# Stability of Halloysite, Imogolite, and Boron Nitride Nanotubes in Solvent Media
## 1 Introduction  
Since carbon nanotubes were discovered in 1991 [1], the nanotube structure has garnered interest and has been widely researched in other types of particles, like metal nanotubes [2; 3], oxide nanotubes [4; 5], boron nitride nanotubes [6; 7] and nanotubular clays, to study the characteristics of tunable chemistry, surface area, and porosity. The need to reach homogeneous particles dispersion in different solvent media represents one of the main conditions for the use and applicability of those systems [8]. With this aim, this review will be focused on the colloidal stability of inorganic nanotubes, as well as on the most used strategies to prepare well-dispersed suspensions. In particular, these aspects will be investigated for halloysite nanotubes (HNTs) [8], imogolite nanotubes (INTs) [9], and boron nitride nanotubes (BNNTs) [10]. Halloysite nanotubes are composed of a silicon oxygen tetrahedron and alumina oxygen octahedrons forming a kaolinite-like sheet that rolls up, giving the clay its own hollow tubular structure (Figure 1a) [11; 12]. Since the lumen and the external surface are chemically different, they are positive and negative in water, respectively, in the 2-8 pH range [13]. HNT size varies with respect to the natural origin, ranging from 0.5 to 1.5 \(\upmu\)m in length, within 50-70 nm for the outer nanotube diameterand 10-20 nm for the internal diameter [14]. Moreover, they have biocompatibility [15] and show no toxicity in vivo [16] and in vitro [17]. These characteristics make HNTs excellent smart materials for the most diverse applications--for example: food packaging [18; 19; 20], drug delivery [21; 22; 23; 24; 25; 26], environment remediation and wastewater treatment [27], cultural heritage [28; 29; 30; 31; 32], and additives for enhancing the performances of polymers [14; 33]. Imogolite, firstly observed in volcanic soils, is another natural aluminosilicate whose shape is nanotubular, with an external diameter of about 2 nm; its length can be expressed in micrometers [34]. INTs are always arranged into bundles, so it is not possible to observe a single nanotube, but rather a network of bundles (Figure 1b) [35]. Their structure, proposed by [PERSON] in 1972, is very similar to halloysite, namely a layer of orthosilicate tetrahedra overlapping a layer with an aluminum in a dioctahedral configuration [36]. INTs are considered the clay counterpart of carbon nanotubes, and they are very similar if dimensions, aspect ratios, and rigidity are considered [9]. Moreover, imogolite nanotubes are easily synthesized using hydrothermal techniques, without purification steps to do post-synthesis, and they form stable colloidal suspensions in aqueous solvents [34; 37]. The good monodispersity of INTs has motivated researchers to investigate their formation process. Preparation routes for imogolite were developed very quickly. For instance, the possibility of controlling their structure and chemical nature makes them very interesting nano-platforms for various applications [37]. BNNTs were predicted in 1994 [38], and first prepared by [PERSON] et al. [39] in 1995 as carbon nanotube inorganic analogs, by alternating boron and nitrogen instead of carbon, almost without changing the atomic spacing of the graphite-like sheet [10; 38]. The key parameters that influence both the length and size of BNNTs are temperature, catalyst, and boron precursor, as well as duration of the heating process [10]. The external diameter varies from 4 to 300 nm, usually reaching 30-100 nm, and the tubes' lengths are in the range of 500 nm to 1 mm, usually 5-10 \(\upmu\)m [10]. Moreover, by changing the condition of synthesis, a single-walled BNNT, a double-walled BNNT, or a multi-walled BNNT can be prepared [40]. Although they are very similar to CNTs, BNNTs show greater mechanical (Young's modulus of 1.2 TPa) [41], and excellent chemical [42] and electrical properties [43]. One of the critical points for the applications of BNNTs, as well for CNTs, is their very poor dispersibility in water and apolar media.