# Stability of Halloysite, Imogolite, and Boron Nitride Nanotubes in Solvent Media
## 2 Halloysite Nanotubes  
### Colloidal Stability in Water  
The colloidal stability of HNTs is a crucial aspect that has been investigated with the aim of improving nanoparticle use and range through several possible applications [8]. HNTs' aqueous dispersions can form liquid crystalline phases when high concentrations are approached [53]. Moreover, the transition can be controlled by pH, providing an interesting system for obtaining birfrangent materials under controlled chemical stimuli [53]. In light of that, different strategies for the manipulation of chemico-physical properties into aqueous media are pursued by the most appropriate functionalization of halloysite internal or external surfaces by electrostatic interactions with differently charged surfactants or polyelectrolytes. [PERSON] et al. [54] reported studies on the adsorption of three biopolymers that possess a different charge--namely positive chitosan, neutral hydroxypropyl cellulose (HPC), and negative pectin--onto halloysite nanotube surfaces in an aqueous environment. It was found that the dispersion stability depends on the charge of particles and their dimensions, as well on the viscosity and inter-particle interactions. For instance, the \(\zeta\)-potential values are not deeply modified by adding the nonionic HPC; meanwhile they are shifted toward more positive or more negative values by the addition of charged biopolymers, namely chitosan and pectin, respectively [54]. Generally, \(\zeta\)-potential experiments are conducted to evaluate the surface properties and stability of functionalized nanoparticles, in order to understand if the charge density of both HNTs and polymers can influence the precipitation process. It was observed that HPC strongly delays sedimentation; meanwhile, chitosan and pectin stabilize nanotube dispersion at acidic and basic conditions, respectively. Pectin interacts with the HNTs' positive lumen and it decreases the HNTs' negative net charge. Chitosan, however, interacts on the outer surface. The HPC mechanism is completely different, because it is adsorbed and it creates a steric barrier that avoids agglomeration and settling [54]. [PERSON] at al. [55] reported the preparation of HNT-based supramolecular complexes by the wrapping of DNA onto halloysite. It was observed that HNTs become highly dispersible in water after their interaction with DNA, because the phosphate groups of DNA are re-orientated and can interact with the silica groups on the external surface of HNTs. Most likely, the enhanced colloidal stability is due to the neutralization of the inner positive charge, leading to an increase of the net negative \(\zeta\)-potential and particle-particle repulsions. These findings are confirmed by more recent studies on anionic surfactants and bio-polyanions (pectins) [54; 56]. The effect on the colloidal stability of halloysite nanotubes has been also studied, considering the functionalization with thermosensitive polymers, namely poly(N-isopropylacrylamide) (PNIPAM) [57; 58]. It was observed that PNIPAM interacted with the external surface of HNT and PNIPAM-NH\({}_{2}\) (amine terminated poly(_N_-isopropylacrylamide)) was adsorbed onto the external surface. Moreover, halloysite nanotubes changed their properties within the polymer/HNTs in comparison with the neat clay, thus indicating a transferring of the thermos-responsiveness from polymers to halloysite in the hybrid system [57]. Furthermore, since the dispersions were stable only under the \"critical temperature\", this allowed for preparing systems where the temperature can be tuned in order to have external stimuli-responsive solubilization and delivery, providing a biocompatible and thermosensitive material for the targeted release of active species [58]. Amphiphilic molecules are often used to stabilize nanoparticle dispersions, exploiting their functional groups.