Protonation Constants of Halloysite Clay Nanotubes

Halloysite is one of the most interesting nano-structured clay materials. It is similar to kaolin but has a hollow tubular structure that can be attributable to particular crystallization conditions. Typically, halloysite nanotubes (HNTs) are formed by 15 – 20 aluminosilicate layers, has a length of 1 ± 0.5 μm and inner and outer diameters of 1 - 30 and 30 - 50 nm, respectively [1,2]. In each layer the SiOH and the AlOH groups are disposed on the external and the internal surfaces, respectively. The particular structure of HNTs makes this kind of material very useful for different purposes in several fields. Among the various applications, worth of mention are: i) their use as additive for devices in the capture tumor cells in blood; ii) nanoparticles for the entrapment and controlled release of active molecules; ii) additive agents in wound and personal care; iv) sorbents for water contaminants etc [1,2]. In all these applications, the knowledge of the acid-base properties of HNTs is of fundamental importance. The acid-base properties of HNTs have been studied by ISE-H+ potentiometric titrations in NaCl medium, in the range 0.025 ≤ I / mol L-1 ≤ 0.750 and at T = 25°C. Two functional groups indicated with HNT1 (SiOH) and HNT2 (AlOH) were considered in the analysis of the experimental data. Potentiometric data were processed with four different models previously used for the study of the acid - base properties of natural and synthetic polyelectrolytes: Högfeldt, Linear, modified Henderson-Hasselbalch and diprotic like models [3]. The results obtained with the four models for the protonation constants of HNT1 groups of HNTs were in good accordance, whilst the acid-base properties of HNT2 groups were well defined by one protonation constant calculated by processing the potentiometric titration data with the computer programs STACO and BSTAC.