Structural features of natural and acids modified chrysotile nanotubes

The structural features of the natural chrysotile have been studied by transmission electron microscopy, scanning electron microscopy, energy-dispersive X-ray spectrometry, thermogravimetric and low-temperature nitrogen adsorption techniques. The chrysotile fibers are present as nanotubes of cylindrical morphology of various forms (rectilinear cylinders, cylinders with cup-like ends, tube twins, cylinder-in-cylinder and cone-in-cone tubes) with the outer diameters of 15–30 nm and the inner ones of 2–6 nm. The surface areas of the raw and the acid leached chrysotile samples obtained by nitrogen adsorption are 15.3 and 63.6 m2/g with the average pore diameter 9.8 and 3.9 nm, respectively. The inner and the outer surfaces of the chrysotile nanotubes are evaluated by the geometric method as 16 and 80 m2/g. The thermogravimetric analysis reveals two main phases of mass loss associated with dehydration and dehydroxylation (with two overlaying steps) processes. The first phase is attributed to the dehydration reaction at low temperature range 293–450 K with activation energy in the range 22–32 kJ/mol. The second phase occurs between 798 and 985 K with activation energy 249–298 kJ/mol for the raw sample and 130–146 kJ/mole for the acid treated one.

► The characterization of the chrysotile porous structure by different methods. ► The determination of the various forms of chrysotile nanotubes. ► The tube twins and cup-like ends of chrysotile nanotubes have been first found. ► The activation energy of the dehydration and dehydroxylation has been evaluated.