Magnetic Amphiphilic Composites Based on Carbon Nanotubes and Nanofibers Grown on an Inorganic Matrix: Effect on Water-Oil Interfaces

New magnetic amphiphilic composites were prepared by the catalytic carbon vapor deposition (CVD) growth of carbon nanotubes and nanofibers using ethanol as carbon source and red mud waste (RM, a by-product of the Bayer process of alumina production) as catalyst and support. Temperature-programmed CVD (TPCVD), analyses by X-ray diffractometry (XRD), Mossbauer spectroscopy, energy dispersive X-ray spectroscopy (EDS), Raman spectroscopy, thermogravimetry (TG/DTA), elemental analysis (CHN), superficial area determination (BET), scanning (SEM) and transmission (TEM) electron microscopies and magnetic measurements showed that ethanol reduces the iron ions in the red mud to form magnetic phases, e.g., Fe3O4 and Fe-0, and carbon deposits (5-42 wt.%), particularly nanotubes and nanofibers. The combination of the hydrophobic carbon nanostructures with the hydrophilic Al, Si and Ti oxides present in the RM produced amphiphilic materials with excellent interaction with the water-oil interface. Soybean oil or decalin mixtures with water (completely immiscible) were easily emulsified in the presence of the amphiphilic composites. When the composites were added to stable biodiesel-water emulsions, the amphiphilic particles diffused to the interface oil-water. These composite particles were attracted by a magnet, carrying the oil droplets with them and leading to the complete demulsification and separation of the biodiesel from the water.