Abstract
Colloidal stability was investigated in two types of particle systems, namely, with bare (h-HNT) and polyimidazolium-functionalized (h-HNT–IP-2) alkali-treated halloysite nanotubes in solutions of metal salts and ionic liquids (ILs). The valence of the metal ions and the number of carbon atoms in the hydrocarbon chain of the IL cations (1-methylimidazolium (MIM+), 1-ethyl-3-methylimidazolium (EMIM+), 1-butyl-3-methylimidazolium (BMIM+), and 1-hexyl-3-methylimidazolium (HMIM+)) were altered in the measurements. For the bare h-HNT with a negative surface charge, multivalent counterions destabilized the dispersions at low values of critical coagulation concentration (CCC) in line with the Schulze–Hardy rule. In the presence of ILs, significant adsorption of HMIM+ took place on the h-HNT surface, leading to charge neutralization and overcharging at appropriate concentrations. A weaker affinity was observed for MIM+, EMIM+, and BMIM+, while they adsorbed on the particles to different extents. The order HMIM+ < BMIM+ < EMIM+ < MIM+ was obtained for the CCCs of h-HNT, indicating that HMIM+ was the most effective in the destabilization of the colloids. For h-HNT–IP-2 with a positive surface charge, no specific interaction was observed between the salt and the IL constituent cations and the particles, i.e., the determined charge and aggregation parameters were the same within experimental error, irrespective of the type of co-ions. These results clearly indicate the relevance of ion adsorption in the colloidal stability of the nanotubes and thus provide useful information for further design of processable h-HNT dispersions.
Highlights
Halloysite nanotubes (HNT)[1−3] are layered aluminosilicates (Al2Si2O5(OH)4·nH2O) with hollow tubular structure possessing opposite signs of charges on the outer surface and inside the lumen.[4]
The h-HNT particles were functionalized with IP-2 polymer, and both systems, bare and IP-2-modified h-HNTs, were characterized by different techniques to verify the successful adsorption of IP-2 on the h-HNT particles
Colloidal stability studies revealed that the particle aggregation mechanism in each system qualitatively followed the prediction of DLVO theory irrespective of the type of counterions
Summary
Halloysite nanotubes (HNT)[1−3] are layered aluminosilicates (Al2Si2O5(OH)4·nH2O) with hollow tubular structure possessing opposite signs of charges on the outer surface and inside the lumen.[4]. The colloidal stability of commercially available HNTs is limited in aqueous environments, i.e., their dispersions can be destabilized by electrolyte-induced aggregation and subsequent sedimentation.[13,14] alkaline treatment of HNTs
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