Purification, size separation and ionic fnctionalization of halloysite nanotubes

Official URL: http://risc01.sabanciuniv.edu/record=b1669817 (Table of Contents)

Halloysite nanotubes (HNTs) are tubular clay minerals, with unique chemical composition and surface charge that can be utilized in composite materials for various applications. However, it is critical to utilize well-defined, non-agglomerated HNTs to obtain homogenous nanocomposites. Additionally, functionalization of HNTs by organosilanes improves their physicochemical and mechanical properties. Here, two studies regarding the preparation of HNTs with enhanced characteristics were carried out. First, the purification and size separation of HNTs were introduced by three hierarchical procedures: alkaline treatment, ultrasonication, and three-step viscosity gradient centrifugation. Secondly, the ionic functionalization of HNTs was examined using an ionic solution though the coupling reaction between an organosilane, 3-(Triethoxysilyl)propyl isocyanate (ISO) and an N-Methyltaurine sodium salt (N-MTSS), as the grafting agent. DLS, FE-SEM, XRD, FTIR, and TGA were used to characterize the size distribution, morphology, structure, chemical and thermal behavior of all HNTs, respectively. Raw HNTs (150 – 1103 nm in length) that exist in the form of relatively large agglomerations were considerably broken, and cut in individual nanotubes during the alkaline treatment and ultrasonication. Impurities have been successfully removed from pure HNTs (average length of 126 – 179 nm) by a three-step centrifugation. Characterization of ionically functionalized HNTs showed that although modification did not affect the structure of HNTs, modified samples were well-dispersed compared to unmodified ones, indicating the improvement of dispersion behavior due to the ionically charged outer surface. In addition, most suitable conditions for ISO and N-MTSS reaction, along with pirhana and oxygen plasma pre-treatment of HNTs illustrated the highest level of grafting on HNT surface.