at SciVerse ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay