Inner- and outer-surface modification of Algerian halloysite nanotubes with triethoxy(octyl)silane and caffeic acid for enhanced functional properties

Abstract

Abstract Halloysite nanotubes (HNTs) face significant challenges in their application due to their aggregation, poor dispersion and high hydrophilicity, which limit their integration into polymer matrices. This study introduces a novel functionalization strategy for Algerian HNTs, targeting their inner and outer surfaces with triethoxy(octyl)silane (OTES) for silanization and caffeic acid (CA) for lumen loading. Comprehensive characterization techniques were used to analyse pristine and OTES-modified HNTs (O-HNTs) and CA-loaded HNTs (CA-HNTs) to evaluate the impacts of both selective agents, which successfully altered the structural, textural, chemical, morphological and thermal HNTs properties. The crystalline structure and changes in crystallite size following surface modification were determined using X-ray diffraction analysis. Brunauer–Emmett–Teller analysis showed that the surface area of O-HNTs increased to 74 m 2 g –1 compared to 54 m 2 g –1 for HNTs, whereas CA-HNTs experienced a surface area decrease to 42 m 2 g –1 owing to pore obstruction, with the pore sizes shifting to 10–12 nm for O-HNTs and to 16 nm for CA-HNTs. Fourier-transform infrared spectroscopy and X-ray fluorescence confirmed effective surface modification through the achievement of successful chemical bonding and a shift in the elemental composition. Morphological analysis using scanning electron microscopy revealed considerable morphological changes in both treatments, and thermogravimetric analysis demonstrated that the thermal stability of HNTs modified with CA was improved, with a higher decomposition peak at 520°C. These modifications effectively improved the dispersion, thermal stability and compatibility of the HNTs, highlighting the potential of the modified Algerian HNTs as promising green nanofillers in polymer nanocomposite applications, such as active packaging and thermal insulation coatings.