MODELLING OF PROTONATION CONSTANTS OF HALLOYSITE CLAY NANOTUBES IN VARIOUS AQUEOUS MEDIA, AT DIFFERENT IONIC STRENGTHS

Abstract

In the last decade nanoparticles have assumed more and more importance because of their particular properties mainly due to the nanometer-scale dimensions that confer them a large surface/volume ratio. Among nanomaterials one of the most studied is the halloysite that, as well as the other natural clay minerals is safe for human and environmental friendly. Halloysite is abundant and cheap and is present in large deposits worldwide like those in New Zealand, France, Belgium and China [1,2]. 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 ~15 and 50 -70 nm, respectively [1,2]. In each layer the SiOH and the AlOH groups are disposed on the external and the internal surfaces, respectively. As consequence, the chemistry of the lumen and of the outer surface of HNTs is completely different. In particular, in each nanotube the inner surface is positively charged and the outer surface has an excess of negative charges in a wide pH range. The particular structure of HNTs makes this kind of clay mineral very useful for different purposes and several papers and reviews have been published on their different applications [1-3]. The behavior of HNTs in all these applications is strictly related to their acid-base properties that here have been studied by ISE-H+ potentiometric titrations in several interacting and non interacting ionic media, in the range 0.025 ≤ I mol L-1 ≤ 1.000, 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 related to the HNT1 groups 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 [4]. The acid – base behavior of HNT2 groups were well defined by one protonation constant calculated by processing the potentiometric titration data with the computer programs STACO and BSTAC.