Halloysite nanotubes functionalization with phosphonic acids: Role of surface charge on molecule localization and reversibility

Halloysite nanotubes (HNT) are aluminosilicates bearing an Al-OH terminated inner lumen and a Si-O-Si exposing outer surface, which hold promise in several research fields due to their intrinsic surface duality. Functionalization with octylphophonic acid (OPA) was here investigated as a means to achieve selective functionalization of the HNT lumen and pH-triggered release. Model oxides were adopted to investigate the role of chemical nature and surface charge on the adsorption mode and reversibility of the OPA bond to the surface. Beside silica and aluminium (oxo)hydroxide used to mimic respectively the outer and inner HNT surfaces, titanium dioxide was also studied due to its intermediate isoelectric point and surface acidity. The effect of the functionalization pH and OPA content, along with the pH-dependent adsorption reversibility, were investigated using both spectroscopic characterization and wetting determinations. Results on both model oxides and HNT support a preferential adsorption of OPA on the Al-OH exposing surface. Functionalized HNT retained their inner lumen porosity and water dispersibility, which are desired properties in terms of application. The specific character of the OPA-HNT interaction is discussed with respect to (oxo)hydroxides, particularly in terms of the pH-dependent adsorption reversibility.